CHAPTER 1. PROPERTIES OF SOILCHAPTER 1. PROPERTIES OF SOIL

Soils are thin mantles of material that cover most of the land surface of the earth. Soil is composed of inorganic and organic matter, water, air, and living organisms. Generally, soils exist as layers known as horizons, commonly called topsoil, subsoil, and parent material or scientifically known as the A, B, and C horizons (Figure 1). Since the A horizon is the top horizon in most cases, it is recognized commonly as being the topsoil, but this association is somewhat inaccurate. Topsoil or surface soil is that portion that is moved by cultivation. Tilling of land to about the same depth for several the years creates a horizon called the plow horizon (Ap) which is also known as topsoil.

Figure 1. Profile of a tilled soil. All of the horizons shown may not appear in all soils. Transition horizons (A3, B1, and B3) and others are not illustrated and may be present in some soils.

In some soils the A horizon is eroded away leaving the B horizon as the cultivated zone and hence the topsoil. Also, in some cases, as with soils in forests, a layer of organic litter covers the top of the soil, and this layer is called the O horizon or Ao horizon. Typically, the A horizon is a zone from which materials are washed downward, and the B horizon is the zone in which accumulation of these materials occurs.

Parent material is the substance from which a soil is derived. Parent material can be inorganic materials, such as various kinds of rocks, deposits left by glaciers, wind, water, or gravitational forces, organic matter from formerly living organisms, or mixtures of materials.

Farmers and gardeners view soil as a habitat for plants. Plants obtain most of their water and nutrients from the soil. Most of the feeding for nutrients occurs in a zone that starts about 3 inches below the soil surface and extends to about 10 inches below the surface (Figure 2). Water is absorbed from this zone and from greater depths in the soil. Soil provides air to roots and also serves to anchor the roots to hold plants in place.

Figure 2. Illustration of fibrous rooting pattern in soil. Herbaceous plants receive most of their nutrition from a zone ranging from 3 inches to 10 inches deep in the soil. Below 10 inches deep, roots function mainly in absorption of water.

Inorganic matterInorganic matter

The inorganic matter of soil is derived from rocks or minerals in various stages of disintegration. Inorganic matter is the major constituent of most soils. Soils that are more than 80% inorganic matter are referred to as mineral soils. On a dry weight basis, most upland soils are 95% or more inorganic matter. If soils are produced from the same kind of rocks or minerals and the inorganic particles differ only in size, the differences among the soils are principally physical properties. If one soil is derived from a different kind of material than another soil, that is, for example, one from sandstone and one from limestone, and if the soil particles are of different sizes in the two soils, the soils will differ in physical and in chemical properties. The chemical and physical properties together determine the agricultural value of a soil and give rise to the term soil fertility. Assessment of soil fertility takes into consideration factors such as water-holding capacity, aeration, drainage, depth, and structural impediments to rooting, as well as chemical factors such as acidity and supply of nutrients.

The inorganic particles of soil are sand, silt, and clay. These three groups of particles are referred to as soil separates or collectively as the fine earth. The physical differences between sand, silt, and clay are the sizes of particles. Within separates, particles also vary greatly in shapes (Figure 3). In agricultural soils (Table 1), particles of sand range from 2 to 0.05 mm in average diameter, that is, they will pass through a sieve with holes ranging from 0.05 to 2 mm in diameter. Silts range in size from 0.002 to 0.05 mm, and clays are smaller than 0.002 mm in diameter. Fragments that are larger than 2 mm are gravel (2 mm to 75 mm), cobbles, stones, flags, and bolders. Soils that have these large fragments are spoken as being gravelly sands, stoney clays, and the like.

Table 1. Size limits of soil separates and some other common mineral fragments in soil.

Separate or fragment Size range, mm

Coarse fragments

Boulders >600

Stonesy >250 to 600

Cobblesy >75 to 250

Gravel >2 to 75

Fine earth

Sand

Very coarse sand >1.0 to 2.0

Coarse sand >0.5 to 1.0

Medium sand >0.25 to 0.5

Fine sand >0.10 to 0.25

Very fine sand >0.05 to 0.10

Silt >0.002 to 0.05

Clay <0.02

Figure 3. Illustration of irregular sizes and shapes of soil particles, coatings on soil particles, and aggregates of soil particles (peds).

