Evaluation of residual fertilizer effects using labelled fertilizers

The residual fertilizer effects of using triple superphosphate (TSP) and diammonium phosphate (DAP) applied under field conditions (100, 200 and 300 kg/ha of TSP and 100 and 200 kg/ha of DAP) were determined on typic Haplustol and lithic Haplustol soils. One year after fertilization, samples were taken from fertilized and unfertilized field plots. Two treatments were applied under growth chamber conditions, one using TSP labelled with 32P (42 mg TSP, with 0.13 MBq/mg P in 450 g of soil) and the other with unfertilized soil. Ray grass seeds were sown. The plants was harvested 30 days after planting and calcinated; then the 32P and 31P concentrations were measured. Calculations were made of the P fractions in plant derived from fertilizer (Pdff) and from soil (Pdfs). The P in plant increased with the increased level of fertilizer in the field treatments and with fertilization under growth chamber conditions. The soil P in fertilized soils included the native pool and residual fertilizer P. The relationship between the Pdff under growth chamber conditions and the Pdfs was chosen in order to compare the P availability in soils with different doses of fertilizer. In the typic Haplustol soil, the relationship between the Pdff and the Pdfs was similar when TSP or DAP was applied. For lithic Haplustol soil, a greater in the available P in soil was found at equal doses using TSP. The relationship between Pdff and Pdfs was similar when TSP was applied to both soils. (author). 6 refs, 1 fig., 4 tabs