Soil
This module is all about soil, as the title implies. Although this may not seem like the most glamorous part of gardening, it is quite literally the foundation of your garden.
"In the spring, at the end of the day, you should smell like dirt."
- Margaret Atwood
- Margaret Atwood
Introduction
So what is soil, anyway? As a medium for plant growth, soil can be described as a complex, natural material derived from disintegrated and decomposed rocks and organic materials that provide nutrients, moisture, and anchorage for plants living on land. Soil is commonly called earth or dirt, though some reserve the term 'dirt' for displaced soil. Soil is a very important part of the Earth's ecosystem; it is a recycling system for nutrients and organic waste, regulates water quality, modifies atmospheric composition through the reserving of carbon gases, and is, most importantly to us gardeners, a medium for plant growth. Because soil lies beneath our feet and is often hidden by paved areas, buildings, and landscaping we do not often think about its impact on our lives, and how much we really depend on it.
Keeping soil healthy is one of the most important things we can do during our lives to support future generations. Most of our food depends on soil, and it is not an easily replaceable resource. Soils form very slowly, as little as 1 cm of soil can be formed in about 500 years. When soil is destroyed, it is not a mistake we can fix in our lifetime. It is of utmost importance that all gardeners really understand what soil is, so that we can be the stewards of this precious resource. There are four principal components of soil:
Keeping soil healthy is one of the most important things we can do during our lives to support future generations. Most of our food depends on soil, and it is not an easily replaceable resource. Soils form very slowly, as little as 1 cm of soil can be formed in about 500 years. When soil is destroyed, it is not a mistake we can fix in our lifetime. It is of utmost importance that all gardeners really understand what soil is, so that we can be the stewards of this precious resource. There are four principal components of soil:
Local soils rarely start out with all four of these principal components, but we can usually make them so even when they don't seem to hold much promise for improvement. With enough effort, you can transform almost any patch of ground into a rich crumbly garden soil in three to five years. To know how much work your soil needs, you have to dig a little! Look at your dirt, feel it, and possibly do some testing. With any luck, you'll end up with the ideal soil composition pictured in the graph. |
Soil Formation
The development of soil from parent rock (also called substratum; the original rock on the Earth) takes thousands of years, and starts with physical and chemical weathering of larger rocks, creating smaller ones. Soil formation is the gradual development of soil from parent materials, and occurs as a dynamic and continuous process of four mechanisms: additions, losses, transformations, and translocations. Additions include the deposition of dust, leaves, animal remains, and the like to the surface of soil. Losses include material lost to erosion and water percolation. Transformation occurs when weathering continues and converts existing minerals into new materials. Finally, translocations occur when clays and dissolved materials are repositioned in the soil thanks to gravity and the action of moving water. These four soil formation processes are controlled and influenced by five essential, naturally-occurring factors:
The development of soil from parent rock (also called substratum; the original rock on the Earth) takes thousands of years, and starts with physical and chemical weathering of larger rocks, creating smaller ones. Soil formation is the gradual development of soil from parent materials, and occurs as a dynamic and continuous process of four mechanisms: additions, losses, transformations, and translocations. Additions include the deposition of dust, leaves, animal remains, and the like to the surface of soil. Losses include material lost to erosion and water percolation. Transformation occurs when weathering continues and converts existing minerals into new materials. Finally, translocations occur when clays and dissolved materials are repositioned in the soil thanks to gravity and the action of moving water. These four soil formation processes are controlled and influenced by five essential, naturally-occurring factors:
The interactions between these five factors is called the CLORPT equation (CLimate, Organisms, Relief, Parent material, and Time). First, changes in climate (mainly temperature) can lead to cracking, chipping, swelling, and shrinking of parent material. These physical changes help to break the rock up into smaller pieces, exposing a larger total surface area. Chemical actions of water, oxygen, carbon dioxide, and acids reduce the rock fragment sizes even further, and also changes their mineral composition. True soil begins to form when organic matter is then added to the weathered rock in the form of decaying plant and animal life, and takes hundreds of thousands of years to fully form. The properties of the resulting soil are closely related to the properties of the parent material in the area. The parent material influences the amount and kinds of nutrients naturally present to support plant growth, the soil's natural texture, and many other physical and chemical properties of the soil. There are many definitions of soil, including:
- "The collection of natural bodies occupying parts of the earth's surface that support plants and that have properties due to the integrated effect of climate and living matter acting upon parent material, as conditioned by relief, over periods of time."
- "A dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms."
Soil Profile
A soil profile is a vertical section through the soil that usually presents as a layered pattern. Individual soil levels are called 'horizons'. Typically, soil will have three general horizons. The top layer includes the soil surface, or topsoil, and is designated as Horizon A. This is the layer most influenced by climate and is where organic matter accumulates. It is usually darker in color due to its organic content. Under Horizon A is Horizon B, the subsoil, a layer that commonly accumulates many of the materials leached and transported from the surface layer. Horizon C, the parent material, is the least affected by physical, chemical, and biological agents; it is considered the original material. Each horizon can vary greatly in depth, ranging from less than an inch to more than a few feet. Additional horizons that are sometimes included in a more in-depth profile are horizons O and E. Horizon O is a soil horizon that is on top of the soil and is most referred to when looking at soil in forested areas or areas with a lot of organic matter sitting on top. A good way to remember this horizon is 'O for Organic Material'. Horizon E is part of the surface layer, under horizon A and at the top of horizon B. It is light in color and typically heavily leached of nutrients, due to rainfall and irrigation. |
Information about a soil's profile is important in the management of nutrients. By examining a soil's profile, gardeners can gain valuable insight into how fertile it is, and how much work is needed to improve it. For example, highly weathered, infertile soil has a small light-colored surface layer from which nutrients have leached away. Conversely, a fertile soil has a deep surface layer that is dark in color, indicating that there are high amounts of organic matter. The photos below are examples of soil profiles. These profiles are easier to see in ground that has not ever been excavated for construction or compacted due to foot or road traffic. For example. the soil around houses or other buildings has often been disturbed by construction activities and may not have the neat soil profile and distinct horizons as seen below. The surface soil at these locations may not even originally have been from the site; many new construction areas will bring in new topsoil to support landscaping after the site had been completed. The best locations to see pristine, natural soil profiles are at road cuts or where new excavation is occurring.
Soil profiles can vary in depth from a few inches to many feet; the typical soil profile extends to 3-6 feet. In the western United States, soil profiles are less developed than those on the East Coast because less water percolates through western soils. As a result, many western soils are higher in calcium, potassium, phosphate, and other nutrient elements than those on the East Coast. The nutrients that exist in soil play a critical role in supporting plant life, as do many other aspects of the soil including texture and acidity.
Physical and chemical properties of soil
It is critical to get a good understanding of the physical and chemical properties of soil in order to know how to best care for the soil in your own garden. Important physical and chemical properties of soil include mineral content, texture, cation exchange capacity, bulk density, structure, porosity, organic matter content, carbon-to-nitrogen ratio, color, depth, fertility, salt content, and pH level.
Soil Minerals and Texture
These terms refer to the composition of your soil, more specifically the ratio of sand, silt, and clay particles in the soil. The United States Department of Agriculture (USDA) and the International Soil Science Society have individually established standards for the size limits of sand, silt and clay particles that do not completely agree, but there are some general guidelines that can be drawn from the two systems:
These terms refer to the composition of your soil, more specifically the ratio of sand, silt, and clay particles in the soil. The United States Department of Agriculture (USDA) and the International Soil Science Society have individually established standards for the size limits of sand, silt and clay particles that do not completely agree, but there are some general guidelines that can be drawn from the two systems:
- The largest sand particles (2mm in diameter) are 1,000 times larger than the largest clay particles (less than 0.002mm in diameter). The smallest sand particles are 10 to 25 times the size of the largest clay particles (the multiplier varies between standards).
- Silt particles are intermediate in size.
- Clay has thousands of times more surface area per gram than silt and almost a million times more surface area per gram than very course sand.
Simply put, sand particles can be seen by the naked eye. silt particles can be seen using a 10x hand lens, and to see clay particles, you'll need an electron microscope. Each soil particle type has certain qualities due to the elements they contain which influence their usefulness in garden soil.
Sandy soils, which are also called 'light soils', are mostly made up of large mineral particles with large pore spaces throughout. Sandy soils drain quickly, and are therefore difficult to keep moist, especially in warm weather. Leaching, or loss of water-soluble plant nutrients, occurs at a more rapid rate in sandy soils as well, meaning that fertilizer needs to be applied more often. Sandy soils do have an upside, however, in that they are easy to dig through, and they warm quickly in the springtime. Sand is composed mostly of the primary minerals of quartz, feldspar, mica, hornblende. and augite.
Silty soils have smaller particles than sandy soils, and smaller pore spaces as well, but contain many of the same primary minerals as sand. These soils do not drain as quickly, but do retain more nutrients as a result. They are slow to warm in the springtime, and can easily compact. When working with silty soil, it is important to not trample over areas in which you will be planting, and it is important to note that this type of soil also requires aeration in high-traffic zones.