Soils that differ with respect to particle size and relative proportions of the various particles of the inorganic fraction of topsoil (A horizon or uppermost mineral horizon) are said to differ in soil texture. Texture is a basic, nearly permanent characteristic of soil. It cannot be altered through ordinary practices, such as tillage; however, erosion, deposition, or mixing of layers may change the texture of the surface zone. Since texture is based only on the inorganic fraction, the amount of organic matter in soil does not affect texture. Soil structure refers to the arrangement of soil particles into groups--aggregates, clods, or larger organizations of particles. Soil structure is not a permanent property and can be affected by management and by organic matter.

Soils in nature are not composed entirely of one separate but are mixtures of sand, silt, and clay. On the basis of the relative amounts of sand, silt, and clay present in the topsoil, soils are classified into three broad groups, sandy soils, loamy soils, and clayey soils (Table 2). Sands when dominant give a coarse-textured or light soil. Light means that the soil is easy to work or to till and does not refer to the actual weight or bulk density of the soil. Soils that have considerable portions of silt, clay, or both separates are fine-textured or heavy soils. Heavy soils are sticky or plastic and are difficult to work or to till. Loamy soils are medium-textured soils, feeling as if they have even proportions of sand, silt, and clay.

Table 2. Soil textural classes based on relative proportions of sand, silt, and clay.

Sandy Light, Coarse Sand

Loamy sand

Loamy Even, Sandy loam

Moderately coarse to Fine sandy loam

moderately fine Very fine sandy loam

LoamSilt loamSiltClay loam

Sandy clay loam

Silty clay loam

Clayey Heavy, fine Sandy clay

Silty clayClay

Sand, silt, or clay do not have an equal influence on determining the specific textural class (Figure 4). Soils that fall in the specific textural class of sand are at least 85% sand by weight. Textural classes with the modifier sandy in their name are 50% or more sand. A soil of the specific class silt is at least 80% silt, whereas soils with the word silt or silty in their class name are 40% or more in silt. On the other hand, a soil of the specific class of clay need have no more than 40% clay, and some soils, for example, sandy clay loam, may have no more than 20% clay. Clay has a much more dominant effect on determining textural class and affecting the physical properties of soils than sands or silts.

In a laboratory, the textural class of a soil is determined by a process called mechanical analysis in which the soil separates are measured quantitatively by weight. In the field, textural class is determined by feeling of the soils with the fingers. Accurate determinations in the field require skill and experience, especially to identify a soil that may be any of the fourteen specific classes (Table 2). Generally, most people do not have the capability to run a mechanical analysis and must rely on the field method to determine soil texture. The Soil Survey Staff of the United States Department of Agriculture has adopted definitions that permit one to determine certain basic textural classes by feeling of the soil with the fingers and palms of the hands (Table 3).

Figure 4. United States Department of Agriculture guide for textural classification of soils based on the mass percentages of sand, silt, and clay in the A-horizon.

Table 3. Determination of soil textural class by feeling of the soil with the hands (Soil Survey Staff. U.S. Dept. of Agriculture, l937. Soil Survey Manual. USDA Handbook No. l8, Washington D. C.).

GUIDE FOR CLASSIFYING THE BASIC TEXTURAL SOIL CLASSESIn the field texture is determined by the feel of the soil mass when rubbed between fingers. The following statements give the obvious physical characteristics of the basic textural grades:

SAND. Sand is loose and single-grained. The individual grains can readily be seen or felt. Single particles may appear shiny. Squeezed in the hand when dry, sand will fall apart when the pressure is released. Squeezed when moist, it will form a cast, but will crumble when touched.

SANDY LOAM. A sandy loam much sand but has enough silt and clay to make it somewhat coherent. The individual sand grains can be seen and felt. Squeezed when dry, a sandy loam will form a cast which will readily fall apart, but if squeezed when moist a cast can be formed that will bear careful handling without breaking.

LOAM. A loam feels to have a relatively even mixture of sand, silt, and clay. It is mellow with a somewhat gritty feel, yet fairly smooth and slightly plastic. Squeezed when dry, loam will form a cast that will bear careful handling, while the cast formed by squeezing the moist soil can be handled quite freely without breaking.

