Effect of certain crops and soil treatments on soil aggregation and the distribution of organic carbon in relation to aggregate size

Studies of the effect of several crops and certain soil treatments on the degree of aggregation of soils in the field and in the greenhouse are presented. The organic carbon contents of various size groups of soil aggregates obtained from a single soil sample were determined. Samples taken from the soil under growing corn and kafir in field and greenhouse showed as good aggregation under the sorghum crop as under corn. When oats succeeded these two crops in the field, however, soil samples removed from the oats stubble revealed a greater degree of dispersion where oats followed sorghum than where corn was the preceding crop. Sweet clover left the soil better aggregated after 1 year's growth than soybeans, while alfalfa and sweet clover gave similar results. Soil fallowed for 2 years in the field was less aggregated than soil fallowed for 1 year. Limed soil supporting sweet clover and red clover in a greenhouse experiment was more highly aggregated than similar soil unlimed but supporting these crops. Unlimed and unleached fallow soil in the greenhouse was as well aggregated as limed fallow soil. It is suggested that perhaps the combined action of lime and a legume crop, or possibly other crops, produces an aggregating force which lime alone may not exert. The grasses failed to produce the aggregation of the soil expected of them, but their failure in this experiment may have been due to the shortness of the growth period. The more aggregated portions of the soil contained significantly more organic carbon than the less aggregated portions. Ultimate particle size in the various size groups of aggregates was quite similar. Hence it is believed these results lend weight to the assignment of an important role to organic matter in the aggregation of the mineral particles of soils.