I. LIMITATIONS OF EXISTING METHODS THE stability of soil crumbs has frequently been characterized by determining their resistance to breakdown in water. This may be done in two ways. In the first, originated by Tiulin (1928), the dry crumbs are placed on top of a bank of sieves, wetted, and the size distribution of the wetted crumbs determined after agitating the sieves under water. Modifications designed to prevent further breakdown when separating the wetted crumbs have been devised by Childs (1940) and by Zeller (195 I ) .The second way, due to Middleton (1930), instead of reducing mechanical breakdown after wetting to a minimum, measures the degree of dispersion roduced by a given amount of mechanical action. This is usually done ! y shaking the crumbs in water in an end-over-end shaker for a fixed time.During the last 20 years, many attempts have been made to correlate the water stability of crumbs determined by either of the above methods with the amounts of organic matter and clay present in the crumbs and the cropping history of the fields from which the crumbs were taken. The results are very variable; but there appear to be two distinct situations according to whether the crumbs come from arable soil or grassland. For grassland, a good correlation has been found between organic carbon and water stability by Ackerman and Myers (1943)~ Low ( I~s o ) , and by Russian workers under Williams. For other crops, several authors, Stauffer et al. (1940), Rogers (1939), Retzer and Russell (1941), and Ackerman and Myers (1943), have shown that the effect of the cropoutweighs any effect of organic matter on aggregate stability, and lucerne appears to be particularly effective in promoting water stability. Regarding clay content, there seems to be general agreement that it is a major factor in determining crumb stability (Puri et aZ. 1939; HCnin, 1938;and Sideri, 1938).The following pa er is an attempt to elucidate the various factors method as a whole to the problem of soil structure in the British Isles.influencing the resu Y ts obtained, and to consider the relevance of the
Causes of Crumb BreakdownThe factors governing the breakdown of dry aggregates when wetted by a liquid can be broadly grouped under two main headings: slaking of the aggregates and dispersion of the clay in the aggregates. For simplicity it has been assumed that the crumbs are effectively calciumsaturated, as is normal in the British Isles. Under these conditions, breakdown due to swelling of the clay has been shown to be unimportant (Emerson and Grundy, 1953).In the following diagram the various factors are subdivided into those