Clay soils, also called 'heavy soils', are made up of tiny mineral particles that are tightly packed together. Clay contains secondary minerals (kaolinite, montmorillonite, and illite) that form as a result of continued weathering. There are very tiny pore spaces throughout this type of soil, meaning it does not drain well, and there is little air. Clay soils are very hard to dig into, are slow to warm, and frequently become waterlogged. On a more positive note, soils that are predominately clay-comprised hold nutrients well, and therefore need to be fertilized much less than sandy soils do.
The USDA classification system of soil recognizes twelve basic soil textural classes that contain varying percentages of sand, silt, and clay. These twelve textures can be divided into three simple categories: coarse. medium, and fine soil. Coarse soil (also known as sandy soil) includes sand and loamy sand. Medium soil (also called loamy soil) includes sandy loam, loam, silt loam, silt, clay loam, sandy clay loam, and silty clay loam. Fine soil (or clay soil) includes clay. sandy clay and silty clay.
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There are two other soil types that are worth mentioning here: peaty and saline. Peat soil began to form about 9,000 years ago, around when the large glaciers that covered the earth began to melt rapidly. Plants were submerged and died quickly during this time, though because they were under the water, they decomposed very slowly which resulted in the surrounding area getting infused with nutrients. This soil is dark in color, soft, and is easily compressed due to its high-water content. Peat can become very dry in the summertime if not watered heavily and can actually become a fire hazard if allowed to dry out. The best quality of peat soil is that it can hold a good amount of water in hot months, and can be protective of plant roots during wet months.
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Saline soil is found in extremely dry regions, and as its name hints, it has a high salt content. Due to this high salt content, it can cause damage to and even stall plant growth, interfere with germination, and can also cause difficulties with irrigation. The salt in this soil is high due to the buildup of salt in the rizosphere, which is the narrow area of soil where root secretions occur and associated microorganisms exist. Plants in saline soil will look like they are being starved of water - and that's because they are. The high salt content in the soil absorbs the water, leaving little water for the plants.
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The ideal soil is called a loam soil, which consists of 40% sand, 40% silt, and 20% clay. With this makeup, the soil has the best of both sand and clay: plant roots have access to water and air, and the soil holds fertilizer well. Loam soils are also very diggable due to their sand content, and contain humus, which is a material formed by the decomposition of leaves and other plant material by soil microorganisms. |
Determining Soil Texture and Type
After reading so much about soil, you may be excited to find out what type of soil you have. There are two tests that you can complete at home that will give you an idea of what type of soil you're working with: the feel method and the mason jar method.
After reading so much about soil, you may be excited to find out what type of soil you have. There are two tests that you can complete at home that will give you an idea of what type of soil you're working with: the feel method and the mason jar method.
Testing your soil type can be as simple as feeling your soil by moistening it and rolling it into a ball. Once you've got a ball, rub it between your thumb and forefinger, flattening it a bit. Notice its characteristics and compare them with the descriptions below:
- Sandy Soil will not form a ball when rolled between your fingers, and will crumble easily. The consistency will be 'rockier' than other soils.
- Silty Soil is smooth to the touch, almost soapy-slick. When rolled between your fingers, this soil will leave dirt on your skin.
- Clay Soil feels sticky when moistened, and will easily form a ball or sausage-shape when rolled between the hands or fingers.
- Peaty Soil, like sandy soil, will not form a ball when rolled. If you have peaty soil, you will be able to squeeze water out of it, like you would with a sponge.
- Loamy Soil is smooth but partly gritty, and will form a sticky ball that crumbles easily.
If you're interested in finding out what type of soil you have in a more visual way, you'll like the mason jar method. Fill a mason jar up halfway with a soil sample from your planting area. Then, fill the rest of the jar with water, leaving a little air at the top for shaking room. Shake vigorously for a few minutes so that all of the particles are suspended in the water. Let the soil settle overnight. The next morning, you'll be able to see clear layers in the jar. The bottom layer is usually sand and any small rocks that may be in your soil. The next layer up will be silt particles. Above that will be your clay particles, and then the added water. There may be some organic matter floating on the top of the water. Next, look at the color of your soil. Lighter soil usually has less organic content than dark soil.
Effect of Soil Texture on Plant Growth
Soil texture directly influences plant growth in many ways. It affects how much water is held in the root zone for plants to use, which nutrients are available in which quantities and the pore space available for air and root growth. Most clay particles formed from the secondary minerals in clay soils are negatively charged. These negatively charged particles attract positively charged ions, or cations, like calcium, magnesium, potassium, ammonium, aluminum, hydrogen, iron, and sodium, among others. Many of these cations are essential elements for plant nutrition, and are attracted via the process of adsorption to the surfaces of solids such as soil particles. Negatively charged clay particles repel negatively charged plant nutrient ions, like nitrate and sulfate, and need to be absorbed alongside water particles from soil pores.
Plant nutrients are constantly being exchanged between clay particles and the rest of the soil solution, making clay particles very important determiners of the physical and chemical properties of soil. Cations replace one another on the surfaces of clay particles, and when this happens, the cation released from the clay particle into the soil solution is then available for absorption by the plant roots. The cation exchange capacity (CEC) measures the amount of cations that can be adsorbed or held by a soil, measured in milliequivalents per 100 grams of soil.
Soils with a high CEC are usually more fertile than those with low CEC, due to the former being more able to resist loss of plant nutrient cations through leaching (the loss of nutrients due to water flow through soil). Clay minerals also have a high affinity for water. The negatively charged clay mineral particles attract the positive charges on the hydrogen and oxygen ions in water. An intermediate amount of clay in a soil, which creates a loamy texture, improves its capacity to hold plant nutrients and water. This is due to both the fact that they are negatively charged (attracting positive mineral and water ions) and the fact that clay has the highest surface area of the various soil composites (a soil of clay particles has almost 100,000 times the surface area than a fine sandy soil of the same weight).
Soil texture directly influences plant growth in many ways. It affects how much water is held in the root zone for plants to use, which nutrients are available in which quantities and the pore space available for air and root growth. Most clay particles formed from the secondary minerals in clay soils are negatively charged. These negatively charged particles attract positively charged ions, or cations, like calcium, magnesium, potassium, ammonium, aluminum, hydrogen, iron, and sodium, among others. Many of these cations are essential elements for plant nutrition, and are attracted via the process of adsorption to the surfaces of solids such as soil particles. Negatively charged clay particles repel negatively charged plant nutrient ions, like nitrate and sulfate, and need to be absorbed alongside water particles from soil pores.
Plant nutrients are constantly being exchanged between clay particles and the rest of the soil solution, making clay particles very important determiners of the physical and chemical properties of soil. Cations replace one another on the surfaces of clay particles, and when this happens, the cation released from the clay particle into the soil solution is then available for absorption by the plant roots. The cation exchange capacity (CEC) measures the amount of cations that can be adsorbed or held by a soil, measured in milliequivalents per 100 grams of soil.
Soils with a high CEC are usually more fertile than those with low CEC, due to the former being more able to resist loss of plant nutrient cations through leaching (the loss of nutrients due to water flow through soil). Clay minerals also have a high affinity for water. The negatively charged clay mineral particles attract the positive charges on the hydrogen and oxygen ions in water. An intermediate amount of clay in a soil, which creates a loamy texture, improves its capacity to hold plant nutrients and water. This is due to both the fact that they are negatively charged (attracting positive mineral and water ions) and the fact that clay has the highest surface area of the various soil composites (a soil of clay particles has almost 100,000 times the surface area than a fine sandy soil of the same weight).
Bulk Density
The soil property referred to as bulk density represents the weight of a volume of soil, which varies depending on the amount of pore space within the soil. A piece of solid quartz has a density of 2.65 grams per cubic centimeter (for every cubic centimeter of volume, a piece of solid quarts weighs 2.65 grams). When solid rock, like quartz, is crushed, its volume then includes air space alongside pieces of rock. As the rock continues to be crushed, the rock particles become smaller and the pore space increases, decreasing the bulk density. Undisturbed sandy soils generally have a bulk density of around 1.6 grams per cubic centimeter. Clay soil, which has much smaller particles (and therefore more pore space) than sandy soil, has a bulk density of about 1.2 grams per cubic centimeter. For the most part, as particle size decreases, bulk density decreases and porosity increases.
The soil property referred to as bulk density represents the weight of a volume of soil, which varies depending on the amount of pore space within the soil. A piece of solid quartz has a density of 2.65 grams per cubic centimeter (for every cubic centimeter of volume, a piece of solid quarts weighs 2.65 grams). When solid rock, like quartz, is crushed, its volume then includes air space alongside pieces of rock. As the rock continues to be crushed, the rock particles become smaller and the pore space increases, decreasing the bulk density. Undisturbed sandy soils generally have a bulk density of around 1.6 grams per cubic centimeter. Clay soil, which has much smaller particles (and therefore more pore space) than sandy soil, has a bulk density of about 1.2 grams per cubic centimeter. For the most part, as particle size decreases, bulk density decreases and porosity increases.
