Humus sources

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. 

Toxicity, although encountered occasionally, is not a very serious factor in the use of wood products as humus sources in the presence of adequate nitrogen and lime, if needed. The few wood products that are known to be toxic should be avoided, or sufficient time after application to soil should be allowed for considerable biological decomposition to occur before crops, particularly seedlings, are exposed to them. 

There has been considerable uncertainty over the years as to the value of wood products as humus sources, although their merits as mulches have been reasonably well evaluated. Their value as a humus source has been questioned primarily because of their deficiency in nitrogen. But considerable uncertainty has also existed as to their merits even where adequate supplemental nitrogen was supplied. 

In terms of economics or cost accounting, no wood products may be worth hauling to the field. Since their nutrient value is negligible, almost their whole value is dependent upon the value put on them as mulches and as humus sources. The situation here is even more unfavorable than for animal manures, discussed above the latter at least have a considerable nutrient value per ton even though the moisture content is much higher. 

Peat is a very valuable material for use in vegetable and flower gardens, as every gardener knows, but its value is primarily as a mulch and for improving soil tilth in heavy clay soils. It also adds to the exchange and absorptive capacity of the soil, adds some available nutrients, and helps to hold nutrients in available form. We are less certain as to its value as a humus source. 

Plant materials that are woody and resistant to decay show higher accumulations of organic matter in soils than do more succulent materials during experimental periods of 1 -2 years. During longer periods there is little evidence that this relationship holds in fact, there is considerable qualitative evidence that woody materials are less suitable as humus sources than are the more readily decomposable materials. Here, we must distinguish between total organic carbon in soil, not all of which is well decomposed, and humus carbon that has lost its plant identity. 

Many of the better peats are being cropped intensively after preliminary treatment as necessary to make them into a satisfactory medium for plant growth. Some of our most valuable soils are peats and mucks. In addition, many tons of peat are now being sold for use as a humus source in mixture with mineral soils. The principal problems involved, and the best methods of using organic soils for plant growth, and for the improvement of mineral soils in gardening operations, are discussed in this chapter. 

Peat was often used in earlier years as both a humus source and as a fertilizer, with the expectation that it would give results similar to those obtained with animal manures. Although increased yields were often obtained, and the physical condition of the soil was improved, it did not act like animal manures. This was due in part to deficiencies in mineral nutrients, but to a greater extent to the comparatively inert nature of the material. As already pointed out, manure is an active material and as it decomposes, nutrients are constantly released through biological action. In contrast, peat is so resistant to decomposition that much of the release of nutrients must be through exchange reactions and as chelated elements. 

The European Thematic Strategy on Soil Protection 45% of the EU soils are poor in humus and composted biowaste could be a source of organic matter for the soils. 

No matter what the source of organic matter or the mechanism of its decomposition, an extremely important compound, humus, is formed during decomposition [6-10,17]. 

Once ionized, the acidic and phenolic molecules carry a negative charge and can thus attract cations and participate in the CEC of soil that are part of humus. The contribution of this source of negative charge will depend on the pH of the soil solution and will change as the pH changes. Thus, the CEC is pH dependent. For this reason, it is essential that the CEC values are measured at the same pH when comparing different soil samples. 

The natural sources of carbazole alkaloids are listed in Table 2.1. Carbazole and various alkylcarbazoles have also been obtained from other sources, such as coal tar, petroleum oil, soil humus, the polluted atmosphere of industrial areas, as well as cigarette smoke. 

C N (or carbohydrate protein) ratios are important with respect to the relative requirements of plants and microbes. Straw has a C N value of approximately 80 1, whereas after ploughing under to form a humus-rich soil, the value narrows to about 12 1. Albrecht found that the humus fraction of the experimental Sanborn Field, cultivated over 50 years, had a C N ratio of about from 2 1 to 3.4 1, where the lower ratio is similar to that of the microbes themselves. Therefore, if microbes feed on straw that has been incorporated into the soil, they will require an additional source of nitrogen, and happen to be more successful at competing with plants for the same nutrient. In Missouri clay, Albrecht found 1.5% C and 0.15% N, which represents a favourable ratio of 10 1, and is an average value for well-weathered soils (Walters, 1989). 

The greater the amount of carbon dioxide in soil, the more hydronium ions and so the lower the pH. Soil that has a low pH is referred to as sour. (Recall from Chapter 10 that many acidic foods, such as lemon, are characteristically sour.) Two main sources of soil carbon dioxide are humus and plant roots. The humus releases carbon dioxide as it decays, and plant roots release carbon dioxide as a product of cellular respiration. A healthy soil may have enough carbon dioxide released from these processes to give a pH range from about 4 to 7- If the soil becomes too acidic, a weak base, such as calcium carbonate (known as lime or limestone), can be added. 

Carbohydrates and proteins serve as an energy source for microorganisms or as materials for forming their body substances while lignin and other phenolic compounds decompose more slowly than the two other main constituents. Therefore, lignin and other phenolic compounds may be regarded as materials essential for forming the dark colored substances in humus. 

Polycyclic aromatic hydrocarbons (PAH), which are ubiquitous in the environment, including surface waters, undergo facile chlorination by hypochlorite when dissolved in humus-poor water to give a suite of chlorinated PAH (1660). It is therefore conceivable that this chlorination can occur under natural conditions, but this is yet to be determined. Another new possible source of natural chlorinated PAH is the reported in vitro reaction of benzo [a pyrene diol epoxide, the ultimate carcinogen of benzo aIpyrene with chloride ion to give chlorohydrin DDD, which has been isolated and identified as an intermediate en route to a benzol a pyrene-DN A adduct (1661). However, DDD is not considered to be a natural compound at this time. 

Xstrom, M. and Corin, N. (2000) Abundance, sources and speciation of trace elements in humus-rich streams affected by acid sulphate soils. Aquatic Geochemistry, 6(3), 367-83. 

Various plant materials that undergo complex biochemical transformations serve as sources of humus substances. 

Source Adapted with permission from Stevenson, F. J. (1994). Humus Chemistry Genesis, Composition, Reactions, 2nd ed., John Wiley and Sons, New York. 

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