2002-07-08_GENERAL DOCUMENTS - M2002004 (3)

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194 SMITH, REDENTE, and HOOPER nique, whereas organic matter content can only be estimated. Therefore, measuring organic C concentration is preferable to determining soil organic matter content because the latter is not, at this point in the development of soil science, an appropriate or accurately measurable soil constituent (58). Effects of organic matter on plant growth and growth media Organic substances play a direct role in the formation of a fertile soil be- cause they provide plant nutrients that become available during mineraliza- tion. Organic matter also has a fundamental effect on the physical proper- ties of soil, such as water - holding capacity, structure, and heat regime. It also influences such physiochemical properties as cation exchange and buf- fering capacities. These properties influence nutrient uptake, water and nu- trient availability, and the deleterious effect of soil acidity and alkalinity (42). Effects on plant growth. The direct effects of organic matter on plant growth and development are well documented. Humus has positive effects on plant physiological and biological processes, promoting development of green tissue and uptake of nutrients (42). Several studies confirm that plants take in CO,, mainly CO, liberated from the soil pool, where considerable CO, is formed as a result of organic matter decomposition. Decomposed organic fertilizers are also a source of CO,. Crop yields can be increased by CO, (8). Organic matter levels in disturbed areas correlate strongly with plant growth after disturbance. The greater the predisturbance level of organic matter, the better the reestablishment of vegetation. In a California study (66), a desert area was seeded to barley and, in some areas, fertilized with N. Yields on soils beneath old shrub areas with moderately high levels of or- ganic matter and without N fertilizer were three times greater than yields on old, bare areas with low soil organic matter levels receiving fertilizer. In a study of reforested areas, growth of cedars (Thuja spp.) and junipers (Juni- perus spp.) was best on soils with a moderately thick to thick organic mat- ter -rich layer (25). In more severely disturbed areas organic matter applications appear to benefit plant growth temporarily. In sorghum and corn fields of Texas and Montana, where land leveling had removed 30 to 90 cm of the upper soil profile, additions of N, P, and /or manure produced a plant response equal to or slightly less than on nearby undisturbed areas (64, 75). In Texas during drier -and warmer- than - normal seasons, yields were considerably lower, probably due to "intensified drought and heat damage related to higher bulk density and lower porosity" (75). Infiltration declined because of the lower porosity and higher bulk density. This resulted in the failure to build a nor- mal topsoil over 31 years, which emphasizes the value of the virgin A horizon. SOIL ORGANIC MATTER Such research underscores an in organic matter can stimulate plant themselves, do not create a self -su plication. Rather, the organic m: and plant growth, which augmer and increased fertility. Similarly, 1 ficial effect on plant growth (69). vegetable crops, similar results w land revegetation species. Only in a few instances has the an undesirable plant growth rest application (400 t /ha) reduced tl creased salinity levels (67). Other tion (7,400 t /ha) have shown simi application rate of 112 t /ha, plar season (49). Salt build -up and N causes of reduced germination an of manure were applied and spra depressed growth and developmt because of prolonged, less- than -o N and other minerals are impoi an economic viewpoint (4). Plant tions in which substances are cycl These elements are scattered thro earth's crust. They accumulate in ment digestors —and heterotroph sequently liberated during the lift Because N makes up more than mosphere, the main reservoir po■ cycling takes place in the soil (86 directly. Other mechanisms are r can use. This process involves mi croorganisms. Most N in surface soils is organ ganic matter and mineral matter mineral forms as nitrates and e (79). Total N content of soils range: more than 2.5 percent in A horiz' tions of the C/N ratio in natural is N relates to the amount of org: in the West, C/N ratios range f found in one 40- year -old mine st cent natural soil (68). The ratio