An ion exchange material, chelating agent, and buffer

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. 

Cation exchange was first observed more than a century ago when it was noted that when a dilute solution of ammonium sulfate was added to a soil the leachate first contained calcium sulfate, rather than ammonium sulfate. When potassium sulfate was similarly added, the percolate also contained calcium sulfate initially and not potassium sulfate. In either case the cation in the added solution replaces the calcium of the soil. Not until most of the readily exchangeable soil calcium has been leached out does the added salt appear in the percolate in appreciable amounts. 

The seat of most of the ion exchange is in the finer portions of the soil which includes the organic and inorganic colloidal fractions, the clay, and at least the smaller silt particles (Hosking, 1948). This involves the particles having a diameter of about 0.02 mm or less. 

There is considerable uncertainty as to the explanation for the development of the negative charge on organic colloids. It is known, however, that charges arise from the presence of carboxyl, phenolic hydroxyl, enolic hydroxyl, imide, and possibly other groups (Broadbent and Bradford, 1952 Broadbent, 1953 and Mortensen, 1963). 

The determination of CEC is commonly made by leaching a soil with a neutral barium acetate solution. It may also be made by extracting a soil with normal ammonium acetate solution, followed by normal potassium chloride. The ammonium removed by the potassium chloride is a measure of the CEC of the.-soil, which includes the cations held by both the organic and inorganic portions. Separate determinations of these portions can be made but less accurately because the separation of one from the other cannot be made without affecting the chemical nature of the organic colloid. 

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