RESIDUAL EFFECTS OF ADDITIONS OF CALCIUM COMPOUNDS ON SOIL STRUCTURE AND STRENGTH

Soil structure was measured from the roughness of soil fracture surfaces created by breaking unsaturated soil clods under tensile stress. Soil clods were collected in 1986 and 1987 from field plots to which calcium compounds had been applied in 1980 (to improve degraded structure) at rates of 0, 2, 4, 10, 14 and 20 t ha-1, and which had not been disturbed since the initial establishment of pasture. A residual effect on the soil structure was sought. In 1987, samples were equilibrated at different water suctions in the laboratory, and physical and chemical measurements including tensile strength, penetrometer resistance, exchangeable cations and dispersible clay were made. Exchangeable cation and dispersible clay values did not correspond with the amounts of calcium added in 1980, indicating that most of the applied calcium had leached by the time measurements were made in 1986 and 1987. The water content at which measurements were made was the dominant factor controlling the tensile strength and penetrometer resistance of the soil clods. The water content also had a major influence on the fracture surface roughness; wetter clods had rougher tensile fracture surfaces. Because most of the applied calcium was leached by the time measurements were made, the residual effect of the calcium amendments was only detectable in the microstructure (undisturbed since 1980). Soils to which greater amounts of calcium had been added tended to have clods with smoother tensile fracture surfaces. This was attributed to stabilization by calcium of aggregates with diameters up to 0.1 mm in the soil structural hierarchy. The importance of the scale at which fracture surface roughness measurements are made is emphasized.