SILT LOAM. When dry, silt loam may appear cloddy, but the lumps break readily. When pulverized, it feels soft and floury. When wet, the soil readily runs together. Either dry or wet it will form casts that can be handled without breaking, but when moistened and squeezed between thumb and finger it will not "ribbon" but will give a broken appearance.

CLAY LOAM. A clay loam breaks into clods or lumps that are hard when dry. When the moist soil is pinched between the thumb and finger, it will form a thin ribbon which breaks readily, barely sustaining its own weight. The moist soil is plastic and will form a cast that will bear much handling. When kneaded in the hand it does not crumble readily but tends to work into a heavy mass.

CLAY. A clay forms very hard lumps or clods when dry and is quite plastic and usually sticky when wet. When the moist soil is pinched out between the thumb and fingers it will form a long, flexible ribbon.

Table 4. Soil physical properties that are governed by textural class

EXPECTED RESPONSES TO SOIL TEXTURAL CLASS

Change in texture from:

Sand --------------------------------------> Loam -------------------------------> Clay

COARSE ---------------------------------> to -----------------------------------> FINE

Particle density ---------------------------------------------------------------> Same

Soil surface area (internal) ----------------------------------------------------> Increase

Total pore volume ------------------------------------------------------------> Increase

Bulk density ------------------------------------------------------------------> Decrease

Water holding capacity -------------------------------------------------------> Increase

Permeability (aeration, drainage) ----------------------------------------------> Decrease

Leaching losses --------------------------------------------------------------> Decrease Potential for aggregation -----------------------------------------------------> Increase

Cation exchange capacity ----------------------------------------------------> Increase

The pore space in fine-textured soils has many small pores or capillaries. Movement of air and water is slow in these soils. Coating of the particles and filling of pores with water restricts diffusion of air; consequently, clayey and silty soils often are poorly drained and poorly aerated. Poorly drained soils when wet will not provide adequate oxygen to roots. On the other hand, fine-textured soils have good water-holding capacity because water is retained in the capillaries. Much of the water in fine-textured soils may not be available to plants because it is held tightly to the surface of clay particles. Partly because of the restricted downward movement of water, leaching of nutrients from the root zone is a minor problem in fine-textured soils.

The surface area of soils increases as the texture becomes finer. The total surface area of a unit weight of colloidal clay, depending on the type of clay, may be about 10,000 times that of the finest sand and about 1,000 times that of the finest silt. Adsorption, the taking up of gases, liquids, or dissolved solids by surfaces, increases as the surface area increases. Fine-textured soils are much more active in adsorbing water and dissolved nutrients than coarse-textured soils. This added retention of water and nutrients enhances the fertility of fine-textured soils relative to that of coarse-textured soils.

As a guideline for the relative proportions of solid matter and pore space in soil, a loamy soil is about 50% solid matter and about 50% pore space by volume. At field capacity level of water retention, about half of the pore space is filled with water, and half is filled with air (Figure 5). Sandy soils will have more total volume occupied by solid matter, and clayey soils will have more total volume occupied by pore space than loamy soils. In the sandy soil, more of the pore space will be occupied by air than by water, whereas the converse will occur in clayey soils.

50 % solid matter including:

25% Air-filled pores

Mineral matter and organic matter

25% Water-filled pores

Figure 5. Illustration of relative volume of solid particles (mineral and organic matter), air-filled pores, and water filled pore space in a medium-textured soil. Fine-textured soils typically will have more total pore space and more water-filled pores than medium-textured soils, whereas coarse-textured soils will have more space occupied by solid matter and less total pore space and less water-filled pore space than either fine-textured soils or medium-textured soils.

Sand particles are fragments of rocks and minerals. Quartz (silicon dioxide, SiO2) grains are the dominant fragments of sand. Quartz provides no essential elements to plants.

Fragments of other primary minerals (such as feldspars and micas) that make up sand may contain essential elements, but these minerals are so insoluble that their capacity to supply nutrients is insignificant in relation to the total requirements of nutrients by plants.

Sandy soils, therefore, have a low capacity to supply plant nutrients from the inherent inorganic materials.

Silt is essentially microfine sand.

Silt is not much more active chemically than sand and has little more capacity to supply plant nutrients. Silt does not impart a good physical structure to soil either.

Silty soils have tendencies to crust. Silts do not form aggregates unless the particles are coated with clay. Large particles in the clay fraction may be fragments of quartz and other minerals, but the major fraction of clays in temperate regions is the secondary silicate minerals.

