The Soil Profile
The soil profile comprises two or more soil layers called horizons, one below the other, each parallel to the surface of the land. Important characteristics that differentiate the various horizons are:
Color, texture, structure, consistency, porosity and soil reaction
Thicknesses ranging from several feet thick to as thin as a fraction of an inch
Generally, the horizons merge with one another and may or may not show sharp boundaries
The uppermost layer in the soil profile or surface soil. It includes the mulch layer and plow layer. Living organisms are most abundant in this horizon, consisting of plant roots, bacteria, fungi and small animals. Organic matter is most plentiful, particularly in the mulch layer. When a soil is tilled improperly, the A Horizon may be eroded away.
Lies immediately beneath the A Horizon and above the C Horizon. It is called the subsoil. The B Horizon has properties of both A and C. Living organisms are fewer in number than in the A Horizon, but more abundant than in the C Horizon. Color is transitional between A and C as well. It is frequently higher in clay than either of the other horizons.
The deepest of the three. This is the material from which the mineral part of the soil forms. It is the parent material of soils. It may have accumulated in place by the breakdown of hard rock, or it may have been placed there by the action of water, wind or ice.
A fertile soil contains an adequate supply of all the nutrients required for plant growth. The full potential of crops is not realized if a shortage of nutrients occurs at any time during the growth cycle. This is true even though plants are capable of remarkable recovery from short periods of starvation.
A fertile soil is not necessarily a productive one. The second major requirement is that the soil must be adequate for plant growth. This soil is based on environmental factors including texture, structure, soil water supply, pH, temperature and aeration.
Soil Texture, Structure and Color
Soil Classes and Textures
An important factor in soil productivity is texture, defined as the relative percentage of sand, silt and clay. Soils are classified on the basis of texture of each of the horizons. The relative proportions of clay, silt and sand determine the soil textural class.
Clays are the smallest particles in soil; silts are somewhat larger in size, followed by sands that are coarse enough that the individual particles are visible to the naked eye. The following table shows the proportion of sand, silt and clay normally found in the various textural classes of soils.
The arrangement of soil particles into groups or aggregates determines the "structure." A single mass or cluster of soil particles held together in a particular way imparts physical characteristics to the soil, such as a clod, prism, crumb or granule. Soil structure is often more important than the texture to the farmer. Soil structure can be changed to produce improved soil conditions for maximum yield and profits. Structure is especially important to water movement and in preventing root growth restrictions, both of which affect nutrient availability to the crop. Examples of various types of soil structure are shown below.
Color in various types of soils is due primarily to the amount of organic matter and the chemical state of the iron and other compounds in the mineral fraction of the soil. Other minerals such as quartz, granite and heavy black minerals may also influence soil color. Unweathered parent materials tend to be gray in color, or else will have the color of the natural minerals from which they are derived.
The color of subsoils can reveal a great deal about the age and drainage conditions in the soil. Iron compounds can exist as oxidized forms (red), hydrated oxides (yellow), and as reduced forms (gray).
The mineral soil harbors a varied population of living organisms that play an important role in the dynamic changes occurring within the soil. Many groups of organisms live in the soil, and range from microscopic to those visible to the naked eye.
Some of the microscopic-sized organisms are the bacteria, fungi, actinomycetes, algae and protozoa. Most soil organisms depend upon organic matter for food and energy. Consequently, they are generally found in the top 12 inches of soil. One of the most important functions of soil microorganisms is the decomposition of crop residue. Some of it is converted into more stable organic compounds that are stable in the soil over long periods of time. But a large percentage of the organic material is released to the atmosphere as carbon dioxide. Also, nitrogen and other essential plant nutrients are released and made available to growing crops.
Rhizobium bacteria form a symbiotic relationship that results in nitrogen fixation in legume plants. These organisms penetrate plant roots, causing the formation of small nodules on the roots. They then live in symbiotic relation with the host plant. The beneficial effect of this process is realized when cultivated legumes, such as alfalfa, clovers, soybeans, etc., are inoculated at seeding with the proper strain of the rhizobium bacteria.
