17 Aggregate Stability and Size

An aggregate is a group of primary particles that cohere to each other more strongly than to other surrounding soil particles. Most adjacent particles adhere to some degree. Therefore, disintegration of the soil mass into aggregates requires imposition of a disrupting force. Stability of aggregates is a function of whether the cohesive forces between particles withstand the applied disruptive force. When comparing size distribution or stability of soil aggregates, it is necessary that the disruptive force(s) be standardized. If the measurements are to have practical significance, forces causing disintegration should be related to forces expected in the field. The better methods to determine aggregate-size distribution have evolved as a result of efforts to standardize the disruptive forces and make them comparable to those in field phenomena. Forces involved in aggregate size and stability studies include (i) impact and shearing forces administered while taking and preparing the samples, (ii) abrasive and impact forces during sieving, and/or (iii) forces involved in the entry of water into the aggregate. These forces are generally related to cultivation, erosion (wind and water), and wetting of soils, respectively. However, the disintegrating forces occurring during sample taking, preparation, and analysis do not duplicate the field phenomena. Consequently, the relationship between aggregate-size distribution obtained in the laboratory and that existing in the field is somewhat empirical. Recognizing the empiricism involved in relating aggregate-