What Does Organic Matter Do In Soil?

By Eddie Funderburg

Of all the components of soil, organic matter is probably the most important and most misunderstood. Organic matter serves as a reservoir of nutrients and water in the soil, aids in reducing compaction and surface crusting, and increases water infiltration into the soil. Yet it's often ignored and neglected. Let's examine the contributions of soil organic matter and talk about how to maintain or increase it.

What is Organic Matter?
Many times we think of organic matter as the plant and animal residues we incorporate into the soil. We see a pile of leaves, manure, or plant parts and think, "Wow! I'm adding a lot of organic matter to the soil." This stuff is actually organic material, not organic matter.

What's the difference between organic material and organic matter? Organic material is anything that was alive and is now in or on the soil. For it to become organic matter, it must be decomposed into humus. Humus is organic material that has been converted by microorganisms to a resistant state of decomposition. Organic material is unstable in the soil, changing form and mass readily as it decomposes. As much as 90 percent of it disappears quickly because of decomposition.

Organic matter is stable in the soil. It has been decomposed until it is resistant to further decomposition. Usually, only about 5 percent of it mineralizes yearly. That rate increases if temperature, oxygen, and moisture conditions become favorable for decomposition, which often occurs with excessive tillage. It is the stable organic matter that is analyzed in the soil test.

How Much Organic Matter Is in the Soil?
An acre of soil measured to a depth of 6 inches weighs approximately 2,000,000 pounds, which means that 1 percent organic matter in the soil would weigh about 20,000 pounds per acre. Remember that it takes at least 10 pounds of organic material to decompose to 1 pound of organic matter, so it takes at least 200,000 pounds (100 tons) of organic material applied or returned to the soil to add 1 percent stable organic matter under favorable conditions.

In soils that formed under prairie vegetation, organic-matter levels are generally comparatively high because organic material was supplied from both the top growth and the roots. We don't usually think of roots as supplying organic material, but a study in the Upper Great Plains showed that a mixed prairie had an above-ground (shoot) yield of 1.4 tons of organic material per acre, while the root yield was about 4 tons per acre. The plants were producing roots that were more than twice the weight of the shoots.

Soils that have developed under forest vegetation usually have comparably low organic-matter levels. There are at least two reasons for these levels:

1. trees produce a much smaller root mass per acre than grass plants, and
2. trees do not die back and decompose every year. Instead, much of the organic material in a forest is tied up in the tree instead of being returned to the soil.

Soils that formed under prairie vegetation usually have native organic matter levels at least twice as high as those formed under forest vegetation.

What Are the Benefits of Organic Matter?

• Nutrient Supply
  Organic matter is a reservoir of nutrients that can be released to the soil. Each percent of organic matter in the soil releases 20 to 30 pounds of nitrogen, 4.5 to 6.6 pounds of P₂O₅, and 2 to 3 pounds of sulfur per year. The nutrient release occurs predominantly in the spring and summer, so summer crops benefit more from organic-matter mineralization than winter crops.
• Water-Holding Capacity
  Organic matter behaves somewhat like a sponge, with the ability to absorb and hold up to 90 percent of its weight in water. A great advantage of the water-holding capacity of organic matter is that the matter will release most of the water that it absorbs to plants. In contrast, clay holds great quantities of water, but much of it is unavailable to plants.
• Soil Structure Aggregation
  Organic matter causes soil to clump and form soil aggregates, which improves soil structure. With better soil structure, permeability (infiltration of water through the soil) improves, in turn improving the soil's ability to take up and hold water.
• Erosion Prevention
  This property of organic matter is not widely known. Data used in the universal soil loss equation indicate that increasing soil organic matter from 1 to 3 percent can reduce erosion 20 to 33 percent because of increased water infiltration and stable soil aggregate formation caused by organic matter.

How Can I Maintain or Improve Soil Organic Matter Levels?
Building soil organic matter is a long-term process but can be beneficial. Here are a few ways to do it.

• Reduce or Eliminate Tillage
  Tillage improves the aeration of the soil and causes a flush of microbial action that speeds up the decomposition of organic matter. Tillage also often increases erosion. No-till practices can help build organic matter.
• Reduce Erosion
  Most soil organic matter is in the topsoil. When soil erodes, organic matter goes with it. Saving soil and soil organic matter go hand in hand.
• Soil-Test and Fertilize Properly
  You may not have considered this one. Proper fertilization encourages growth of plants, which increases root growth. Increased root growth can help build or maintain soil organic matter, even if you are removing much of the top growth.
• Cover Crops
  Growing cover crops can help build or maintain soil organic matter. However, best results are achieved if growing cover crops is combined with tillage reduction and erosion control measures.

