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 Australia. 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 1988-1990 to monitor changes in pH(CaCl2) and in concentrations of exchangeable aluminium (Al) and manganese (Mn). The sites received no fertiliser or cultivation and therefore allowed for the estimation of natural temporal 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 weather conditions. The larger changes in soil pH were associated with more extreme climatic conditions than normal, e.g. following the break of season after a hot, dry summer (autumn 1988) or during periods of above-average rainfall in autumn and early spring (1990). Temporal variation in pH was less than the spatial variability at the sites but greater than the long-term net acidification rate reported for the region. Temporal variation in the concentration of exchangeable Al ranged from 0 to 0 4 cmol(+)/kg during a year and varied primarily with the inverse of pH. Variations in the concentration of exchangeable Mn ranged from 0.05 to 0.35 cmol(+)/kg during a year. The concentration of exchangeable Mn increased over summer to an extent dependent on the drying of the soil. At the 2 sites with duplex profiles, maxima in the concentration of exchangeable Mn also occurred in spring, particularly in the warm wet spring of 1990. Soil tests for soil acidity therefore represent guides to probable risks of toxicity, as pH and the concentrations of exchangeable Al and Mn may change between soil sampling, sowing, and the period of crop or annual pasture growth. Such variations will alter the responsiveness of crops and pastures to lime.