The millions of microorganisms in the soil play critical roles in plant nutrition, although many are unidentified. The improved understanding of the microbiology of plant nutrition is one of the important unmet challenges of crop nutrient management.
Some soil microorganisms are harmful to soils and growing plants, in the form of diseases, toxins produced and denitrification. When the supply of air in a soil is limited, certain aerobic soil organisms can get their supply of oxygen by reducing highly oxidized compounds, such as nitrates. Further reducing action may result in free nitrogen (N₂) being produced and lost to the atmosphere. This is not an environmental problem, because 78 percent of the atmosphere is N₂ gas, but the result is a net loss of N available for the crop. Other microorganisms contribute to loss of N as NOx gases, potent greenhouse gases that can be an environmental problem.
Adapted from "Fertilizers and Soil Amendments" by Follett, Murphy and Donahue.
Soil organic matter represents an accumulation of partially decayed and partially resynthesized plant and animal residues. Such material is in an active state of decay by soil microorganisms. Consequently, it is transitory and must be renewed constantly by additional plant residues.
The organic matter content of a soil is only about 3 to 5 percent by weight in most topsoils. However, it may actually be less than 0.5 percent in very sandy soils. Organic matter serves as a "granulator" of the mineral particles, being largely responsible for the loose, friable condition of productive soils. Also, organic matter is a major source of two important mineral elements, phosphorus and sulfur, and is essentially the sole source of inherent soil nitrogen.
Through its effect on the physical condition of soils, organic matter also tends to increase the amounts of water a soil can hold and the proportion of this water that is available for plant growth. The capacity of decomposed organic matter (humus) to hold water and nutrient ions greatly exceeds that of clay, its inorganic counterpart. Therefore, small amounts of humus can greatly improve the soil’s capacity to promote plant production.
Humus and Clay — The Seat of Soil Activity
Much of the dynamic nature of soils is attributed to the portions of the finer components, humus and clay. Both of these soil constituents exist in the colloidal state. The individual particles of each are characterized by extremely small size, large surface area per unit weight, and the presence of surface charges to which ions and water are attracted. Clay and humus act as centers of activity around which chemical reactions and nutrient exchanges occur. By attracting ions to their surfaces, they temporarily protect many essential nutrients from leaching and then release them slowly for plant use. Because of their surface charges, they are also thought to act as "contact bridges" between larger particles, thus helping to maintain stable granular structure that results in a soil that is easily tilled and has good air and water movement.
On a weight basis, the humus colloids have greater nutrient and water-holding capacities than clay. Clay is generally present in larger amounts, however. For that reason, the total contribution of clay to soil chemical and physical properties will generally equal or exceed that of humus. The best agricultural soils contain a good balance of humus and clay.
Humus is a highly complex substance that plays an important role in moisture and nutrient retention in the soil, and encourages the formation of good soil structure. It is black or dark brown in color and spongy or jelly-like in consistency. Oftentimes, humus is referred to as the life force of the soil because it boosts soil fertility. Soil organisms feed and reproduce within humus. Because humus is a colloid, it increases the cation exchange capacity of the soil.
Because of its critical role in nutrient availability in the soil, it is important to know some of the basic characteristics of clay. Clay minerals are composed of layers, or sheets, of silica and alumina — two of the most prominent elements of the Earth's crust. The edges of these sheets expose negative charges that attract positively charged nutrients.
Understanding the clay mineral makeup of the soil for any given field provides important information to help determine nutrient management practices best suited for that field. The type of clay minerals, along with texture, structure and organic matter, helps guide nutrient decisions.
There are predominantely two broad types of clays, montmorillonite and kaolinite. They are found in the temperate regions, which include most of the important agricultural soils of the world and practically all of the agricultural soils in the United States. Other types of clays, such as illite, are present in smaller quantities.
