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SMITH, REDENTE, and HOOPER <br />. cent of the C in humus remains <br />new humus (37). <br />or in the soil vary widely in their <br />i by microorganisms under aero- <br />and proteins decompose rapidly. <br />resistant to biological decay (4). <br />be digested by soil microorga- <br />lecompose more rapidly than the <br />'lose and hemicellulose are con - <br />luickly by soil organisms (69). <br />higher levels of protein and fat <br />s not contain lignified tissues as <br />:d by other animals, reaching the <br />lecomposition rates. Fresh plant <br />avorable conditions for rapid mi- <br />c microflora (organic compound <br />tuna use organic substances for <br />hworms (Lumbricus spp.), speed <br />anic debris and /or soil and leav- <br />wever, generally do not occur in <br />ral and riparian areas where they <br />'crease, there is a corresponding <br />t measure of organic matter de- <br />s are added, the CO2 emitted de- <br />•sition will be equal to or greater <br />yes are added (69). Therefore, the <br />)y relates to the soil's C content. <br />I affect the size and composition <br />itures and moist soil conditions <br />). But decomposition also occurs <br />during the winter when soils are <br />to conditions that totally change <br />'sited on, soils affect decomposi- <br />entrations in saline or sodic soils <br />imposition and size of organism <br />id possibly the end 'products of <br />tate dispersion of organic matter <br />the organic residues contributed <br />grates (cellulose, hemicellulose, <br />SOIL ORGANIC MATTER 189 <br />sugars), protein, alcohols, lignins, fats (waxes and resins), tannin, <br />glucosides, alkaloids, and pigments (4). Humus is produced as these <br />residues decompose. <br />Physical and chemical properties. The physical nature of soil organic <br />matter freshly added to the soil depends largely upon the type of plant or <br />animal residue. For example, forest ecosystems contribute different plant <br />and animal litter than do prairie ecosystems (4). During decomposition, the <br />material is altered physically and will, many times, be unlike the original <br />material. <br />Organic residues constitute a minor percentage of soil organic matter, us- <br />ually less than 35 percent. Sources of these organic residues include such <br />materials as plant roots, animal manure, dead animals, microorganisms, <br />and aboveground plant remains. Of these materials, plant tissue contributes <br />more to soil organic matter than animal tissue (4). <br />Humus, the primary constituent of soil organic matter, is amorphous, <br />dark brown to black, and nearly insoluble in water, but soluble in dilute <br />alkali (NaOH or KOH) solutions. Humus contains about 30 percent each of <br />N -rich proteins, the slow -to- decompose lignins, and complex sugars. The <br />complex sugars are a large percentage of the varied organic substances that <br />cement soil aggregates together. Humus has about 60 percent C, 5 percent <br />N, some 0, and lesser amounts of S, P, and other elements. Its cation ex- <br />change capacity, on a dry weight basis, is many times greater than that of <br />clay colloids (28). The particle density of humus is nearly half that of clay or <br />quartz minerals, ranging from 1.3 to 1.5 g /cm <br />Humic acid (proteins and lignins), the major constituent of humus, is that <br />fraction of humus that is soluble in alkali solution and precipitated by acid <br />(pH 1.0 to 2.0). It is dark brown to black in color. <br />Of the several chemical elements found in organic matter, C and N are <br />the most important. While the N level in humus is about 5 percent (4), C <br />levels range from 48 to 58 percent (70). Hence, the organic C content of <br />various soil horizons is a useful indicator of the extent of organic matter ac- <br />cumulation. <br />Biological properties. A productive soil includes a myriad of microorga- <br />nisms and small animals that represent a source of C, energy, and nutrients <br />for plants and other microorganisms. Such factors as the food material <br />available, plant species, amounts and kinds of organic amendments ap- <br />plied, soil texture, pH, moisture, aeration, and amounts of salts influence <br />the numbers, kinds, and activities of these organisms. They live by the rule <br />of the survival of the fittest. That is, those organisms active in any particu- <br />lar microenvironment are best adapted to the conditions present or repre- <br />sent the most successful competitors (28). These organisms include bacteria, <br />actinomycetes, fungi, algae protozoa, nematodes, insects, and small mam- <br />mals. <br />The chief role of soil fauna seems to be mixing soil and organic matter, <br />