Humus buffering capacity

Humus/SOM enter into a wide variety of physical and chemical interactions, including sorption, ion exchange, free radical reactions, and solubilization. The water holding capacity and buffering capacity of solid surfaces and the availability of nutrients to plants are controlled to a large extent by the amount of humus in the solids. Humus also interacts with solid minerals to aid in the weathering and decomposition of silicate and aluminosilicate minerals. It is also adsorbed by some minerals. 

Figure 5.12 depicts a hypothetical soil pH-buffer capacity curve. It is clear from this relationship that the soils most likely to reach very low pH and toxic Al solubilities even with modest inputs of acidity are those with low inherent buffer capacities (arising from the lack of carbonates, clay, or humus content) or those whose buffers have already been expended in neutralizing past acid inputs. These are termed acid-sensitive soils, and are typically acid soils with little clay and humus. They are the most likely soils to reach extremely low pH (<4) on exposure to acid rain or other sources of acidity. Since primary and secondary aluminosilicate mineral dissolution is very pH sensitive, phytotoxic concentrations of soluble AP" are increasingly likely as the pH lowers. 

What is the buffering capacity of humus expressed in terms of CaC03 equivalent Assume 50% dissociation of humus acidity over the pH range of 4 to 7. 

The most important chemical characteristics of a soil are (l) its content of essential nutrients and their availability to plants (2) the exchange capacity (3) the buffering capacity (4) acidity or alkalinity and (5) content of inorganic and organic colloids (humus). Perhaps the state of oxidation or reduction of the soil should be mentioned this is ordinarily not of major importance but may be if a waterlogged or poorly-drained soil is under consideration. 

In the preceding chapter frequent reference was made to the role played by soil organic matter as a cation exchange material, and to its ability to chelate nutrients. It is also an excellent buffer. Since thes properties account for many of the most important benefits derived from its presence in soil it is well to consider at some length just what is meant by these terms, and how and to what extent humus functions in these processes. Since plant roots also possess cation exchange capacities that in many respects resemble those of soil organic matter, reference will also be made to this phase of ion exchange. 

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. 

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