A good supply of soil organic matter is beneficial in crop or forage production. Consider the benefits of this valuable resource and how you can manage your operation to build, or at least maintain, the organic matter in your soil.

References
Barber, S. A. Soil Nutrient Bioavailability: A Mechanistic Approach. New York: Wiley, 1984.
Brady, N. C. The Nature and Properties of Soils. New York: Macmillan Publishing Co., 1974.
Plaster, E. J. Soil Science and Management. 3rd ed. Albany: Delmar Publishers, 1996.
Tisdale, S. L. and W. L. Nelson. Soil Fertility and Fertilizers. 3rd ed. New York: Macmillan, 1975.
Source: Noble Foundation

Soil Basics

Soils are vital, fragile, finite natural resources that are essential for the sustained production of food and fiber. Soils, however, are subject to degradation and erosion when mismanaged. Between 1950 and 1993, grain area per person worldwide decreased from 0.58 to 0.33 acres (0.23 to 0.13 hectares). As human populations increase, soil resources are used more intensively, with increasing probability that many practices will lead to deterioration of the resource. Competition between agricultural uses and non-agricultural uses of land, such as support of structures, disposal of wastes, and growing plants for recreational and aesthetic purposes will increase.
In ecosystems, soils, water, air, plants, animals and people have interdependent relationships. Soils are dynamic, living systems whose productivity, through management that often includes additions of nutrients, organic materials and water, can be sustained indefinitely. Soils exhibit unique physical and chemical sorptive qualities and dynamics reflective of their inorganic and organic composition. Cycling of carbon, nitrogen and other nutrient elements in nature involves transformations in soils.
Great diversity occurs among soils, sometimes in very small geographical areas, such as building lots in urban areas. The rise and fall of civilizations sometimes has been related to the wise use and misuse of natural resources including soil and water.
Definitions of soil vary, but one view is that the unconsolidated material at the earth's surface becomes soil when biological activity results in a noticeable accumulation of organic matter as revealed by a dark surface color. Soils that form in loose, fine-grained material weathered from the rock immediately below them are called residual soils. More frequently, soils are formed in materials that have been transported away from the source rock. Examples of such materials are alluvial materials, which have been deposited from running water as in flood plains or deltas; lacustrine material which is deposited in lakes; glacial material which has been moved by ice, and aeolian material which has been transported and deposited by wind.
Many of the intensively used soils in the world are formed in transported materials. Their usefulness often is associated with topography, or with physical and chemical properties inherited directly from the transported material.

Soils are porous natural bodies composed of inorganic and organic matter. They form by interaction of the earth's crust with atmospheric and biological influences. They are dynamic bodies having properties that reflect the integrated effects of climate (atmosphere) and biotic activity (microorganisms, insects, worms, burrowing animals, plants, etc.) on the unconsolidated remnants of rock at the earth's surface (parent material). These effects are modified by the topography of the landscape and of course continue to take place with the passage of time. Soils formed in parent materials over decades, centuries, or millennia may be lost due to accelerated erosion over a period of years or a few decades.
The exposed surfaces of soils are a common sight on almost any landscape not dominated by rock. The surface of a soil reveals very little about the depth of the soil or its subsurface characteristics. A vertical cross-sectional view of a soil is called a soil profile. Each of the horizontal layers which can be seen in the vertical section is called a soil horizon. Horizons are formed because of the integrated effects of climate and biosphere change and generally become less pronounced with depth. The depth of soils, usually 0.6 to 1.8m, is determined by the depth to which the mantle material has been altered in a significant way. That part of the three-dimensional soil body in which the effects of climate and biological activity are most pronounced is the soil solum. In succeeding pages the nature and properties of soils, their management, and environmental public policy issues will be discussed. 