Soil Structure
Soil structure refers to the way grouped particles are arranged in soil. Except for sand grains, soil particles will typically form groups, or aggregates (also called peds). Groups of particles are formed through physical forces like freezing, drying, and thawing, and binding agents, usually products of decomposition of organic matter. Plant growth is strongly influenced by soil structure, as it dictates water movement, moisture availability to plants, fertility, aeration, porosity, heat transfer, bulk density, and mechanical resistance to root growth. Good soil management practices can improve soil structure and create a better environment for your plants to grow. Well- structured soil will have good water infiltration, drainage, aeration, and overall tilth. |
A soil aggregate is a clump of soil particles held together in a unit so that it functions a bit like a single, larger particle. They are naturally occurring, and vary from a fraction of an inch to several inches in diameter. They have characteristic shapes and sizes, according to their makeup, and those that are closer to the surface layer (A Horizon) are more easily changed than those in subsoil horizons. The development of these aggregates can occur as a result of many variables. Aggregation is enhanced by clay and organic matter because they both act as binding agents. Physical forces like tilling operations and plant root growth and climatic forces like rainfall and temperature can also affect how soil binds together. The following chart outlines the varying sizes and shapes of aggregates that occur in soils.
Soil Aggregate Type
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Description
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Common Soil Location
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Aggregates in this group have a thin, vertical dimension when compared to its lateral dimension. This aggregate type is formed when silty soil is deposited in thin layers over time. This type can also be formed through repeated compression of soil in high-traffic areas.
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Compacted layers of soil, or near waterways. |
These structural aggregates have a greater vertical than lateral dimension. Aggregates are formed around a vertical axis and are bounded together by relatively flat vertical surfaces. Formed by shrinking and cracking caused by drying out.
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Found in B Horizons and are common in subsoils in arid regions. |
A soil profile may have a single type of aggregation, but it is more likely that each horizon will have its own unique structural pattern, largely dictated by the degree of soil weathering (referring to the factors in the CLORPT equation). Soil structure takes many years to form, but is rapidly broken down through soil mismanagement, improper tilling, or intensive cultivation. It can also be affected by working, walking, or driving on soil when it is too wet. To avoid causing damage to soil structure when it is wet, it is best to wait for soil to dry for at least 2 days before physically working it. The way soil is treated and cared for can maintain, or even greatly improve, soil structure.
Soil Porosity
Even in the most compacted soils, there are tiny pores that still exist within the structure. These pores are often connected, allowing for the movement of oxygen, water, and dissolved mineral nutrients and allowing room for root growth. Pores are filled with air and water; in ideal moist soil there is a 50/50 mix of these elements in pore spaces. For the most part, larger pores (also called macropores) are filled with air, because gravity has caused the water to flow downward. Small pores (micropores) are primarily responsible for holding water. The ideal soil has a balance of micropores and macropores that allow for free transfer of water and nutrients, and also has air space for plant root growth and aeration. Many plant issues stem from problems within the soil pores in the root zone. Both plant roots and microbial respiration consume oxygen and produce carbon dioxide. Air-filled pores provide a way for oxygen to penetrate the soil; it is how the soil 'breathes'. When soil gets overwatered, the pores fill with water. This creates a lack of usable oxygen and roots cannot respire efficiently; they become starved of oxygen and cannot use available nutrients. Nitrogen also becomes less available as pores fill with water.
Even in the most compacted soils, there are tiny pores that still exist within the structure. These pores are often connected, allowing for the movement of oxygen, water, and dissolved mineral nutrients and allowing room for root growth. Pores are filled with air and water; in ideal moist soil there is a 50/50 mix of these elements in pore spaces. For the most part, larger pores (also called macropores) are filled with air, because gravity has caused the water to flow downward. Small pores (micropores) are primarily responsible for holding water. The ideal soil has a balance of micropores and macropores that allow for free transfer of water and nutrients, and also has air space for plant root growth and aeration. Many plant issues stem from problems within the soil pores in the root zone. Both plant roots and microbial respiration consume oxygen and produce carbon dioxide. Air-filled pores provide a way for oxygen to penetrate the soil; it is how the soil 'breathes'. When soil gets overwatered, the pores fill with water. This creates a lack of usable oxygen and roots cannot respire efficiently; they become starved of oxygen and cannot use available nutrients. Nitrogen also becomes less available as pores fill with water.
Soil Color
Soil color is one of the most useful characteristics in soil identification and classification. It is a function of the original parent material, the amount of organic material present, the degree and kind of weathering the soil has endured, the amount and type of salts present in the soil, and the aeration of the soil. Soil color can provide a clue as to the chemical makeup or drainage status of the soil. The following soil colors have varying qualities:
Soil color is one of the most useful characteristics in soil identification and classification. It is a function of the original parent material, the amount of organic material present, the degree and kind of weathering the soil has endured, the amount and type of salts present in the soil, and the aeration of the soil. Soil color can provide a clue as to the chemical makeup or drainage status of the soil. The following soil colors have varying qualities:
Gray and brown soils are commonly found in California. These soils are relatively low in organic matter, but do have some content due to the presence of alluvial silt deposits. In the Central Valley and our coastal valleys, nearly all of the soil is alluvial. In the higher elevation areas of California, by contrast, the soil is formed from the weathering of underlying rocks. |
If soil is white or light grey, this usually indicates a highly leached sandy soil or one that is calcareous (containing lime). If the soil does contain lime, this can mean that it has an iron deficiency and will not be great for orchard crops. White-colored salts can also accumulate on the top of existing soils, creating a white crust. |
Layers of blue or blue-grey in soil indicate that there is organic matter that is decaying without air (anaerobically). This color of soil can have a bit of a smell to it due to the gases released from decomposition. This soil should not be mixed into topsoil or used as the main soil for planting because it contains matter toxic to plant roots. If this type of soil is extensively aerated, allowing oxygen to get to the decomposing matter. it can be turned into adequate planting soil. |
Soil Depth
Soil depth refers to the vertical distance from the soil surface to a layer past which plant roots and water cannot penetrate. This impermeable layer can be made of rock, gravel, a claypan, a hardpan, or a partially cemented layer of soil. Deep soil will provide more nutrients and water to plants than shallow soil. There are categories for soil depth, outlined as follows:
Soil depth refers to the vertical distance from the soil surface to a layer past which plant roots and water cannot penetrate. This impermeable layer can be made of rock, gravel, a claypan, a hardpan, or a partially cemented layer of soil. Deep soil will provide more nutrients and water to plants than shallow soil. There are categories for soil depth, outlined as follows:
Very Shallow
Shallow Moderately Deep Deep Very Deep |
Less than 10 inches
10 to 20 inches 20 to 36 inches 36 to 60 inches Deeper than 60 inches |
Soil Reaction (pH)
Soil reaction, or pH, refers to the suitability of soil for growing crops and ornamental plants. It is a measurement of the concentration of hydrogen and hydroxyl ions in soil. Soil is referred to as acid, neutral, or alkaline, depending on its pH measurement. Soil reaction is measured on a scale, which ranges from 1.0 to 14.0. 7.0 is neutral (with an equal amount of hydroxyl and hydrogen ions), below 7.0 is acid (with more hydrogen than hydroxyl ions), and above 7.0 is alkaline (with more hydroxyl than hydrogen ions). Soil reaction is important to understand because it affects nutrient availability, solubility of toxic ions, and microbial activity. It also dictates which plants will grow well in your yard, making it an important step in planning and executing your garden. Most plants can adapt to a soil pH level of between 6.0 and 7.5, though some plants are a bit more picky. pH testing for your garden should be done every autumn; this will allow you ample time to correct any issues in the soil before the next spring growing season.
There is a simple, at-home way to test soil pH that will give you a general idea of which end of the spectrum your soil is on. To complete this DIY soil test, you'll need a clean, medium sized glass bowl, a clean, glass liquid measuring cup, a hand towel, distilled water, vinegar and baking soda. Once you've gathered your supplies, follow these instructions to complete your test:
Soil reaction, or pH, refers to the suitability of soil for growing crops and ornamental plants. It is a measurement of the concentration of hydrogen and hydroxyl ions in soil. Soil is referred to as acid, neutral, or alkaline, depending on its pH measurement. Soil reaction is measured on a scale, which ranges from 1.0 to 14.0. 7.0 is neutral (with an equal amount of hydroxyl and hydrogen ions), below 7.0 is acid (with more hydrogen than hydroxyl ions), and above 7.0 is alkaline (with more hydroxyl than hydrogen ions). Soil reaction is important to understand because it affects nutrient availability, solubility of toxic ions, and microbial activity. It also dictates which plants will grow well in your yard, making it an important step in planning and executing your garden. Most plants can adapt to a soil pH level of between 6.0 and 7.5, though some plants are a bit more picky. pH testing for your garden should be done every autumn; this will allow you ample time to correct any issues in the soil before the next spring growing season.
There is a simple, at-home way to test soil pH that will give you a general idea of which end of the spectrum your soil is on. To complete this DIY soil test, you'll need a clean, medium sized glass bowl, a clean, glass liquid measuring cup, a hand towel, distilled water, vinegar and baking soda. Once you've gathered your supplies, follow these instructions to complete your test:
- Dig a hole 4-6 inches deep in your garden soil with your hand trowel in a few areas of your bed and remove some soil from each location. You'll need about a cup of soil total for each test you do. You can use your glass measuring cup to measure out your sample. Pour the soil into your glass bowl.
