TEMPORAL VARIATION IN POTASSIUM CHLORIDE EXTRAC- TABLE ALUMINUM, SODIUM, AND SOIL pH, AND THE EFFECTS ON INTERPRETATION OF SUBSOIL ALUMINUM TOXICITY IN YELLOW EARTHS

Temporal variation in two soil tests ([Al] and the ratio [Al]:[Na] in 1:5 0.005M KCl extract) and pH were evaluated on six occasions during a twelve month study of eleven yellow earths in the Merredin region (31°5, 118°E) of Western Australia. The [Al] and ratio [Al]:[Na] in 1:5 0.005M KCl extracts are useful soil tests capable of distinguishing between productive and non-productive (Al toxic) yellow earths in Western Australia. The aim of the study was to determine the most appropriate time to sample yellow earths, in order to predict accurately which soils contained concentrations of Al in the 15-25 cm layer that were toxic to wheat. The concentration of Al differed (p<0.05) at three or more sampling times at four of the eleven sites. Temporal variation in [Al] did not affect the identification of Al-toxic and non-toxic yellow earths. The ratio [Al]:[Na] also differed (p<0.05) at three or more sampling times and at six of the eleven sites. The ratio [Al]:[Na] was affected by temporal variation to such an extent, that incorrect diagnosis of potential subsoil Al toxicity could have been made, at four of the eleven sites. Spatial variation was a confounding factor in assessing temporal variation in all soil tests. If [Al] alone is used to predict Al toxicity in wheat, soil samples could be collected at any time of the year. The most appropriate time to sample soil and use the more accurate soil test, [Al]:[Na], is after summer, but prior to the commencement of winter rainfall patterns and the growing season. I