These minerals are formed from the recrystallization of the decomposed products of primary minerals. Silicate clays generally are laminated, being made up of mica-like layers of plates (Figure 6).

The particles vary greatly in sizes and shapes and in composition. The different kinds of clays have differing effects on the physical and chemical properties of soils. Some generalizations of the effects of clays are possible.

Figure 6. Exchangeable and nonexchangeable cations helt to negatively charged (A) micaceous layer silicate clays and (B) kaolinitic layer silicate clay.

The platelike structures of some micaceous clay have the capacity to swell and shrink and have considerable internal and external surface areas.

These clays are called expanding-lattice clays, in contrast to clays which do not have expanding lattices, such as kaolinite which do not swell when wetted or shrink when dried.

The expanding types of clays dominate in mineral soils.

Positively charged ions of nutrients such as calcium, magnesium, potassium, and ammonium may enter into the internal spaces between the plates and become trapped or fixed.

Fixation increases the nutrient content of clays. Fixed ions are held rather strongly in the plates and are not removed ordinarily by exchange with other ions.

Fixed ions are often referred to as nonexchangeable cations.

Clays have negative charges.

These charges come about generally from two sources.

One source of charge is from the unbalanced charges at the edges of the broken particles.

The more the particles are broken, the more the negative charges. Another source of charge is the substitution of one ion for another in the structure of the plates, for example, the substitution of Mg+2 for Al+3, which results in an unbalanced negative charge.

The unbalanced negative charges from the exposure of edges and ionic substitution attract positively charges ions in a loosely held swarm around the clay particle.

These ions that are attracted by the electronegativity of clays are called exchangeable cations.

The capacity of colloids to hold exchangeable cations is called cation exchange capacity.

Among the plant nutrients, these cations are calcium, magnesium, potassium, and ammonium.

Generally, clay soils are more fertile with respect to capacity to supply nutrients because of the exchangeable and nonexchangeable cations that are held on the surface or in the framework of the clays.

Expanding-lattice clays have greater cation exchange capacity than nonexpanding clays.

Sands and silts as such because of their chemical and physical constitution do not have much capacity to hold cations in exchangeable or nonexchangeable forms.

In humid, temperate regions, sandy soils are likely to be infertile because they do not contain minerals that will supply nutrients readily, they have only limited capacity to hold nutrients in exchangeable form, and they lose nutrients rapidly with water that drains through the soil.

Application of fertilizers to maintain an adequate supply of nutrients will have to be much more frequently on sandy soils than on loamy or clayey soils.

Organic matter

Most soils range from 1 to 6% organic matter by weight in the topsoil.

A soil composed of greater than 20% organic matter is defined as an organic soil.

Soils of lowland, wet area may have 80% or more organic matter. These soils are called peats and mucks.

The organic matter of soils is living and dead matter.

Living matter includes microorganisms of various kinds, but roots of living plants and macroorganisms are not included.

Soil organic matter includes any fresh and partly decomposed residues of plants and animals.

Humus is a specific form of soil organic matter.

It is a relatively stable organic material, dark in color, and remains after the major portion of the original organic matter has decayed. Humus is an amorphous, not crystalline, colloidal material, which has a very high adsorptive capacity. One cannot add humus directly to the soil. It must be formed in the soil from additions of other forms of organic matter.

Organic matter

The cation exchange capacity and water-holding capacity of humus is several times, perhaps three or more, that of colloidal clays.

Soil organic matter is a major source of essential elements for plants. It contains some of every plant nutrient. Almost all of the nitrogen in the soil is in the organic matter.

About a third to a half of the phosphorus in the top horizon of soil is in the organic matter. Organic matter is an important source of sulfur.

These nutrients become available to plants after the organic matter has decomposed.

Decomposition of soil organic matter, including humus, is accomplished by the microbiological and animal life in the soil.

Essential elements are held in exchangeable sites of humus or in other complexes with organic matter and may be released by exchange with other ions or by decomposition of the organic matter.

The decomposition of organic matter acidifies the soil and facilitates the release of nutrients from inorganic soil minerals.

Another function of soil organic matter is in the binding of mineral particles into aggregates.

Formation of aggregates improves the structure of soils, particularly those that have a substantial clay content

Additions of sand to improve the structure of clay is not a productive process.

