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 />
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