- Take out any sticks and stones in the soil sample, and break up large clumps of soil with your fingers or a fork.
- Add enough distilled water to get a muddy consistency.
- Rinse out your glass measuring cup, dry it, and then measure out 1/2 cup of vinegar. Add the vinegar to your bowl of mud and gently mix it together.
- If the mixture fizzes, foams, or bubbles, you have alkaline soil.
- If the mixture does not react, pour your test out and start over from step 1. Once you reach the end of step 3, jump to step 7.
- Rinse out your glass measuring cup, dry it, and then measure out 1/2 cup of baking soda. Add it to your bowl of mud and gently mix it together.
- If the mixture fizzes, foams or bubbles, you have acidic soil.
- If you get no response from either the vinegar or the baking soda, your soil is most likely neutral.
- Complete this test in different areas or beds in your garden as many times as you'd like. While this test does not give you a numerical reading as to what your soil pH is, it will give you an idea of where your soil is on the acid to alkaline range.
If you're looking for a more accurate reading of your soil's pH level, pick up a strip test. Odds are that your local nursery or hardware store sells a soil pH testing kit (they're also available on Amazon). To use a basic strip test, you'll need a hand trowel, a clean. small glass bowl, distilled water, a spoon, pH testing strip(s), a glass measuring cup, and a coffee filter. After gathering your materials, follow these instructions to complete your strip soil pH test:
- Dig a hole 4-6 inches deep in your garden soil with your hand trowel in a few areas of your bed and remove some soil from each location. You'll need about three teaspoons of soil total for each test you do.
- Take out any sticks and stones in the soil sample, and break up large clumps of soil with your fingers, and then place your soil sample in your glass bowl.
- Fill the bowl with distilled water up to the level of the soil sample.
- Vigorously stir the mixture together with a spoon.
- Pour your mixture through a coffee filter into a glass measuring cup.
- Dip your test strip into the liquid. Once the strip shows color, take it out and compare it to the guide provided by the testing strip manufacturer.
- Repeat the process in a few different areas of your garden to get a better idea of how much your soil's pH varies.
Consult the profiles of each individual plant you have in your garden, and ensure that their recommended pH is being maintained by comparing them to your soil test results. Most garden soils in California have a pH of between 5.0 and 8.5. Soils strongly leached of nutrients in wet climates will have a pH of below 5.0. Most agricultural soils have a pH of between 5.5 and 7.0, and soils in arid climates usually come in above a pH of 7.0. Saline, arid soils can have a pH of above 8.0, and in some circumstances (often associated with pollutants present in soil), the pH in some areas can be under 4.0. Soils inherit most of their acidity from their parent material; weathering of granite or rhyolite will produce acidic soil, while weathering of limestone or chalk will produce more alkaline soils. Soils can become acidic through the leaching of nutrients from high levels of rainfall.
Buffering capacity of soil refers to the tendency of soils to resist acidification. The soils with the largest buffering capacity are clay soils and those with good quality organic matter. These soils are able to hold many nutrients on the edges of their particles, which are then available to be picked up by water and moved. This leaves behind hydrogen ions. The acidic environment in most soils aids in the breakdown of parent materials, which releases many valuable nutrients, and slows down the effect of acidification. This buffering capacity is most resilient at a pH of between 5.0 and 6.5. If you need to amend your soil to reach a certain pH, there are a variety of different soil amendments that can be used to achieve this goal. The addition of limestone to your soil is a great way to lower pH, though be careful when amending your soil with limestone as it is much easier to lower pH than increase it. If your soil is too alkaline, the addition of elemental sulfur, iron sulfate or aluminum sulfate can be added to soil.
Buffering capacity of soil refers to the tendency of soils to resist acidification. The soils with the largest buffering capacity are clay soils and those with good quality organic matter. These soils are able to hold many nutrients on the edges of their particles, which are then available to be picked up by water and moved. This leaves behind hydrogen ions. The acidic environment in most soils aids in the breakdown of parent materials, which releases many valuable nutrients, and slows down the effect of acidification. This buffering capacity is most resilient at a pH of between 5.0 and 6.5. If you need to amend your soil to reach a certain pH, there are a variety of different soil amendments that can be used to achieve this goal. The addition of limestone to your soil is a great way to lower pH, though be careful when amending your soil with limestone as it is much easier to lower pH than increase it. If your soil is too alkaline, the addition of elemental sulfur, iron sulfate or aluminum sulfate can be added to soil.
Soil Salts and Salinity
As salts in soil become more and more concentrated through the actions of evaporation and added fertilizers, it becomes more and more difficult for plants to take up water. Chemically speaking, a salt is the neutral product of a reaction between an acid and a base. They contain a balance of cations (positively charged ions) and anions (negatively charged ions). Soluble salts are minerals dissolved in water, and in moist climates they naturally get washed out through rainfall, but in Mediterranean or arid climates, they are added to soils through dust deposits and evaporation. Salt concentration in soil is measured as electrical conductivity, or EC. It is expressed in units of decisiemens per meter (dS/m). EC levels can be formally measured using a portable EC meter. An EC level of 2.0 dS/m or more cam reduce the growth of salt-sensitive plantings. though there are many plants that can tolerate an EC of 4.0 dS/m. Plants directly affected by high soil salinity levels (with an EC of between 8.0 dS/m and 16.0 dSm) will exhibit symptoms like leaf burn, leaf scorch, and leaf drop. Soil salinity and pH are related; soils with high levels of soluble salts usually have a high pH.
In the semi-arid areas in California, saline soil can develop because evaporation of water vapor from the soil surface is more prevalent than leaching. Salts dissolved in the water accumulate in large quantities in the root zone, and can cause a crust of white-looking soluble salts on the soil surface. When a soil has a high concentration of salts, plant roots will have difficulty absorbing water molecules due to the fact that water and salt molecules are so strongly held together. Plants will become dehydrated and wilt. grow slowly, and eventually die. To correct this issue in soil, excessive irrigation is recommended. Another common western soil with salinity issues is referred to as sodic soil. This soil has a pH that is too high because of an excessive accumulation of sodium without an adequate concentration of other salts. This concentration is toxic to plants, and can physically disperse soil particles, destroying soil structure and resulting in poor water and air infiltration. Some soils can be both sodic and saline.
As salts in soil become more and more concentrated through the actions of evaporation and added fertilizers, it becomes more and more difficult for plants to take up water. Chemically speaking, a salt is the neutral product of a reaction between an acid and a base. They contain a balance of cations (positively charged ions) and anions (negatively charged ions). Soluble salts are minerals dissolved in water, and in moist climates they naturally get washed out through rainfall, but in Mediterranean or arid climates, they are added to soils through dust deposits and evaporation. Salt concentration in soil is measured as electrical conductivity, or EC. It is expressed in units of decisiemens per meter (dS/m). EC levels can be formally measured using a portable EC meter. An EC level of 2.0 dS/m or more cam reduce the growth of salt-sensitive plantings. though there are many plants that can tolerate an EC of 4.0 dS/m. Plants directly affected by high soil salinity levels (with an EC of between 8.0 dS/m and 16.0 dSm) will exhibit symptoms like leaf burn, leaf scorch, and leaf drop. Soil salinity and pH are related; soils with high levels of soluble salts usually have a high pH.
In the semi-arid areas in California, saline soil can develop because evaporation of water vapor from the soil surface is more prevalent than leaching. Salts dissolved in the water accumulate in large quantities in the root zone, and can cause a crust of white-looking soluble salts on the soil surface. When a soil has a high concentration of salts, plant roots will have difficulty absorbing water molecules due to the fact that water and salt molecules are so strongly held together. Plants will become dehydrated and wilt. grow slowly, and eventually die. To correct this issue in soil, excessive irrigation is recommended. Another common western soil with salinity issues is referred to as sodic soil. This soil has a pH that is too high because of an excessive accumulation of sodium without an adequate concentration of other salts. This concentration is toxic to plants, and can physically disperse soil particles, destroying soil structure and resulting in poor water and air infiltration. Some soils can be both sodic and saline.
Organic Matter Matters
The organic fraction of soil is the solid phase that originates from living organisms (which differs from the mineral fraction, which is added to soil through weathering of parent material). This organic matter consists primarily of carbon (50-55%) and nitrogen (7-8%) and is a reservoir of plant nutrients. This organic fraction includes living organisms like fungi, bacteria, earthworms, and related animals, and organic matter derived from plant and animal residues as they decompose. Residue left over from decomposition activities and derived from plant and animal material is called humus. The organic matter content of soil varies according to climate as well as soil type. Organic matter in the soil of warm, dry regions comprises just 1% of the soil, and in cool, moist regions comes in between 1% and 10% of soil content. In poorly drained valley soil that was originally swampland, soil contains between 5% and 50% organic matter. Frequent tilling activities and crop removal tend to reduce the natural organic matter content of soil. Organic matter in soil is critical to soil productivity or fertility. It improves soil structure, aeration and water infiltration, increases the capacity of the soil to hold water, and provides a cushion against outside agents changing the soil too much. It also increases the availability of nutrients that are then available to plants, and provides a food source for beneficial microorganisms in the soil. There are many important entities that comprise organic matter, outlined in the following paragraphs.
