The Base Exchange Capacity of Decomposing Organic Matter 1

T organic matter fraction of soils has been shown by various investigators (3, ~9, 2, 4, ~, 5, 9, 2o, x6)~ to be responsible for a large part of the base exchange capacity of soils. McGeorge (4) noted a linear relation between the exchange capacity and the carbon content of ~o soils, but no such relationship existed between the base exchange capacity and the nitrogen content or the carbon-nitrogen ratio. He concluded that lignin, ligno-hemicellulose, and ligno-cellulose fractions function largely as the exchange compounds of soil organic matter. He observed (5) that the exchange capacity of ligno-humate was larger than the exchange capacity of lignin extracted from the same soil. Xylan, a constituent of hemicellulose, was found to have an exchange capacity which was increased by chemical humification. The increase in exchange capacity of decomposing alfalfa was attributed to a differential decomposition resulting in a concentration of lignin-like bodies in the residues and the variation in the properties of lignin was attributed to the number of hydroxyl and methoxyl groups in the molecule. Mitchell (9) concluded that two fractions of soil organic matter have base exchange properties, namely, the hemicellulose-containing fraction and the lignin fraction of which the latter is more important. There appeared to be no relation between the nitrogen content of the lignin-humus and its base exchange capacity. Powers (~5), working with sweet clover, oat straw, flax shires, sphagnum moss, sphagnum peat, saw grass, peat, and wheat during decomposition, noted that the exchange capacity increased, that conditions favoring decomposition favored an increase in exchange capacity, and that the increase in lignin content tended to parallel the increase in base exchange capacity. MeGeorge (6) pointed out that lignin may exist in the plant active and inactive forms with the base exchange capacity varying accordingly. Waksman and Iyer (2 ~) observed that the base exchange capacity of ligno-protein preparations increased with an increase in the protein content. Mflller (xo), studying manure and straw, found an increase exchange capacity with decomposition. He concluded that the property of base exchange in isolated fractions of organic matter may be attributed to certain groups, such as the hydroxyl and carboxyl,