The proportions of these components may vary between horizons in a soil or between similar horizons in different soils. The ratio of soil water to soil air depends upon whether the soil is wet or dry. The mineral matter, composed of particles ranging in size from the submicroscopic to gravel or even rocks in some cases, accounts for the bulk of the dry weight of the soil and occupies some 40 to 60% of the soil volume. Organic matter, derived from the waste products and remains of plants and animals, occurs in largest amounts in the surface soil, but even here seldom accounts for more than 10% of the dry weight of the soil. Soils are very porous bodies. Some 40 to 60% of the volume is interparticle space, or pore space. The pores, highly irregular in shape and size but almost all interconnected by passages, contain soil water, soil air, or both of these. The soil water reacts chemically with the soil solids and usually contains dissolved substances and perhaps suspended particles. The soil air approaches equilibrium with atmospheric air through movement of individual gases. Bedrock is the ultimate source of the inorganic component in soils. When rock is exposed at the surface of the earth's crust, it is broken down into smaller and smaller fragments by physical forces. The fragments may be altered or decomposed by chemical reaction of mineral matter with water and air. Hundreds, thousands, or even millions of years may be required for the weathering or physical and chemical alteration of rock to produce the ultimate end products in soils. Once particles reach a sufficiently small size they can be moved by wind, water or ice when exposed at the surface. It is common, therefore, for small particles to be moved from one location to another. A single particle might occur in several different soils over a period of 100,000 years. Eventually, these particles or their decomposition products reach the ocean where they are redeposited as marine sediments. The silicate group of minerals is dominant in soil systems. The terms, clay mineral and layer silicate, are used almost interchangeably. The dominant chemical elements in silicate clays are oxygen, silicon, aluminum and iron. Important constituents in relatively small amounts are potassium, calcium, magnesium and sodium. Other elements occur in very small amounts in silicates. Carbonates, oxides, phosphates and sulfates are other mineral groups that occur commonly in parent materials. Soils are porous and open bodies, yet they retain water. They contain mineral particles of many shapes and sizes and organic material which is colloidal (particles so small they remain suspended in water) in character. The solid particles lie in contact one with the other, but they are seldom packed as closely together as possible. Texture The size distribution of primary mineral particles, called soil texture, has a strong influence on the properties of a soil. Particles larger than 2 mm in diameter are considered inert. Little attention is paid to them unless they are boulders that interfere with manipulation of the surface soil. Particles smaller than 2 mm in diameter are divided into three broad categories based on size. Particles of 2 to 0.05 mm diameter are called sand; those of 0.05 to 0.002 mm diameter are silt; and the <0.002 mm particles are clay. The texture of soils is usually expressed in terms of the percentages of sand, silt, and clay. To avoid quoting exact percentages, 12 textural classes have been defined. Each class, named to identify the size separate or separates having the dominant impact on properties, includes a range in size distribution that is consistent with a rather narrow range in soil behavior. The loam textural class contains soils whose properties are controlled equally by clay, silt and sand separates. Such soils tend to exhibit good balance between large and small pores; thus, movement of water, air and roots is easy and water retention is adequate. Soil texture, a stable and an easily determined soil characteristic, can be estimated by feeling and manipulating a moist sample, or it can be determined accurately by laboratory analysis. Soil horizons are sometimes separated on the basis of differences in texture. Structure Anyone who has ever made a mud ball knows that soil particles have a tendency to stick together. Attempts to make mud balls out of pure sand can be frustrating experiences because sand particles do not cohere (stick together) as do the finer clay particles. The nature of the arrangement of primary particles into naturally formed secondary particles, called aggregates, is soil structure. A sandy soil may be structureless because each sand grain behaves independently of all others. A compacted clay soil may be structureless because the particles are clumped together in huge massive chunks. In between these extremes, there is the granular structure of surface soils and the blocky structure of subsoils. In some cases subsoils may have platy or columnar types of structure. Structure may be further described in terms of the size and stability of aggregates. Structural class is based on aggregate size, while structural grade is based on aggregate strength. Soil horizons can be differentiated on the basis of structural type, class, or grade. What causes aggregates to form and what holds them together? Clay particles cohere to each other and adhere to larger particles under the conditions that prevail in most soils. Wetting and drying, freezing and thawing, root and animal activity, and mechanical agitation are involved in the rearranging of particles in soils--including destruction of some aggregates and the bringing together of particles into new aggregate groupings. Organic materials, especially microbial cells and waste products, act to cement aggregates and thus to increase their strength. On the other hand, aggregates