The organic fraction of soil is the solid phase that originates from living organisms (which differs from the mineral fraction, which is added to soil through weathering of parent material). This organic matter consists primarily of carbon (50-55%) and nitrogen (7-8%) and is a reservoir of plant nutrients. This organic fraction includes living organisms like fungi, bacteria, earthworms, and related animals, and organic matter derived from plant and animal residues as they decompose. Residue left over from decomposition activities and derived from plant and animal material is called humus. The organic matter content of soil varies according to climate as well as soil type. Organic matter in the soil of warm, dry regions comprises just 1% of the soil, and in cool, moist regions comes in between 1% and 10% of soil content. In poorly drained valley soil that was originally swampland, soil contains between 5% and 50% organic matter. Frequent tilling activities and crop removal tend to reduce the natural organic matter content of soil. Organic matter in soil is critical to soil productivity or fertility. It improves soil structure, aeration and water infiltration, increases the capacity of the soil to hold water, and provides a cushion against outside agents changing the soil too much. It also increases the availability of nutrients that are then available to plants, and provides a food source for beneficial microorganisms in the soil. There are many important entities that comprise organic matter, outlined in the following paragraphs.
Humus
Most soil physical and chemical properties (even in mineral soil that contains little organic matter), are functions of humus and clay minerals. Like clay, humus particles are negatively charged and attract water and plant nutrient cations. Humus increases a soil's CEC and improves soil fertility due to adsorbed cations remaining in the root zone for use by plants. Humus also assists in forming granular aggregates, which greatly improves soil structure. Humus tends to be brown or very dark brown (appearing black) and its main components are humic and fulvic acids. |
Beneficial Soil Organisms
Most beneficial soil organisms, including earthworms and saprophytes, live close to the surface in the root zone of plants. A single gram of soil may contain as many as 4 billion bacteria, a million fungi, 20 million actinomycetes, and 300,000 algae. Some organisms are microscopic, and others are visible to the naked eye.
Most beneficial soil organisms, including earthworms and saprophytes, live close to the surface in the root zone of plants. A single gram of soil may contain as many as 4 billion bacteria, a million fungi, 20 million actinomycetes, and 300,000 algae. Some organisms are microscopic, and others are visible to the naked eye.
Earthworms are segmented worms that are part of the beneficial group if soil macrofauna. They feed on plant residues as they move through the soil, stirring and aerating as they go. Their excretions and casts are very high in plant-available nutrients (phosphates. potassium. nitrate nitrogen, and exchangeable calcium and magnesium). These easily recognizable wigglers are true garden heroes. |
Saprophytic soil bacteria and fungi also have important roles in the development and maintenance of healthy soil. As these organisms feed on dead and decaying plant residues from the previous seasons' crop, saprophytes use enzymes to break them down and metabolize them into usable, mineral forms of nitrogen, sulfur. and phosphorus. Because plants cannot absorb nutrients in their complex forms, these soil organisms are critical recyclers. In this way, soil organic matter serves as a great slow-release fertilizer that provides a continual supply of garden nutrients. |
As saprophytes feed on organic residue, they release a gummy substance that glues together and stabilizes aggregates and improves soil structure. Organic matter in soil promotes a crumb-like granular soil structure. This improved aggregation is associated with improved pore structure and a more even distribution of micropores and macropores, which in turn improves water-holding capacity, water infiltration rate, and aeration. Saprophytic bacteria and fungi also release carbon dioxide as a byproduct into the soil, which can either move directly into the atmosphere or be combined with water to form carbonic acid. This acid, together with others, facilitates weathering of soil minerals, further increasing the amount of available nutrients in the root zone for use by plants.
Other non-saprophytic bacteria and fungi are also beneficial to the soil biome, and work by forming associations (called mycorrhizae) with plant roots. Over 75% of all terrestrial plants form these beneficial relationships between their roots and soil fungi. Plant roots provide carbon-containing compounds that serve as a food source for the fungi, and in turn, the fungi improve root absorption or phosphorus and other nutrients in soil for the plants to use. |
The study of microbially-derived plant hormones and their regulatory effect on plant growth and development is an active area of research in soil science. Many beneficial soil microorganisms like bacteria, fungi, and actinomycetes that are associated with plant roots and synthesize plant hormones like auxins, gibberellins, cytokinins, ethylene, and abscisic acid. These hormones positively affect plant growth and development.
The Carbon to Nitrogen Ratio
Organic matter is known to improve soil fertility, and is often added to garden soil in large amounts. Although this may not transform soil immediately, the cyclical relationship between the activities of decomposers in the soil and availability of nitrate nitrogen to plants will begin to make small but lasting changes to the structure and makeup of garden soil. The ratio of carbon to nitrogen (C:N) is the amount of carbon compared with the amount of nitrogen present in soil's organic matter. In most fertile, productive soils, this ratio is constant, varying from around 10:1 to 12:1, indicating that the level of carbon is about 10-12 times greater than the nitrogen level.
Maintaining a good C:N ratio is important for soil microorganisms that need both elements to grow. Lack of available carbon in soil will limit microbial activity, but adding too much carbon-rich organic matter (like leaf litter, bark, straw or sawdust) can also cause some issues in plant growth due to a process referred to as immobilization. When this occurs, microbial decomposers in soil receive an enormous food supply that contains a lot of carbon, but little nitrogen. These decomposers will use up both the carbon and the available nitrogen in the soil as they process the organic matter. Once nitrogen levels are depleted, the competition between plant roots and microbes for inorganic ammonium and nitrate nitrogen begins, and the plants are the ones that usually lose. To remedy this situation, it is best to apply an inorganic nitrogen fertilizer to the soil when undecomposed organic residues with a high C:N ratio are incorporated just before planting or during plant growth.
The length of time that nitrogen is limited due to immobilization varies and can be limited through the amounts and types of organic residues added to the soil. As the process of humus formation comes to an end, many of the decomposers will die and their activities will decrease. Other soil bacteria will feed on their bodies. and convert the stored nitrogen into nitrate nitrogen, supplying a form that garden plants can use. When an organic soil amendment has a high C:N ratio and is applied as a mulch, nitrogen deficiency is not usually as much of a problem because it is decomposed slowly on the surface. When an amendment with a low C:N ratio is added into soil, it decomposes rapidly. Nitrogen levels are not limited in this case, and the decomposition process releases excess nitrogen from organic matter into inorganic forms that garden plants can absorb, and a period of nitrogen unavailability doesn't occur.
Adding organic matter is a great way to improve your soil. By adding enough over time, say a few years, you can change the structure of your soil by increasing humus content, providing necessary nutrients to plant life, and encouraging beneficial soil organisms to take up residence. Organic soil amendments can be purchased, or they can be locally produced (meaning in your own backyard). Compost is the ideal organic amendment, however other materials like barnyard manure, green plants, and fallen leaves can be effective as well. Other materials that can be used as organic amendments are straw, sawdust, rice hulls, shredded bark, and peat moss. However, these materials are not as effective as they take longer to decay than the former three options. When you are buying organic matter, look at the label and check to see if the material has been composted. If it has, this means that what you buy is probably already on its way to becoming humus. A great alternative to purchased organic soil amendments is to make your own compost. To learn more about composting, visit the link below:
Organic matter is known to improve soil fertility, and is often added to garden soil in large amounts. Although this may not transform soil immediately, the cyclical relationship between the activities of decomposers in the soil and availability of nitrate nitrogen to plants will begin to make small but lasting changes to the structure and makeup of garden soil. The ratio of carbon to nitrogen (C:N) is the amount of carbon compared with the amount of nitrogen present in soil's organic matter. In most fertile, productive soils, this ratio is constant, varying from around 10:1 to 12:1, indicating that the level of carbon is about 10-12 times greater than the nitrogen level.
Maintaining a good C:N ratio is important for soil microorganisms that need both elements to grow. Lack of available carbon in soil will limit microbial activity, but adding too much carbon-rich organic matter (like leaf litter, bark, straw or sawdust) can also cause some issues in plant growth due to a process referred to as immobilization. When this occurs, microbial decomposers in soil receive an enormous food supply that contains a lot of carbon, but little nitrogen. These decomposers will use up both the carbon and the available nitrogen in the soil as they process the organic matter. Once nitrogen levels are depleted, the competition between plant roots and microbes for inorganic ammonium and nitrate nitrogen begins, and the plants are the ones that usually lose. To remedy this situation, it is best to apply an inorganic nitrogen fertilizer to the soil when undecomposed organic residues with a high C:N ratio are incorporated just before planting or during plant growth.
The length of time that nitrogen is limited due to immobilization varies and can be limited through the amounts and types of organic residues added to the soil. As the process of humus formation comes to an end, many of the decomposers will die and their activities will decrease. Other soil bacteria will feed on their bodies. and convert the stored nitrogen into nitrate nitrogen, supplying a form that garden plants can use. When an organic soil amendment has a high C:N ratio and is applied as a mulch, nitrogen deficiency is not usually as much of a problem because it is decomposed slowly on the surface. When an amendment with a low C:N ratio is added into soil, it decomposes rapidly. Nitrogen levels are not limited in this case, and the decomposition process releases excess nitrogen from organic matter into inorganic forms that garden plants can absorb, and a period of nitrogen unavailability doesn't occur.
