Maintaining Organic Matter

W. V. Bartholomew.

Soil organic matter is dynamic material. It changes continually through further decomposition, but it maintains a degree of stability in quantity and in quality through the additions of new raw materials.

Organic matter is a temporary product a stage in a natural cycle of elements. Each increment remains in the Soil while it passes through the several Slow biological oxidation changes that eventually reduce it to carbon dioxide, water, and mineral elements. As it passes through the cycle, it is replaced by organic matter formed from fresh residues.

Organic matter is formed in the biological decomposition of plant and animal residues. In the decomposition process, some of the plant substances are converted rapidly to carbon dioxide, water, and mineral elements (mineralization), and other substances may be only chemically altered at first.

The microbiological activity is high when fresh plant residues begin to decay. As the micro-organisms consume the more easily decomposable materials, the level of activity gets less and less. When only the more resistant plant substances remain along with the series of new organic materials synthesized by the micro-organisms, the microbial activity becomes slow akin to a smoldering fire and is the cause of constant loss of organic matter from soil.

The amount of organic matter in soil at any time hinges on the speed of the microbiological activity and the amount of fresh residue material that is added each year. The principles that regulate microbiological decomposition which affect mechanical losses of soil and determine the amount and kind of residues returned to the soil therefore are the principles that govern the level of organic matter in soil.

A number of things affect the speed of activity of soil microbes. We can control some of them. Others depend on the weather. Some are determined by early geological processes and the kind of plant cover that prevailed before man became interested in soil organic matter. Among the factors are temperature, moisture, aeration, acidity, supply of plant nutrients, tillage, and the kind and the amount of crop residues and manures returned to the soil. Cropping systems and soil management exert strong influences on most of these factors.

Microbes are most active in a moist soil. Microbial activity is depressed when a soil is extremely wet or dry. Air is excluded from the soil pore spaces in a wet soil, and the lack of air slows decomposition. Although microbial activity is less when moisture is low, many microbes remain rather active when soil moisture is below the wilting point for crop plants. Decomposition, in fact, proceeds about half as fast at the wilting point for plants as it does when the soil is at field-moisture capacity.

Soil temperature is determined generally by the temperature of the air, but it can be modified by man's use and manipulation of the soil. Operations that influence moisture also influence temperature. Dry soil has a low heat capacity and warms up faster than a wet soil.

Soil temperatures are generally cooler in daytime and warmer at night when the soil is covered by a growing crop. Similar modifications occur when crop residues are on the surface. In fact, under mulch culture in spring, increases in temperature are noticeably lower than those under clean cultivation.

The optimum temperatures for most soil microbes are between 75 and 95 F. The soil temperature seldom goes higher than that unless the soils are bare and dry, but in many areas temperatures often fall far below the optimum. The activity of soil microbes drops as the soil temperatures drop, but the activity of most microbes does not stop completely until the soil freezes. General decomposition of soil organic matter proceeds about two-thirds as fast at 75 as at 90 , and about one-third as fast at 60 as at 90 .

Aeration remains generally optimum for microbiological processes in soils that are not saturated with water.

When the soil pores are filled with water, as when drainage is poor or after a soaking rain, aeration the exposure to air limits the general rate of decomposition in that the slower acting anaerobic organisms are stimulated and most of the fast acting aerobic organisms are eliminated. Artificial drainage of wet soils greatly improves aeration and thus tends to speed up microbial activity.

SOIL REACTION may influence microbial activity in two ways. Soil organic matter is slightly less available as food for micro-organisms when the soil is acid than when it is neutral or alkaline. Further, because organisms differ in optimum pH for growth, soil reaction markedly influences the kinds of microbes that predominate in the flora and fauna in the soil. The general range in soil reaction, pH 5 to pH 8, however, has only minor effects on the overall rate of decomposition. When one organism is repressed, another generally arises to take its place.

The plant nutrients, except nitrogen, probably have only minor direct effects on the decomposition of plant residues and the activities of soil organisms. Enough calcium, magnesium, and potassium generally is present in the organic matter, plant residues, and manures to supply the needs of microbes. Phosphorus also is normally sufficient for microbiological activity, but minor elements may not be in some soils.