Montmorillonite clays, found largely in arid regions and in colder climates such as the western and Midwestern states, are composed of one alumina layer between two silica layers. The layers of silica and alumina are not held together tightly, and they tend to expand when wet and contract upon drying. This expanding nature gives them a high surface area relative to weight (like opening the "pages" of a book), resulting in a high capacity to hold water and nutrients. Soils with a high percentage of montmorillonite are very difficult to cultivate when wet, being sticky and hard to manage. When these soils dry, cracks appear on the surface.
The interlayer space expands and contracts with wetting and drying. Negative charges are on particle surfaces, interlayers and broken edges of the colloid.
Kaolinite clays are found generally in the more humid and temperate climates, such as the southeastern United States, and are more weathered. These clays are composed of one layer of silica and one layer of alumina, often referred to as a 1:1-type clay. The layers are held together more tightly than montmorillonite and, therefore, do not tend to expand when wet and contract upon drying. Their negative charges are primarily along the broken edges of the colloids. As a result, kaolinite-type clay soils are easier to cultivate and hold less water than montmorillonite clays.
Cation Exchange Capacity
Each soil colloid contains a net negative electrical charge due to its structural and chemical makeup. Soil colloids have the ability to attract and hold positively charged elements by electrical attraction. Most chemical compounds when in solution dissolve into electrically charged particles called ions. Ions with positive charges are called cations and ions containing negative charges are referred to as anions. Consequently, positively charged cations such as potassium (K⁺), calcium (Ca⁺⁺), magnesium (Mg⁺⁺) and ammonium nitrogen (NH₄⁺) are attracted and held to the surface of soil colloids much like a magnet attracts and holds iron filings.
Montmorillonite clay and organic colloids have more surface area exposed than kaolinite-type colloids and, therefore, have a higher net negative electrical charge. Thus, montmorillonitic soils have more capacity to hold positively charged nutrient ions, or cations. This characteristic is called cation exchange capacity (CEC). Knowledge of a soil’s CEC is basic to understanding how to manage lime and fertilizer additions. Since kaolinite clays have less surface area exposed, they have lower (CEC) values, meaning less capacity to hold nutrients.
CEC helps to explain why certain fertilizer elements such as positively charged potassium, calcium and magnesium, as well as ammonium nitrogen are not as easily leached from the soil as the negatively charged ions, or anions, of nitrate nitrogen, sulfates or chlorides.
Cations adsorbed on the surface of soil colloids, and those contained in the soil solution, are available for plant use. Adsorbed cations, however, can be replaced by other cations present in the soil solution through the process of cation exchange. These replaced cations may then combine with an anion and be leached from the soil.
For example, when large amounts of a fertilizer material such as muriate of potash (KCI) are applied to the soil, the KCI dissolves in soil moisture and disassociates into K⁺ and Cl⁻ ions. The K⁺ in solution tends to exchange with Mg⁺⁺ adsorbed on the clay and organic matter. The K⁺ is held on the soil particles, and the Mg⁺⁺ combines with Cl⁻ to form MgCl, a soluble compound that is then leached from the soil with rainfall. As plants remove nutrients from the soil solution throughout the growing season, the concentrations change, and this dynamic exchange of nutrients continues.
The force by which cations are held by soil colloids will depend upon several factors. The smaller the cation and the less water it has adsorbed, generally the tighter the cation is held on the soil particles. Hydrogen ions, therefore, are more tightly held and more difficult to replace than larger and more hydrated cations such as ammonium, calcium, magnesium and potassium. Divalent cations (two charges) are generally held tighter by soil colloids than monovalent cations (one charge). Therefore, calcium and magnesium, divalent cations, are more difficult to replace than the monovalent cations such as potassium and ammonium. Soils with high sand and silt content have a lower percentage of clay and organic matter, and thus have lower CEC. This explains why coarse-textured soils require more frequent applications of lime and fertilizer.