may be destroyed by poor tillage practices, compaction, and depletion of soil organic matter. The structure of a soil, therefore, is not stable in the sense that the texture of a soil is stable. Good structure, particularly in fine textured soils, increases total porosity because large pores occur between aggregates, allowing penetration of roots and movement of water and air. Consistence Consistence is a description of a soil's physical condition at various moisture contents as evidenced by the behavior of the soil to mechanical stress or manipulation. Descriptive adjectives such as hard, loose, friable, firm, plastic, and sticky are used for consistence. Soil consistence is of fundamental importance to the engineer who must move the material or compact it efficiently. The consistence of a soil is determined to a large extent by the texture of the soil, but is related also to other properties such as content of organic matter and type of clay minerals. Color The color of objects, including soils, can be determined by minor components. Generally, moist soils are darker than dry ones and the organic component also makes soils darker. Thus, surface soils tend to be darker than subsoils. Red, yellow and gray hues of subsoils reflect the oxidation and hydration states or iron oxides, which are reflective of predominant aeration and drainage characteristics in subsoil. Red and yellow hues are indicative of good drainage and aeration, critical for activity of aerobic organisms in soils. Mottled zones, splotches of one or more colors in a matrix of different color, often are indicative of a transition between well drained, aerated zones and poorly drained, poorly aerated ones. Gray hues indicate poor aeration. Soil color charts have been developed for the quantitative evaluation of colors.  Major Elements Eight chemical elements comprise the majority of the mineral matter in soils. Of these eight elements, oxygen, a negatively-charged ion (anion) in crystal structures, is the most prevalent on both a weight and volume basis. The next most common elements, all positively-charged ions (cations), in decreasing order are silicon, aluminum, iron, magnesium, calcium, sodium, and potassium. Ions of these elements combine in various ratios to form different minerals. More than eighty other elements also occur in soils and the earth's crust, but in much smaller quantities. Soils are chemically different from the rocks and minerals from which they are formed in that soils contain less of the water soluble weathering products, calcium, magnesium, sodium, and potassium, and more of the relatively insoluble elements such as iron and aluminum. Old, highly weathered soils normally have high concentrations of aluminum and iron oxides. The organic fraction of a soil, although usually representing much less than 10% of the soil mass by weight, has a great influence on soil chemical properties. Soil organic matter is composed chiefly of carbon, hydrogen, oxygen, nitrogen and smaller quantities of sulfur and other elements. The organic fraction serves as a reservoir for the plant essential nutrients, nitrogen, phosphorus, and sulfur, increases soil water holding and cation exchange capacities, and enhances soil aggregation and structure. The most chemically active fraction of soils consists of colloidal clays and organic matter. Colloidal particles are so small (< 0.0002 mm) that they remain suspended in water and exhibit a very large surface area per unit weight. These materials also generally exhibit net negative charge and high adsorptive capacity. Several different silicate clay minerals exist in soils, but all have a layered structure. Montmorillonite, vermiculite, and micaceous clays are examples of 2:1 clays, while kaolinite is a 1:1 clay mineral. Clays having a layer of aluminum oxide (octahedral sheet) sandwiched between two layers of silicon oxide (tetrahedral sheets) are called 2:1 clays. Clays having one tetrahedral sheet bonded to one octahedral sheet are termed 1:1 clays. Cation Exchange Silicate clays and organic matter typically possess net negative charge because of cation substitutions in the crystalline structures of clay and the loss of hydrogen cations from functional groups of organic matter. Positively-charged cations are attracted to these negatively-charged particles, just as opposite poles of magnets attract one another. Cation exchange is the ability of soil clays and organic matter to adsorb and exchange cations with those in soil solution (water in soil pore space). A dynamic equilibrium exists between adsorbed cations and those in soil solution. Cation adsorption is reversible if other cations in soil solution are sufficiently concentrated to displace those attracted to the negative charge on clay and organic matter surfaces. The quantity of cation exchange is measured per unit of soil weight and is termed cation exchange capacity. Organic colloids exhibit much greater cation exchange capacity than silicate clays. Various clays also exhibit different exchange capacities. Thus, cation exchange capacity of soils is dependent upon both organic matter content and content and type of silicate clays. Cation exchange capacity is an important phenomenon for two reasons: exchangeable cations such as calcium, magnesium, and potassium are readily available for plant uptake and cations adsorbed to exchange sites are more resistant to leaching, or downward movement in soils with water. Movement of cations below the rooting depth of plants is associated with weathering of soils. Greater cation exchange capacities help decrease these losses. Pesticides or organics with positively charged functional groups are also attracted to cation exchange sites and may be removed from the soil solution, making them less subject to loss and potential pollution. Calcium (Ca++) is normally the predominant exchangeable