Adding organic matter is a great way to improve your soil. By adding enough over time, say a few years, you can change the structure of your soil by increasing humus content, providing necessary nutrients to plant life, and encouraging beneficial soil organisms to take up residence. Organic soil amendments can be purchased, or they can be locally produced (meaning in your own backyard). Compost is the ideal organic amendment, however other materials like barnyard manure, green plants, and fallen leaves can be effective as well. Other materials that can be used as organic amendments are straw, sawdust, rice hulls, shredded bark, and peat moss. However, these materials are not as effective as they take longer to decay than the former three options. When you are buying organic matter, look at the label and check to see if the material has been composted. If it has, this means that what you buy is probably already on its way to becoming humus. A great alternative to purchased organic soil amendments is to make your own compost. To learn more about composting, visit the link below:
Soil Water
Soil water occupies the pore space not taken up by air. Water is used as a solvent for plant nutrients and minerals, which are transported in the water to plant roots. How water moves through and is held in soil is of utmost importance to gardeners, and requires a basic understanding of physics and chemistry. To begin with, the forces of attraction between water particles is called cohesion and the forces of attraction between soil particles and water particles is called adhesion. The water in soil is said to be adsorbed onto the soil particle surfaces, and is held there against the force of gravity. The force of adhesion is greatest at the surface of the soil particle and decreases with distance from the soil particles. Both cohesion and adhesion act to hold soil water against other forces and are responsible for the upward and lateral movements of soil water.
For plant roots to be able to take in water, they need to pull it away from its adhesion to soil particles and other water molecules. The total pore volume, referred to as porosity, and the size of soil pores both play roles in the availability of water to plants because they determine how much water the soil will hold in the root zone (water-holding capacity) and how tightly the water is held. Forces of cohesion and adhesion are much stronger in fine-textured soil (like clay) because the small, negatively charged particles have a large amount of surface area that attracts positive charges of the hydrogen ions in water. The water is held close to particle surfaces as well, where the force of adhesion is strongest. Coarse-textured soils (such as sandy soils) have larger mineral particles, less clay, and larger pores with less total pore volume. This leads to a situation where the forces of cohesion and adhesion are lessened. Therefore, the water-holding capacity is much less than that of clay. The transpiration process pulls water from the soil, through plants, and into leaves. For every water molecule that evaporates from the leaf, one is pulled into the root from the soil via the capillary forces in the plant's vascular system. Because plant roots must exert force to extract soil water, scientists measure soil water in terms of quantity (soil moisture content) as well as in units of pressure or tension (atmospheres, bars, or pascals). These units quantify how tightly a soil holds water.
For plant roots to be able to take in water, they need to pull it away from its adhesion to soil particles and other water molecules. The total pore volume, referred to as porosity, and the size of soil pores both play roles in the availability of water to plants because they determine how much water the soil will hold in the root zone (water-holding capacity) and how tightly the water is held. Forces of cohesion and adhesion are much stronger in fine-textured soil (like clay) because the small, negatively charged particles have a large amount of surface area that attracts positive charges of the hydrogen ions in water. The water is held close to particle surfaces as well, where the force of adhesion is strongest. Coarse-textured soils (such as sandy soils) have larger mineral particles, less clay, and larger pores with less total pore volume. This leads to a situation where the forces of cohesion and adhesion are lessened. Therefore, the water-holding capacity is much less than that of clay. The transpiration process pulls water from the soil, through plants, and into leaves. For every water molecule that evaporates from the leaf, one is pulled into the root from the soil via the capillary forces in the plant's vascular system. Because plant roots must exert force to extract soil water, scientists measure soil water in terms of quantity (soil moisture content) as well as in units of pressure or tension (atmospheres, bars, or pascals). These units quantify how tightly a soil holds water.
Water Availability
In moist soils, the soil particles are surrounded by coats of water. As soil is wetted further, the water films on the surface of soil particles thicken. The water particles closest to the soil particles are held tightly by adhesion forces, and are not available to plant roots. Water particles farthest from the soil particles is held more loosely, and are able to be used by roots. The water farthest from soil particles is also more susceptible to the force of gravity, which pulls water down out of the root zone. This water is called gravitational water. The water that remains in the root zone against the force of gravity is held there by the forces of cohesion and adhesion. When all pores are completely filled with water, the soil is said to be saturated. The strength of soil water adhesion (soil moisture tension, or SMT), is 0 kilopascals (kPa) in this situation. Field capacity (FC) is the maximum amount of water that can be held by a soil against the downward force of gravity after a soil has been fully saturated. Soil water at field capacity is held at 30 kPa.
In moist soils, the soil particles are surrounded by coats of water. As soil is wetted further, the water films on the surface of soil particles thicken. The water particles closest to the soil particles are held tightly by adhesion forces, and are not available to plant roots. Water particles farthest from the soil particles is held more loosely, and are able to be used by roots. The water farthest from soil particles is also more susceptible to the force of gravity, which pulls water down out of the root zone. This water is called gravitational water. The water that remains in the root zone against the force of gravity is held there by the forces of cohesion and adhesion. When all pores are completely filled with water, the soil is said to be saturated. The strength of soil water adhesion (soil moisture tension, or SMT), is 0 kilopascals (kPa) in this situation. Field capacity (FC) is the maximum amount of water that can be held by a soil against the downward force of gravity after a soil has been fully saturated. Soil water at field capacity is held at 30 kPa.
The permanent wilting percentage, or PWP, is the amount of water remaining in the soil when water particles are held too strongly to soil particles to be used by plants. In this situation, plants will wilt, even though there is still technically water present in the soil. At PWP, water is held at 1,500 kPa in medium and fine textured soil and at 1,000 kPa in sandy soils. The soil water held between field capacity (30 kPa) and permanent wilting percentage (1,000 kPa - 1,500 kPa) is called plant available water. In practice, more than 90% of plant available water is extracted from a soil when the soil moisture tension (SMT - the strength of soil water adhesion) reaches 100 kPa. The majority of soil water is held at 30 kPa - 100 kPa in medium and fine textured soils.
Soil texture is an important factor in determining water availability at field capacity. Clay holds the greatest amount of water at field capacity, followed by clay loam. silt loam, loam, and fine sand. At field capacity, sand can hold between 5% and 10 % water, but clay loam can hold from 25% - 35% water. Greater amounts of moisture is held in heavier soils at the PWP as well; clay holds more water that is unavailable to plant roots than does clay loam. Although fine-textured soil can technically hold more water, a medium textured soil will hold more water that is actually usable by plants. A soil's FC can be changed by the addition of organic amendments and improved soil management techniques; both fine-textured and sandy soils improve in the amount of plant available water they can hold when organic matter is added.
Soil texture is an important factor in determining water availability at field capacity. Clay holds the greatest amount of water at field capacity, followed by clay loam. silt loam, loam, and fine sand. At field capacity, sand can hold between 5% and 10 % water, but clay loam can hold from 25% - 35% water. Greater amounts of moisture is held in heavier soils at the PWP as well; clay holds more water that is unavailable to plant roots than does clay loam. Although fine-textured soil can technically hold more water, a medium textured soil will hold more water that is actually usable by plants. A soil's FC can be changed by the addition of organic amendments and improved soil management techniques; both fine-textured and sandy soils improve in the amount of plant available water they can hold when organic matter is added.
The concept of water availability applies directly to gardening. Enough water needs to be available to our plants so that their roots can absorb it quickly enough to maintain optimal growth and health. Most plants grow best at soil moisture tension (SMT) of between 30 and 50 kPa. Over 50 kPa, many plants will have difficulty pulling enough water from the soil to support maximum growth. SMT can drop below 30 kPa for a day or two after irrigation or rainfall and reach or exceed 50 kPa just before irrigation or rainfall.
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Soil Fertility and Plant Nutrition
Fertile soil contains nutrient elements essential to plant life in amounts favorable for optimal growth of plants in a form readily absorbed by roots. There are many variables at play that imbue certain soils with mixtures of these nutrients, making some perfect for gardening and others difficult to grow in. There are 17 essential plant nutrients (that we humans know of) that support plant growth and functioning. Three are found in the air (carbon (C), hydrogen (H), and oxygen(O)) and the remaining 14 are absorbed from soil through plant roots.
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Those found in soil are divided into three groups, categorized by the amounts required by plants. The primary nutrients include nitrogen (N), phosphorus (P), and potassium (K). The secondary nutrients include calcium (Ca), Magnesium (Mg), and Sulfur (S). The primary and secondary nutrients, collectively called the macronutrients, are all measured as a percentage (parts per 100) of dry-weight tissue. The remaining eight nutrients are called the micronutrients, as they are needed in small quantities for plant growth, and are measured on a parts-per-million (ppm) basis. The micronutrients include boron (B), chlorine (Cl), copper (Cu), Iron (Fe), manganese (Mn), molybdenum (Mo), Nickel (Ni), and zinc (Zn). The nutrient most recently discovered as essential for plant growth is nickel, required by plants in such a small quantity (50-100 ppb) that it wasn't added to the list until 1987.
