Some aspects of soil shrinkage and the effect of cracking upon water entry into the soil.

Abstract

The shrinkage of natural soil aggregates that accompanies water withdrawal has been examined. Four main stages operate: (1) structural shrinkage, (2) normal shrinkage, (3) residual shrinkage, (4) no shrinkage: The pF ranges over which these stages operate and the fraction that they constitute of the total shrinkage has been measured for a range of soils. The influence of structure and texture upon shrinkage was assessed. These affect both the shrinkage phase and the range over which it operates. Structural development affects shrinkage markedly, giving a very different shrinkage pattern from that found for puddled soil or remoulded soil blocks. The difference in porosity between a natural aggregate and a puddled block of the same soil corresponding to a definite pF value is suggested, therefore, as a means of assessing the degree of structural development in the soil. It provides a suitable index for assessing the rate of structure improvement under various ameliorative treatments. The influence of cracking in increasing the rate of water entry into fine textured soils is discussed. In the particular cases of irrigated soils, where soil water contents are maintained at relatively high levels (above wilting point), it seems that cracking may not have progressed sufficiently to compensate for the low inherent permeability of such soils. Cultivation of the surface soil to form a mulch before irrigation tends to eliminate any contribution to conduction by the smaller cracks which are present at higher soil water contents. Large increases in permeability, due to the presence of gross soil cracks, are attained only when water contents are reduced to levels which are not practicable in continuous irrigation culture.