Changes in soil Olsen P over six years with annual applications of triple superphosphate or reactive phosphate rock (Sechura)

Abstract

The effects of annual applications at four different rates of either triple superphosphate (TSP) or Sechura phosphate rock (SPR) on the Olsen P test in soil under pasture were measured over 6 years at 13 sites in New Zealand. Averaged over the 13 sites, TSP applied at the calculated maintenance (M) rate maintained an almost constant Olsen P level throughout the 6 years. However, there were marked differences between sites, particularly at the 2.0 M application rate which at some sites raised Olsen P by over 30 μg P/ml soil but caused no increase at other sites. No obvious explanation such as soil group, phosphate retention, rainfall, or pH, or the TSP rates used could be found for these site differences. Olsen P tests were always markedly lower with SPR than with TSP; averaged over the 13 sites, even the 2.0 M SPR treatment barely maintained the initial Olsen P test values. This difference between TSP and SPR in their effect on Olsen P test contrasted with pasture production data (reported elsewhere) which show dry matter yields from these fertilisers converging after 6 years. It was concluded that the Olsen P test underestimated the available soil P pool derived from the dissolution of SPR. A dynamic model was proposed as a quantitative mechanistic explanation of the effect of P fertiliser and time on the soil test data. The proposed model provided a good fit to Olsen P data from TSP treatments. Olsen P tests were markedly lower with the higher rates of SPR than model predictions, further supporting the view that the Olsen P test underestimated available soil P derived from dissolution of SPR. Olsen P in no-P control treatments declined less rapidly than predicted by the model. © 1994 Taylor & Francis Group, LLC.