STRENGTH DEVELOPMENT DURING DRYING OF A CULTIVATED, FLOOD-IRRIGATED HARDSETTING SOIL .1. COMPARISON WITH A STRUCTURALLY STABLE SOIL

A comparison was made between the physical behaviour of aggregate beds of a structurally unstable, hardsetting soil and a soil of similar texture but more structural stability. The soils were flood-irrigated and then allowed to drain and dry. Tensile strength, penetrometer resistance, vertical movement, matric potential, and water content were monitored. The hardsetting soil slaked during flooding and its tensile strength increased steadily during drying. The more stable soil did not slake and aggregates retained their structural integrity. This soil was still a bed of loose aggregates with zero tensile strength after drying. Penetrometer results did not indicate clearly the contrast in physical behaviour between the two soils, and can only be used to indicate physical impedance to root growth in soils like the hardsetting soil that are structureless. In combination, air porosity and penetrometer resistance indicate that in the hardsetting soil there was only a narrow range of Potentials, between -30 and -100 kPa, in which soil physical conditions were unlikely to limit root growth.