Hunger Signs in Plants
Since ancient times, humans have been using the outward appearance of plants to diagnose issues. Plants speak though the distress signals they send, and their messages can indicate pest infestations, climate-caused injury, physical damage, water issues, or nutrient deficiencies, among other problems. Recognizing the general signs and symptoms of nutrient deficiency is an invaluable skill developed by many gardeners to better care for their plants and correct issues before the situation becomes irreversible. The most common nutritional problems found in California gardens are related to deficiencies of nitrogen, phosphorus, potassium, zinc, and iron or excesses in boron, chloride or sodium. Issues with the three airborne essential nutrients are not common, unless a plant is not getting enough air in its root zone. |
For information on diagnosing nutrient issues in plants, see the Plant Issues page on this website.
Most plant nutritional disorders are difficult to diagnose visually; in many cases, tissue and soil testing need to be done to narrow down the culprit of plant disease. In some instances, plants might not show any outward symptoms of nutritional deficiencies until severe stress has occurred and is irreparable. If a nutrient issue needs to be diagnosed, a commercial laboratory can perform a diagnostic test using the specimen tissue at about $50 a sample. A cheaper way to diagnose plant issues is to plant sweet corn in the area where plants are having trouble. Corn plants express deficiency symptoms very clearly, and a diagnosis can be made.
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Effect of Soil pH on Availability of Plant Nutrients
Most plants grow best when the pH of the soil is between 5.5 and 7.5 (acidic to neutral), because mineral nutrients that are essential to plant growth derived from the soil are in chemical forms that plant roots can absorb in this range. At lower or higher pH levels, some nutrients become insoluble in water and cannot be absorbed by roots. Soil reaction also affects which detrimental or toxic mineral elements are soluble. For example, moderately to very acidic soil (with a low pH) will have inadequate levels of plant available nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, and molybdenum, and also can contain toxic levels of aluminum and manganese. Very alkaline soils (where the pH level is too high) are deficient in plant-available nitrogen, phosphorus, iron, and manganese, and can also contain excessive concentrations of soluble salts or sodium (which are detrimental and toxic to plant growth). |
Fertilizing Garden Soil
Soil is the starting point for all life in the garden, serving to support growth, provide water, and anchor plants in place. Garden soils rarely start out with all nutrients needed for optimal plant growth, but it is also rare to find soils that are missing several of the key elements. California soils contain most of the elements needed for plant growth, and it is only necessary to fine tune them. There are many types of fertilizer to choose from when amending garden soil.
Soil is the starting point for all life in the garden, serving to support growth, provide water, and anchor plants in place. Garden soils rarely start out with all nutrients needed for optimal plant growth, but it is also rare to find soils that are missing several of the key elements. California soils contain most of the elements needed for plant growth, and it is only necessary to fine tune them. There are many types of fertilizer to choose from when amending garden soil.
Inorganic Fertilizer
These amendments are commercially available and can be purchased at garden centers and hardware stores, and are divided into two main categories: complete and incomplete. Complete inorganic fertilizers are mixes that contain the three primary nutrients (NPK) and incomplete inorganic fertilizers are those that contain a single nutrient (e.g., nitrogen only) or double compounds (e.g., ammonium phosphate, which contains nitrogen and phosphorus). By law, the guaranteed content of the fertilizer (expressed as a percentage of each plant nutrient supplied) must be stated on the container. |
Quick Question: What do the numbers mean?
Fertilizer packages usually will have three hyphenated numbers on the packaging, These three numbers stand for the percentages by weight of nitrogen, phosphorus, and potassium, always in that order. For example, the fertilizer pictured here has the ratio 19-19-19. This means that it is 19 percent nitrogen, 19 percent phosphorus, and 19 percent potassium. You may notice that this only adds up to 57 percent. The remaining 43 percent is filler material and other macro- and micro-nutrients. Incomplete fertilizers will have a zero as one of the percentages. |
These fertilizers are fast-acting and are relatively low in cost. Some can, over time, change the pH of local soils, as well. Disadvantages of using inorganic fertilizers are that they have the potential to contaminate the environment due to runoff and leaching, and that they can burn plants if they are not applied in the correct amounts. A way to minimize these environmental effects is to purchase a slow-release nitrogen fertilizer. These fertilizers are more expensive, and their release rate is dependent on factors like soil moisture and temperature. They are sometimes called water-insoluble nitrogen (WIN) fertilizers. When used correctly, these fertilizers can provide a consistent supply of nitrogen and fewer applications are required for plant growth. In addition to macronutrient inorganic fertilizers, micronutrient inorganic fertilizers can be used to change soil nutrient availability. In many cases, however, micronutrient amendments are not needed because raising or lowering the pH of soils will create an environment in which they are more readily available. A major limitation to the use of these micronutrient fertilizers are that applications are required frequently.
Manure and Organic Fertilizer
Manure is comprised of varying mixtures of plant-eating animal excreta and plant remains. They can make great, organic garden fertilizers when used correctly, and supply garden plants with many essential nutrients. Manure also improves soil structure. For organic nutrient sources to be beneficial to plants, however, the nutrients provided must be changed into chemical forms that plants can readily absorb, and this takes time. Manures can also contain unwanted weed seeds, so they should be used in areas where weeds will not be a problem. Other organic fertilizers include fish meal, blood meal, bat guano, seaweed, and peanut hulls. Because two of the breakdown products of organic matter are ammonium ions and carbon dioxide (or carbonic acid), when it is dissolved in water, additions of organic materials tend to lower the pH of soil over time. Principal limitations of organic fertilizers are their bulk, water content, availability, associated odor, potential salt and weed seed hazards, and expense per pound of nutrient. The value of manures and organic fertilizers are not solely nutritional, however. These amendments also have beneficial effect on soil structure, decreasing soil bulk density, improving water infiltration and nutrient-holding capacities, and adding small amounts of micronutrients.
Manure is comprised of varying mixtures of plant-eating animal excreta and plant remains. They can make great, organic garden fertilizers when used correctly, and supply garden plants with many essential nutrients. Manure also improves soil structure. For organic nutrient sources to be beneficial to plants, however, the nutrients provided must be changed into chemical forms that plants can readily absorb, and this takes time. Manures can also contain unwanted weed seeds, so they should be used in areas where weeds will not be a problem. Other organic fertilizers include fish meal, blood meal, bat guano, seaweed, and peanut hulls. Because two of the breakdown products of organic matter are ammonium ions and carbon dioxide (or carbonic acid), when it is dissolved in water, additions of organic materials tend to lower the pH of soil over time. Principal limitations of organic fertilizers are their bulk, water content, availability, associated odor, potential salt and weed seed hazards, and expense per pound of nutrient. The value of manures and organic fertilizers are not solely nutritional, however. These amendments also have beneficial effect on soil structure, decreasing soil bulk density, improving water infiltration and nutrient-holding capacities, and adding small amounts of micronutrients.
Knowing when and how to apply fertilizer is a key step in caring for a thriving garden. With so many different amendments and fertilizers on the market, it is overwhelming at times to be faced with the multitude of decisions to be made about what fertilizer to choose, when to apply it, how it should be applied, and how to monitor the effects. Luckily, there's a handy page on this website dedicated solely to fertilizing:
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MAnaging and amending soil physical and chemical properties
Successful gardening depends on both the chemical and physical properties of local soils. Soil with good physical structure and good tilth can hold and provide adequate quantities of nutrients, water and air to plant roots. When soil has been overworked, graded, layered, compacted or otherwise greatly disrupted, problems can crop up in establishing and maintaining landscape and garden plant materials. In mature landscapes, problems can crop up with soil properties as well. Soil compaction in high foot-traffic areas or from vehicles driving over non-paved areas is one of the most common and potentially harmful changes to soil structure. Overfertilization can also severely damage soil, creating high levels of soluble salts and lowering pH levels. Adding one or more soil amendments regardless of the starting soil situation is considered essential to building great garden soil. It is usually impractical, labor-wise and cost-wise, to amend the soil across an entire landscape, unless it is in really bad shape. Physical soil amendments are more commonly used in the home garden in specific areas such as small beds, ornamental plant sections of the landscape, or where crops are regularly removed and replanted. Occasional additions of organic matter, such as compost, can really help these areas, ensuring that microbes have adequate food supplies and preserving the soil biome. Before amending any garden areas, it is first prudent to evaluate and analyze your planting site to determine whether an amendable problem exists.
Investigate soil with augers, probes or shovels to do a quick check for layered soil or compaction zones below the soil surface. If there has been any work done on the property, the original grading specifications should be consulted to subsurface changes that could affect soil structure. Soil should also be evaluated for its relative high clay or sand content, and rates of water infiltration and drainage should also be estimated. In investigating a garden space, the following issues should be noted, as they indicate a need for soil amendment:
Investigate soil with augers, probes or shovels to do a quick check for layered soil or compaction zones below the soil surface. If there has been any work done on the property, the original grading specifications should be consulted to subsurface changes that could affect soil structure. Soil should also be evaluated for its relative high clay or sand content, and rates of water infiltration and drainage should also be estimated. In investigating a garden space, the following issues should be noted, as they indicate a need for soil amendment:
- Soil texture analysis that reveals an extremely high percentage of clay (>30%) or an extremely high percentage of sand (>70%).