Mixing of sand and clay often leads to a concrete-like material of poor structure.

Organic matter

Also, building the humus content of soils increases their abilities to retain nutrients and water (Table 5).

A good organic matter content is essential to the maintenance of fertility in sandy soils.

Fresh organic matter added to the soil usually decomposes away in a year or two.

The rate of decomposition depends on the physical, chemical, and biological properties of the soil and of the organic matter and on the amount of material added.

Decomposition of organic matter is a biological, largely microbiological, process.

The better that a soil is aerated, the faster organic matter will decompose. For this reason, organic matter decomposes faster in sandy soils than in loamy or clayey soils, which are not as well aerated as sandy soils.

Sandy soils generally have lower organic matter contents than loamy or clayey soils.

Maintenance of organic matter in sandy soils requires more frequent additions of fresh organic matter than loamy or clayey soils.

Table 5. Effects of organic matter on soil physical properties.

Response to Increase in Soil Organic Matter

Bulk density -------------------------------------------------------------------> Decrease

Moisture retention -------------------------------------------------------------> Increase

Field capacity -----------------------------------------------------------------> Increase Wilting percentage ------------------------------------------------------------> Increase

Available water ---------------------------------------------------------------> Increase

Cation exchange capacity -----------------------------------------------------> Increase

Aggregation (structure) -------------------------------------------------------> Increase

Infiltration rate ----------------------------------------------------------------> Increase Leaching ---------------------------------------------------------------------->Decrease

Table 5. Effects of organic matter on soil physical properties.

The composition of the organic matter also governs its rate of decomposition.

The most important factor of composition is the relative amount of carbon and nitrogen in the organic matter, the carbon to nitrogen ratio (C:N).

In general, but with many exceptions, materials with wide ratios are slower to decompose than those with narrow ratios (Table 6).

All of these materials are of biological origin, and although a definite C:N value is given, the actual value will vary depending on the origin and handling of the specific materials.

Table 6. Approximate carbon to nitrogen ratios of some common organic substances

---------Material--------------- ---------------C:N---------------- ------------Material------------ ---------------C:N----------------

Newspaper 1,000 Farm manures, large animalsz 30

Sawdust, wood chips 500 Green plants 20

Straw (small grain) 180 Poultry manure 17

Pine needles, dead 180 Soil 12

Broad leaves, dead 180 Seed meals 8

Peatmoss 100 Soil microorganisms 7

Composts 30 Dried blood 5

zLarge farm animals no bedding (cattle, hogs, horses, sheep

Fresh sawdust may require three or more years to decay in the soil.

Paper, which has a wider C:N, may decay more quickly because the cellulose of paper will be more prone to decay than the complex mixture of carbohydrates, cellulose, lignin, and other substances of sawdust.

Conversely, peatmoss decomposes slowly in the soil, because the easily decomposable substances have been rotted away leaving behind skeletal material, mostly lignin, which is resistant to decay.

Manures and composts will require two or more years to break down in the soil.

A green manure crop decomposes about 75% in the first year. Seed meals and dried blood are fertilizer-grade organic matter and will be decomposed almost entirely in one year.

Decomposition of organic matter may release nitrogen that is in the organic matter into a form that plants can use. This process is called nitrogen mineralization (Figure 7).

Figure 7. Partial nitrogen cycle in soils, illustrating mineralization of soil organic matter (or organic fertilizers), oxidation of ammonium to nitrate (nitrification), losses of ammonia by volatilization, losses of gaseous nitrogen products by denitrification of nitrate, absorption of plant nutrients, and microbial immobilization.

The forms of nitrogen that are available to plants are ammonium (NH4+) and nitrate (NO3

-).

In well-aerated soils, nitrate is the dominant form, because nitrification proceeds rapidly in these soils.

Decomposition takes place over a period of time, and the organic matter changes chemically and physically as decomposition occurs.

Different kinds of organisms participate in the decomposition as the organic matter changes with time.

The first group of microorganisms live on the easily decomposable sugars, starches, and cellulose.

As that energy supply becomes depleted, these organisms die and become part of the organic matter, effectively narrowing its C:N ratio.

Another group of fungi and bacteria then will dominate in the decomposition, using the dead fungal tissue and the remaining organic residue.

