Soil amendments modify phosphate sorption in an acid soil: the importance of P source (KH2PO4, TSP, DAP)

Abstract

Soil acidity is a widespread problem in Victoria, Australia, affecting at least 4 million ha of agricultural land. Soil amendments such as lime and organic materials may ameliorate acid soils, resulting in raised soil pH and increased availability of plant nutrients such as phosphorus (P). The addition of lime, lignite, and compost significantly modified P sorption in an acid soil, with the degree of change highly dependent upon the source of P applied. The application of 2.5 t/ha of lime increased P sorption for all P sources, while P sorption was decreased in the lignite and compost treatments when di-ammonium phosphate (DAP) was applied. Lime and compost addition increased the solution pH, with no change in pH in the lignite treatment. Addition of TSP decreased the pH in all treatments, while DAP addition only increased solution pH in the untreated soil and the lignite treatment. The addition of soil amendments had a significant effect on solution cation concentrations, due to both the influx of cations, and the resultant changes in solution pH. The source of P applied (KH2PO4, TSP (triple superphosphate), DAP) also had a significant effect due to both the counter-ions present and the pH of each P source (e.g. TSP pH 2.7; DAP pH 7.4). The lignite treatment decreased total P sorption relative to the other amendments. The combination of lignite and DAP resulted in both the greatest decrease in P sorption, and the formation of soluble Al–organic complexes. Therefore, a combination of lignite and DAP may be of use in decreasing P sorption in acid soils.