The origin, nature, and importance of soil organic constituents having base exchange properties

The results of these studies may be summarized as follows: 1. The calcium-magnesium equilibrium constants for the organic base exchange reactions have not the same value in all soils. This fact indicates that more than one compound is responsible for base exchange reactions in soil organic matter. 2. The organic base exchange complexes may be destroyed by ignition at 350 degrees to 400 degrees C without apparently affecting the inorganic exchange material. 3. A large proportion of the exchange capacity of mineral soils is often due to organic matter, ranging from 41 to 65% of the total in the soils studied. 4. The organic matter of different soils differs greatly in its base exchange capacity. 5. Extraction with common organic solvents appears neither to dissolve nor otherwise to affect the base exchange material of soil organic matter. 6. Two fractions of soil organic matter have base exchange properties, properties, namely, the hemicellulose-containing fraction and the "lignin-humus" fraction. The latter fraction is the more important, since it appears to give soil organic matter its more or less permanent base exchange properties. In the peats studied, 60 to 80% of the base exchange capacity was due to this fraction. 7. It is suggested that lignin, or a derivative of very similar nature, is the constituent responsible for the base exchange reactions of the "lignin-humus" fraction of soil organic matter. Supporting this possibility is evidence that there appears to be no relationship between the nitrogen content of the "lignin-humus" fraction and its base exchange capacity, suggesting a nitrogen-free substance as the active constituent; that lignin prepared by a chemical method has base exchange capacity; and that the calcium-magnesium equilibrium constant of the "lignin humus" material is similar to that found for the prepared lignin.