Bacterial decomposition narrows the C:N ratio toward a limiting value of 4:1 to 9:1, but the C:N ratio seldom reaches this value.

The organic matter will stabilize in humus at a C:N ratio that is characteristic of the particular soil, but a ratio of 10:1 to 15:1 is common in soils.

If environmental factors are constant, humus is built and decomposed at about the same rate.

illage accelerates the rate of decomposition by loosening the soil thereby increasing aeration.

Rate of decomposition of humus declines with increasing time of cultivation, eventually reaching a constant rate.

In a newly cultivated field in temperate regions, the decomposition of humus may be about 4% in the first year.

This rate falls 1 to 2% per year in fields under prolonged cultivation.

A soil that has not been cultivated for a long period of time may have 100,000 pounds of humus per acre (tilled zone of the soil), which holds about 5,000 pounds of nitrogen--a ratio of total humus to nitrogen of about 20 to 1 is common (this is not the C:N ratio).

In the first year, the release of nitrogen will be about 200 pounds per acre (4% of 5,000 lb N).

This amount of nitrogen will support a productive crop, which will be able to recover about half of the released nitrogen.

After 30 or 40 years of cultivation with no deliberate effort made to maintain soil organic matter, the humus in this soil will fall to 60,000 or few pounds per acre (3,000 lb N/acre), and the decomposition will be at the slower rates.

Now the release of nitrogen from humus will be only 30 to 60 pounds per acre (1% or 2% of 3,000 lb N).

If the crop recovers half of the nitrogen (15 to 30 lb/acre), insufficient nitrogen will be available to support a productive crop, and nitrogen from fertilizers will be necessary.

Humus in a soil can be maintained by large additions of manures or composts or by a green manure crop in a rotation.

The management of green manures, farm manures, and composts is discussed in chapters that follow this one.

On small plots of land, the use of manures and composts is better, for the green manure crop may take the land out of production for a year.

On large acreages, green manures are better, for the quantities of manures and composts and the labor required to obtain and spread them may exceed the supplies of material and labor.

Materials with a wide C:N ratio will build up the humus content of soil faster than ones with a narrow C:N ratio due to differences in rate of decomposition of the organic matter and the amount of stable organic matter that remains after the easily decomposable materials are gone.

Most of the nitrogen in the added organic matter becomes part of the bodies of the microorganisms that are living on the organic matter.

The carbon goes into the microbial bodies, but a lot of it is lost to the air as carbon dioxide.

Thus, with time the C:N ratio of the organic matter narrows.

If the C:N ratio is very wide as in sawdust, straw, dead leaves and garden residues, and manures with bedding, the organic matter may not supply enough nitrogen to meet the needs of the microorganisms.

In this case, the soil is depleted of available nitrogen by the microorganisms.

In other words, the deficiency of nitrogen in the organic matter is made up by the microorganisms using nitrogen from the soil.

This process is called nitrogen immobilization.

Microorganisms are better competitors for soil nitrogen than plant roots; hence, nitrogen deficiency will develop on plants grown in soils to which organic matter with a wide C:N ratio is added.

The solutions to nitrogen immobilization are to delay planting of crops until the C:N ratio has narrowed enough so that nitrogen is released by mineralization or to add nitrogen as fertilizer to compensate for the nitrogen that is consumed by the microorganisms.

Generally, the amount that needs to be added is one pound of nitrogen for each 100 pounds of organic matter with a wide C:N ratio.

These applications will need to be continued until the organic matter had decomposed sufficiently to lower the C:N ratio to 35:1 or lower.

Generally, this amount of time is 3 years in the case of sawdust, 2 years in the case of straws and coarse dead residues, and 1 year in the case of manures with bedding.

Trying to accelerate the rate of decomposition of highly carbonaceous organic matter by additions of nitrogen in excess of 1 pound per 100 pounds per year usually leads to problems of salinity or ammonium toxicity.

A C:N ratio of 35:1 is about the breaking point between mineralization and immobilization.

Materials well below this value will begin to release nitrogen soon after their addition to the soil, whereas materials well above this value will immobilize nitrogen until the C:N ratio is narrowed to 35:1.

Materials that are near a C:N ratio of 35:1 may have a short period in which nitrogen is immobilized before it is released.

The major purpose of composting is to narrow the C:N of organic matter to a value at which immobilization will be short lived after the addition of the organic matter to the soil.