Nitrogen often influences microbiological activity. Too little nitrogen may occur in many crop residues to provide for the needs of the microbes during the early stages of decomposition. If additional nitrogen is not supplied from the soil or from fertilizer, the speed of decay of fresh plant residues may be lessened.

High rates of loss of soil organic matter have been associated with intensive tillage, particularly with the production of row crops. Stirring and disturbance of the soil seem to stimulate the microbiological population, but the reasons therefor are not known.

The kind of plant or animal residue from which soil organic matter is formed also influences the activity of the microbes, which depend on it as food. Some plant materials resist decomposition. Others decompose quickly. Decomposition of young, succulent grasses and legumes may be almost complete in one season, but the roots of some grasses and sedges may undergo little decomposition in many years.

Decomposition of the soil organic matter can be increased by the addition of easily decomposable substances like green manures. In fact, in soils that have moderate or high amounts of organic matter, green manure may so stimulate the metabolism of soil organisms that the loss of organic matter may more than offset that added from the fresh residues.

Every crop that grows makes a contribution in residues and therefore helps to maintain the soil organic matter. Top growth from some crops is wholly or partly returned to the soil.

Only the roots and stubble of other crops may be left.

Because annual crops die each year, both tops and roots may contribute to the organic matter. The contributions from perennials vary with the kind of crop and the conditions of growth. Sod crops harvested for hay contribute only through root residues, but a large part or all of the top growth may be returned to the soil if they are pastured or used as green manure.

The entire root of an annual dies and begins to decompose each season. Some dying and regeneration of roots occurs annually with all perennials, but the amount and proportion of root dieback are greater in some plants than in others. The major parts of the root systems of some grasses perform much like annuals in that they die at the end of each growing season and are replaced by new roots the next.

Plant residue materials, because of differences in composition and structure, vary in susceptibility to decomposition and therefore in the contribution they make to the soil organic matter as they decompose.

Residues of mature plants generally decompose more slowly than immature plants. Roots and stubble rot more slowly than top growth. Nonlegumes rot more slowly than legumes. The residues that decompose most slowly remain longer in the soil and contribute more to the supply of organic matter.

THE INFLUENCE of these principles can be seen by comparing the organic matter contents of different soil areas. The Prairie soils of the Midwest attained high levels of organic matter under virgin conditions. The vegetation was prairiegrass. Fertility was high, and moisture and the growing season were adequate for high production. Decomposition processes were not excessive, because microbial activity during a large part of each season was arrested by cold temperatures. Soil organic matter therefore accumulated to near maximum amounts for arable mineral soils.

Less soil organic matter has accumulated in the Southern States than in the North. Soil temperatures in the South are warm during the growing season, and they remain generally favorable for microbial activity during the winter. The mild, moist climate accelerates soil weathering and causes some loss of plant nutrients. It does not increase plant growth and increments of organic matter as much as it stimulates decomposition by microbes.

The marked influence of high soil temperature in stimulating decomposition of organic materials more than the production of residue is demonstrated in the tropical forest regions. Annual production of residue materials is higher there than under any other current vegetation cover, less residue accumulates on the forest floor, and less organic matter is found in the soil than in forests of the temperate region. The teeming activity of the soil organisms under continuous optimum temperature and moisture more than keeps pace with the high production of organic debris.

A different set of conditions exists in the and regions. The shrublike vegetation produces little residue, and the annual input of organic residues to the soil is small. The heat and the drought of the hot desert has not arrested the activity of the micro-organisms and the decomposition processes as much as it has curtailed the growth of the plants. Little organic matter has accumulated therefore in or on the hot desert soils.

In cold climates, which have short growing seasons and low annual rainfall, decomposition is slow; despite the small annual additions, a thin layer of organic debris accumulates on the surface soil.

The influence of crop and of treatment is illustrated in the nitrogen changes in the Morrow plots in Illinois. (The Morrow plots, named for George E. Morrow, who established them, were started in 1876. They are the oldest experimental plots in America.) H. P. Rusk, of the Illinois Agricultural Experiment Station, in 1947 reported a decline of 1,525 pounds of nitrogen an acre-37 percent of the original between 1904 and 1943 from the untreated plot in continuous corn. All top growth was removed at the end of each harvest season. Only roots and stubble were left.