Humus crop residues

In addition to maintaining soil health, microbes are also important in converting manures, crop residues and other organic materials into composts, humus and plant available nutrients (Welbaum ef al. 2004, see also Chapter 2 and Special topic 2), providing biological control of certain pests and diseases (Trejo-Estrada ef al. 1998, Brimner and Boland 2003) and decomposing crop residues in the paddock (Vazquez ef al. 2003). 

FIGURE 6.2. Idealized diagram for the decay of crop residues in soil under conditions that are optimal for microbial activity. (From E J. Stevenson. 1982. Humus Chemistry. Wiley, New York.)... 

The characteristics and merits of organic materials that are commonly used as humus sources in agriculture are discussed below. These include crop residues that never leave the soil, portions of plants that are removed and returned, outside carbon sources, and crops that are incorporated into soil for soil improvement. These humus sources are not of equal value per unit of carbon, and maximum benefits are realized only by varying the methods of use to fit the individual soil and cropping system. These benefits may include not only the maintenance or build-up of soil organic matter but also better tilth, increased water use efficiency, improved nutrient status of the soil, increased buffer and exchange capacity, and erosion control. 

In many sections of the country wood wastes in the forms of sawdust, bark, or chopped-up limbs are available in large quantities at little or no cost other than hauling. The questions often asked are of what value are these materials as sources of nutrients and soil organic matter Are they toxic to plant growth or can they be used satisfactorily as substitutes for crop residues as source materials for humus maintenance Recent researches in the fields of soil organic matter, soil microbiology and soil aggregation serve as a basis for rather positive answers to these questions. 

Although green manure carbon is rapidly oxidized by the soil microorganisms there is little evidence that this carbon is any less valuable in humus formation per unit of carbon added to the soil than is carbon from more carbonaceous crop residues (Pinck et al., 1950 Giddens et al., 1965). This is contrary to past prevailing opinion (Russell, 1961) which considered green manure carbon to be decomposed so rapidly and completely that little was left to contribute to total organic matter. It does decompose rapidly but the residual end products seem to be very well suited for rapid humus formation. 

A few years ago an occasional article was published that expressed the idea that the addition of abundant nitrogen to crop residues would result in holding more of the carbon, and thereby form more humus. This is a fallacy, as pointed out in Chapter 21. 

Semotan (1962), in his discussion of the reclamation of the spoils from brownstrip-mines of Czechoslovakia, emphasized the importance of using the right technology of strip mining, including the selection of soil materials that will, when placed on the top of the spoil, serve as satisfactory subsoil and topsoil ingredients. The next step is to get humus into these soil materials. Since the addition of crop residues is a very slow method of building up humus, he recommended the addition of farm manures, fertilizers, moor-... 

When soils are well aggregated and remain in this condition they are somewhat less subject to wind erosion than are poorly-aggregated ones. However, a high level of thoroughly-decomposed crop residues (humus) may, according to Chepil and Woodruff... 

Soils can be categorized as substrate-rich or substrate-deficient in terms of their capacity to produce ethylene. Crop residues provide natural substrates but there is no correlation betv/een combined organic matter (humus) present in the soil and ethylene production. To provide the substrate, localized variation in oxygen concentrations around crop residues probably favour the accumulation of methionine under anaerobic conditions, A similar reaction may take place at the anaerobic centres of crumbs vdth outward... 

Humus by means of exchange reactions and chelation mechanisms is able to hold most of them except copper in an available form as they are either released from soil minerals or are added to the soil in plant and animal residues and in fertilizers. The ability of soil humus to hold microelements for a considerable time and release them as iweded to crops is undoubtedly one of the most important benefits derived from its presence in soils. 

The colloidal materials in soil also protect plants against many of the numerous pesticides that are now in common use in agriculture Mader, 1959). Only rarely do these materials, at the rates commonly used, cause injury to crops. But as the colloid content of soils decreases the possibility of injury increases. Humus is especially effective in protection against insecticide residues that contain copper. It has been shown repeatedly that where pesticides are applied directly to soils the effective dose increases markedly as the colloid content of the soil increases. This shows how effective colloids are in inactivating toxic materials. The literature is covered rather fully by Bailey and White (1964). 

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