Effects of frequency of application of triple superphosphate and Sechura phosphate rock on pasture DM production, herbage P concentration, and Olsen P soil tests

Abstract

Triple superphosphate (TSP) and Sechura phosphate rock (SPR) were applied to developed pastures annually at 0.75 times the estimated annual pasture maintenance requirement (M) and triennially at 2.25 times M in small‐plot trials at 12 sites for 6 years. A control treatment receiving no phosphorus (P) fertiliser was also included. Pasture dry matter (DM) production and soil Olsen P tests were measured annually at all sites and pasture P concentration at two sites. Total DM production over 6 years did not differ significantly between annual and triennial treatments. Triennial applications resulted in higher DM production in Years 1 and 4, similar production in Years 2 and 5, and lower production in Years 3 and 6, compared with annual applications. Averaged over two 3‐year cycles of fertiliser application, the amplitude of the cyclic effect on DM production from triennial as compared to annual application was + 4% for TSP and ± 2% for SPR. Differences in annual production between triennial and annual applications were not affected by soil phosphate retention. Olsen P levels from triennial TSP applications were higher than from annual applications in Years 1,2, 4, and 5 but were similar in Year 3 and lower in Year 6. With SPR, there was no significant difference in Olsen P levels between annual and triennial applications in any year. At the two sites where herbage P concentration was measured on all treatments at every cut, the patterns of mean annual herbage P concentration over 6 years closely followed those observed with DM production. Annual applications of TSP and SPR maintained a near constant mean annual herbage P concentration over 6 years, whereas P concentrations declined markedly in the nil P control, particularly in the first 3 years. Herbage P concentrations from triennial TSP application were higher than from annual applications in Years 1 and 4, equal in Years 2 and 5, and lower in Years 3 and 6. Application frequency of SPR did not significantly affect herbage P concentration. A mechanistic model of P in pastoral systems was able to account well for the lack of effect of TSP application frequency on total DM production and also for the cyclic pattern of DM production from triennial TSP application relative to annual application. While correctly predicting a very small cyclic effect with SPR, it did not account well for the observed pattern.