- Infiltration/percolation less than a rate of 1/4 inch per hour.
- Layers of soil with widely differing textures.
- Compaction of soil surface or layers below the soil surface.
- A pH of beyond an acceptable range for supporting plant life (below 5.0 or above 8.0).
- A high level of soluble salts (EC test value of 2.0 dS/m or greater for sensitive plants, 4.0 dS/m or greater for normal plants).
Quick Question: What is soil tilth?
Soil tilth is defined as the physical condition of soil as related to its ease of tillage, fitness as a seedbed, and its promotion of seedling emergence and root penetration. Soil with good tilth is loose, friable (crumbles with ease), well-granulated, and has natural mulching abilities.
Soil tilth is defined as the physical condition of soil as related to its ease of tillage, fitness as a seedbed, and its promotion of seedling emergence and root penetration. Soil with good tilth is loose, friable (crumbles with ease), well-granulated, and has natural mulching abilities.
Best Practices for Preventing the Breakdown of Soil Structure
The structural breakdown of soil is much easier to prevent than remedy; maintaining or improving soil structure is one of the most important aspects of soil management in a landscape or garden area. Although some disruption to soil is inevitable where land is intensively cultivated, there is frequent foot or heavy equipment traffic, or where soil has been disturbed by construction or grading, there are ways to ensure that soil stays in the best condition possible:
The structural breakdown of soil is much easier to prevent than remedy; maintaining or improving soil structure is one of the most important aspects of soil management in a landscape or garden area. Although some disruption to soil is inevitable where land is intensively cultivated, there is frequent foot or heavy equipment traffic, or where soil has been disturbed by construction or grading, there are ways to ensure that soil stays in the best condition possible:
- Cultivate or till soil only when it has a medium moisture content (it will crumble easily.) Working soil when it is too dry creates dust issues and working it when it is too wet creates puddling and packing.
- Till garden soil only when required to turn under organic matter, control weeds, make irrigation furrows, or loosen a volume of soil for planting seeds or transplants.
- Avoid recompaction of freshly loosened, ripped, or plowed soil. The less tillage and the less traffic after loosening, the better.
- Designate traffic areas in landscapes and gardens with paths, and keep foot and equipment traffic over the soil to a bare minimum, especially when it is very wet.
- Apply organic mulch to bare areas around landscape and garden plants.
Soil Physical & Chemical Problems & Solutions
Soil physical problems usually stem from issues with soil content (too much clay or too much sand) or soil compaction. If issues cannot be prevented, physical soil problems can be amended and improved. Amendments are meant to dilute existing soil particles, which enhances structure (aggregation), alters porosity (improves aeration, drainage, and moisture-holding conditions), and reduces bulk density. If soil structure is poor, the following factors and practices favor the formation of a more granular structure:
Soil physical problems usually stem from issues with soil content (too much clay or too much sand) or soil compaction. If issues cannot be prevented, physical soil problems can be amended and improved. Amendments are meant to dilute existing soil particles, which enhances structure (aggregation), alters porosity (improves aeration, drainage, and moisture-holding conditions), and reduces bulk density. If soil structure is poor, the following factors and practices favor the formation of a more granular structure:
- Bacterial decomposition of organic materials and plant residues produces gums that help to bond soil particles together, and organic materials added to clayey soils can also help break up soil structure. Adding organic matter to sandy soils will improve their structure as well, encouraging the formation of aggregates.
- Planting fibrous-rooted cover crops (grasses) promotes the bonding of soil particles. which yields aggregates with continuous pore spaces between them.
- Cycles of wetting and drying cause swelling and shrinking of soil, generally resulting in improved aggregation.
- Sand can be added to clayey soils so that the composition is at least 45% sand to the depth requiring amendment.
- If excessive sodium is present, adding gypsum to the soil can improve structure.
- It is best to correct pH levels before planting any crops or ornamentals.
- If a soil's pH is too low, add lime (calcium carbonate, calcium hydroxide, calcium oxide or dolomite) or wood ashes.
- If a soil's pH is too high, add sulfur (at a rate of 2-4 pounds per 100 square feet at a depth of 6-8 inches) or aluminum sulfate. Acidifying soil takes longer than raising pH because the conversion of sulfur to acid is mediated by soil microorganisms; beneficial results may not be noticed for months. In existing landscapes, apply sulfur in monthly increments of a few ounces per 100 square feet of garden space and follow each application with irrigation to avoid damaging nearby plant roots.
- In high-sodium situations gypsum (calcium sulfate) and sometimes sulfur are used. In new sites, spread a total of 20 pounds per 100 square feet of garden space, and wait a few months to retest soil. In existing landscapes, apply small amounts over a few months and irrigate heavily after application.
- Soluble salts in soil can be leached simply by overwatering an area
Lead Contamination of Garden Soil
In urban settings, lead can build up in soil as a result of accumulated paint scrapings from buildings coated with lead-based paint and from airborne depositions of lead particles (from the leaded gasoline they use). Although many cities in California have made a concerted effort to remove lead-based paint and other products due to their poisonous qualities, there are some areas, even here in the Bay Area, that still have issues with lead contamination. Under normal soil conditions, lead is inert because it is bound to other oxides, clays, and organic matter. However, soil deficient in phosphate or soil that has a low pH might contain more soluble lead than plants can take up. When lead is absorbed by plants, most of it goes into root tissue and leaf tissue, and very small amounts are found in fruit. To lessen lead contamination in ornamental and crop plants, there are some best practices to follow:
In urban settings, lead can build up in soil as a result of accumulated paint scrapings from buildings coated with lead-based paint and from airborne depositions of lead particles (from the leaded gasoline they use). Although many cities in California have made a concerted effort to remove lead-based paint and other products due to their poisonous qualities, there are some areas, even here in the Bay Area, that still have issues with lead contamination. Under normal soil conditions, lead is inert because it is bound to other oxides, clays, and organic matter. However, soil deficient in phosphate or soil that has a low pH might contain more soluble lead than plants can take up. When lead is absorbed by plants, most of it goes into root tissue and leaf tissue, and very small amounts are found in fruit. To lessen lead contamination in ornamental and crop plants, there are some best practices to follow:
- Locate plantings as far from streets and roads as possible, at least 75 feet is best. Avoid planting in areas where older buildings stand or have been demolished.
- Maintain a soil pH of around 7.0 and keep phosphate levels in the higher normal range.
- Add as much organic material to soil as possible.
- Wash all produce thoroughly; discard outer leaves of leafy vegetables. Peel all root crops grown under contaminated conditions.
- Avoid growing leafy crops near streets or in highly contaminated soils.
- If high concentrations of lead in soil are known or suspected. reduce or eliminate possible uptake by plants by removing and replacing the topsoil, establishing high raised planter beds, or growing crops in containers.
the mulch effect
Mulch is considered to be any material that a gardener lays on top of the soil surface to conserve water, modify the soil temperature, prevent soil crusting, lessen erosion, keep weeds down, and reduce the spread of disease. Mulch also has the added benefit, when used around the bases of certain plants, of suppressing the development of root rot issues. Mulch can be an organic or inorganic material. Organic mulches act like a layer of leaf litter on the forest floor, decaying slowly into the soil to improve its structure and improve fertility. This layer often gets dug into the soil throughout the season and acts as another great source of organic matter to help build up humus in the soil. When organic material is used for mulch, soil fauna will slowly churn it back into the soil and move it through the soil profile. There are many materials that can be used for mulching, including:
To reap all of the benefits of using mulch, it is important to choose a mulching material that is appropriate for your unique garden biome. In general, mulches are best when they are coarse, woody material with pieces that are between 1 and 3 inches long. If leafy or grassy materials (like straw or fallen leaves) are used as mulch, they will need to be applied more frequently because they break down more quickly than other mulches. Also, in rainy weather, these grassy materials can become soggy and anaerobic, which causes issues for surrounding plant life. Regardless of the material chosen, keep mulch around 4-6 inches away from plant stems to avoid trunk fungal and bacterial diseases, and should be spread between 2 and 4 inches deep to reap the maximum benefits.
Quick Question: What's the difference between mulch and topdressing?
Although these two groups of materials are applied similarly, there are some key differences that are worth mentioning. Topdressing is a thin layer of material applied to the surface of garden soil for the purpose of assisting with soil drainage and structure and is commonly used in lawn maintenance practices. Materials commonly used to topdress are compost, peat moss, and sand.
Although these two groups of materials are applied similarly, there are some key differences that are worth mentioning. Topdressing is a thin layer of material applied to the surface of garden soil for the purpose of assisting with soil drainage and structure and is commonly used in lawn maintenance practices. Materials commonly used to topdress are compost, peat moss, and sand.
Conclusion
Congratulations, you've officially made it through the soil module! This module was pretty information-heavy, but you'll be pleased to know that the next few modules are a little less intense. Before moving on to the Plant Identification Modules, take some time to check out the Key Terms page for this module, review the Resources & References section below, and do the homework activities.
Homework
Click the button below to access the activities and homework questions for this Module. If you'd like to print the homework page or any other resources, visit the printables page.
Resources & REferences
Videos
All about dirt.
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What's dirt, anyway?
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How to improve your soil
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Tips on how to create healthy soil for your garden
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Links