TEMPORAL VARIATION IN SOIL ACIDITY

Seasonal variation in the response of crops and pastures to limestone application has been observed on acidic soils in south-eastern Austral ia. Our hypothesis was that temporal variation in soil acidity related factors may contribute to this variable response. Soils from 4 annual pasture sites were sampled at least monthly for 3 years during 1988-1 990 to monitor changes in pH(CaCl2) and in concentrations of exchangea ble aluminium (Al) and manganese (Mn). The sites received no fertilise r or cultivation and therefore allowed for the estimation of natural t emporal variation. Temporal variation in soil pH during a year ranged from 0 to 0.45 pH units depending on the site, soil depth, and the wea ther conditions. The larger changes in soil pH were associated with mo re extreme climatic conditions than normal, e.g. following the break o f season after a hot, dry summer (autumn 1988) or during periods of ab ove-average rainfall in autumn and early spring (1990). Temporal varia tion in pH was less than the spatial variability at the sites but grea ter than the long-term net acidification rate reported for the region. Temporal variation in the concentration of exchangeable Al ranged fro m 0 to 0 4 cmol(+)/kg during a year and varied primarily with the inve rse of pH. Variations in the concentration of exchangeable Mn ranged f rom 0.05 to 0.35 cmol(+)/kg during a year. The concentration of exchan geable Mn increased over summer to an extent dependent on the drying o f the soil. At the 2 sites with duplex profiles, maxima in the concent ration of exchangeable Mn also occurred in spring, particularly in the warm wet spring of 1990. Soil tests for soil acidity therefore repres ent guides to probable risks of toxicity, as pH and the concentrations of exchangeable Al and Mn may change between soil sampling, sowing, a nd the period of crop or annual pasture growth. Such variations will a lter the responsiveness of crops and pastures to lime.