cation in soils, even in acid, weathered soils. In highly weathered soils, such as oxisols, aluminum (Al+3) may become the dominant exchangeable cation. The energy of retention of cations on negatively charged exchange sites varies with the particular cation. The order of retention is: aluminum > calcium > magnesium > potassium > sodium > hydrogen. Cations with increasing positive charge and decreasing hydrated size are most tightly held. Calcium ions, for example, can rather easily replace sodium ions from exchange sites. This difference in replaceability is the basis for the application of gypsum (CaSO4) to reclaim sodic soils (those with > 15% of the cation exchange capacity occupied by sodium ions). Sodic soils exhibit poor structural characteristics and low infiltration of water. The cations of calcium, magnesium, potassium, and sodium produce an alkaline reaction in water and are termed bases or basic cations. Aluminum and hydrogen ions produce acidity in water and are called acidic cations. The percentage of the cation exchange capacity occupied by basic cations is called percent base saturation. The greater the percent base saturation, the higher the soil pH. Soil pH Soil pH is probably the most commonly measured soil chemical property and is also one of the more informative. Like the temperature of the human body, soil pH implies certain characteristics that might be associated with a soil. Since pH (the negative log of the hydrogen ion activity in solution) is an inverse, or negative, function, soil pH decreases as hydrogen ion, or acidity, increases in soil solution. Soil pH increases as acidity decreases. A soil pH of 7 is considered neutral. Soil pH values greater than 7 signify alkaline conditions, whereas those with values less than 7 indicate acidic conditions. Soil pH typically ranges from 4 to 8.5, but can be as low as 2 in materials associated with pyrite oxidation and acid mine drainage. In comparison, the pH of a typical cola soft drink is about 3. Soil pH has a profound influence on plant growth. Soil pH affects the quantity, activity, and types of microorganisms in soils which in turn influence decomposition of crop residues, manures, sludges and other organics. It also affects other nutrient transformations and the solubility, or plant availability, of many plant essential nutrients. Phosphorus, for example, is most available in slightly acid to slightly alkaline soils, while all essential micronutrients, except molybdenum, become more available with decreasing pH. Aluminum, manganese, and even iron can become sufficiently soluble at pH < 5.5 to become toxic to plants. Bacteria which are important mediators of numerous nutrient transformation mechanisms in soils generally tend to be most active in slightly acid to alkaline conditions.  Soils, like other naturally occurring things, come in great variety and exhibit great ranges of properties. Using measurable and observable properties, such as the kind and arrangement of soil horizons, soils can be characterized and named. The soil series is the lowest category within soil taxonomy (classification system). All soils within a single series have uniform differentiating characteristics and arrangement of horizons. This does not mean that all soils within a series are identical; it does mean that they have the same horizonation, but the horizons may be of different thickness, color, structure, etc. within prescribed limits. Some 15,000 soil series have been described and named in the United States. Most series names are taken from the name of a town, city, county, river or other constructed or natural feature near the location where the soil is first described and named. All of the soils within a series will have developed in the same kind of parent material with comparable drainage characteristics and will be of similar age. The effects of climate and biological activity will have been very similar. Consequently, the soils within a series exhibit like properties and respond in like fashion to usage or manipulation. Higher levels of classification are the family, subgroup, great group, suborder and order. All these categories are given generic names which convey as much information as possible about the soil series to be classified within the group. Eleven soil orders form the highest level of classification. Soils classified within each order show only small differences in the kinds and relative strengths of processes that tend to develop soil horizons. Considerable effort by soil scientists has been expended, and continues to be expended, in surveying soil resources. These surveys, when done in a detailed manner, require a soil scientist to traverse the landscape at frequent intervals, stopping periodically to auger or dig into the soil. The soil surveyor plots the occurrence of soils on a map which is subsequently formalized and eventually published in a soil survey report which includes not only the soil maps for a given area, usually a county, but all types of information about the county as well as descriptions of the properties of the soils in the area, their present and potential uses, and potential problems associated with the utilization of the soils for both agricultural and engineering purposes. Such reports are very expensive in terms of labor and money, but they contain information that is of great value to those who utilize soils for any purpose. Unfortunately, many who could use the information in soil survey reports to great advantage do not know that the reports are available. With increased emphasis on planning, which includes land use, on county, state and regional bases, more utilization of soil survey information is being made and even more detailed and more modern soil surveys are being urgently requested in many areas. Much of this information is being digitized for electronic transmission.