Determining Soil Cation Exchange Capacity
The cation exchange capacity (CEC) of a soil is typically expressed in terms of milliequivalents. A milliequivalent is defined as "one milligram of hydrogen or the amount of any other element that will displace it." When applied to soils, milliequivalents are generally expressed on the basis of 100 grams of oven-dried soil. One milligram of hydrogen per 100 grams of soil equates to 10 parts of hydrogen per one million parts of soil. An acre (top 6 2/3 inches) of soil weighs about 2,000,000 pounds. Therefore, 10 parts per million of hydrogen (whose atomic number is one) equals about 20 lb/acre of hydrogen.
This calculation provides a standard of measurement for converting the milliequivalent of other elements to pounds per acre. The standard is one milliequivalent of hydrogen equals 20 lb/acre of hydrogen. Since the atomic weight of hydrogen is 1, to convert a milliequivalent of other elements to pounds per acre, multiply its atomic weight by 20. Remember, divalent elements have two positive electrical charges and replace two hydrogen ions; therefore, to arrive at the equivalent atomic weight of divalent cations, divide its atomic weight by 2.
Example: Atomic weight of calcium = 40 Valence = 2 Equivalent weight = 40/2 = 20
Therefore, one milliequivalent of calcium is equal to the equivalent weight of calcium multiplied by 20 lb/acre of hydrogen. Calcium equivalent weight of 20 x 20 lb/acre hydrogen = 400 lb/acre.
CEC by a Lab
One laboratory method of determining the exchange capacity of a soil is to remove all of the adsorbed cations by leaching a weighed portion of soil with a salt solution such as one normal ammonium acetate. All of the adsorbed cations are replaced by the ammonium ions. All excess ammonium ions are then removed by leaching with alcohol. The adsorbed ammonium ions are then removed from the soil by extracting with a different salt, such as one normal potassium chloride. The potassium ions replace the adsorbed ammonium ions. The quantity of ammonium ions in the leachate can then be measured, and is then expressed as milliequivalents per 100 grams of soil — the CEC value. This laboratory procedure is laborious and time consuming. Generally, an estimate of the soil's CEC value is sufficient.
Estimate of CEC
An estimate of the cation exchange capacity of a soil can be made from soil test results. This can be accomplished by dividing the pounds per acre of the element as determined by the soil test by the milliequivalent weights of the cations. First, the equivalent weights of cations must be converted into pounds per acre. The cations used in the calculation of CEC are hydrogen, potassium, magnesium and calcium.
To arrive at the estimated cation exchange capacity of this soil, divide the lb/acre of each element as determined by soil test by one milliequivalent (m.e.) in lb/acre of each element. As shown in the table below, for calcium, divide the 800 lb/acre soil test value by the 400 m.e. value, which yields a value of 2.0 m.e. per 100 grams of calcium. The sum of the m.e. per 100 grams for each of the four nutrients is the calculated CEC for that soil.
The proportion of adsorbed base cations (calcium, magnesium and potassium) relative to hydrogen is expressed in terms of percent base saturation. Generally, the higher the percent base saturation of a soil, the higher the soil pH and fertility level. In the above example, the percent base saturation would be:
((Ca 2.0 + Mg 0.5 + K 0.32) /5.32) X 100 = 53% Base Saturation
This base saturation number is then used with the appropriate calibration database for the area to guide fertilizer recommendations.
The table at right shows the CEC values for representative soils across the United States and illustrates the wide range of values that can occur.
Anions are the opposite of cations, in that they contain a net negative charge. The most common anions in soils are chloride, sulfate, phosphate and nitrate.
In addition to cation-adsorbing capacity, soils also have the ability to adsorb anions, but to a lesser extent than cations. Anion adsorption is pH dependent and increases with a decrease in soil pH. Phosphates and sulfates are adsorbed more strongly than nitrates and chlorides. Anion adsorption is not as important agriculturally as cation adsorption. Most agricultural soils have a pH higher than that at which anion adsorption is at its maximum strength; and with the exception of phosphate, and to a lesser degree sulfate, anions are largely lost from the soil by leaching.