RATE OF RESPONSE OF STRUCTURAL STABILITY TO A CHANGE IN WATER-CONTENT - INFLUENCE OF CROPPING HISTORY

Hardsetting is an undesirable behaviour of soil that is influenced by soil structural stability. Water content has been shown to influence stability. The temporal change in stability subsequent to a change in water content and the degree to which those temporal changes can be modified by cropping practices are, however, unknown. Changes in the structural stability subsequent to a change in water con tent were determined using aggregates from different treatments in rotation experiments on a Humic Calcaric Gleysol (Humaquept). Aggregates less than 4 mm were dried from a water content of theta = 0.32 kg H2O kg-1 dry soil to different water contents (theta = 0.12-0.32), and incubated at constant theta for different lengths of time. The structural stability was assessed on two scales by measuring the percentage of aggregates larger than 0.25 mm which were water-stable (WAS) and the percentage of material less than 0.002 mm which was dispersible, i.e. unstable (DCF). There was a net gain in structural stability with time following drying and the increase was predicted to continue from 40 to 86 days after the water content was decreased. The gain was particularly pronounced at low water contents (theta = 0.12-0.22) and was attributed to non-microbial age-hardening processes. The gain in stability with time subsequent to drying was not influenced by cropping history. Soil under long-term bluegrass production exhibited, however, the greatest structural stability. Because the soil under long-term blue-grass was probably already close to its maximum stability, it was found not to be sensitive to the stabilising action of drying. The increase in stability observed after 20 days of incubating at theta = 0.12 or 0.22 was shown to persist for less than 10 days for WAS and more than 10 days for DCF following rewetting to theta = 0.32. The data suggest that management practices which enhance age-hardening and which reduce the loss of stability at high water contents will enhance the structural stability of hardsetting soils.