An investigation of reduction in wheat yields after use of a high level of ammonium sulphate for a number of years

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Abstract
Investigations were carried out to determine reasons for reduced yields in two out of three multiple wheat cropping trials, where ammonium sulphate (S/A) was applied each year at 376 kg ha-1 (76 kg N ha-1). These included chemical analysis of soil and plant samples from the trials, and a pot trial on soil from the two affected sites at Merredin and Wongan Hills. At all three sites the SIAtreatment lowered soil pH and exchangeable bases and increased exchangeable H in the soil. Available Al was markedly increased at Merredin and Wongan Hills, the sites where yields were reduced, but to a much lesser degree on the third site at Beverley. Available Mn was increased at Beverley. The reductions in soil Mg and K were reflected in the contents of these nutrients in wheat tops from the sites, but Ca content was affected only at Merredin. Plant Mn levels were increased by S/A at Wongan Hills and Beverley, but Al levels did not show any consistent effects. The level of plant Mo was reduced by S/A at all sites. In pots, on soil from the high S/A plots at Merredin and Wongan Hills a large growth response to lime was obtained, but no growth responses to Ca, Mg or K. The response to lime was associated with a decrease in Al in the plants at both sites and an increase in P at Merredin. It was considered that Al toxicity was the most likely cause of the yield reductions, but that levels of K and Mo could also be marginal for healthy growth.