Comparison of phosphate fertilizers for pasture and their effect on soil solution phosphate

A field experiment was conducted over a period of six years to evaluate the agronomic effectiveness on permanent pasture of (i) an unground North Carolina phosphate rock (PR), and (ii) the PR treated with elemental sulfur (S) and Thiobacillus spp. bacteria (PR/S). Single superphosphate (SSP) was used as the reference fertilizer. Changes in soil solution phosphate (P(i)) with time at 0-20 mm, 20-40 mm, and 40-75 mm depths, as effected by fertilizer treatments at different rates, were also determined. Application of PR/S gave the same dry matter yield as SSP from the first year onwards whereas PR gave less dry matter yield than SSP in the first 3 years and then again in year 5. The differences between fertilizers were not significant in years 4 and 6. Phosphate concentration in soil solution in the control treatment was greater in summer than in spring as a result of low pasture growth in summer, which was due to low rainfall. On application of SSP, P(i) concentration in solution increased with increasing rate of application and the values rapidly declined with time. At 30 kg P ha(-1) rate, P(i) in solution returned to similar concentration to that of control in about 3 to 6 months after application each year and there appeared to be no build up in soil labile phosphorus (P) (soil P in equilibration with solution P), as indicated by solution P(i), with time after 6 annual applications. At greater rates of SSP application there was evidence of accumulation of labile P as indicated by increased solution P(i) concentrations. Application of PR increased solution P(i) concentration up to 90 kg P ha(-1) rate and the value marginally decreased at 120 kg P ha(-1) rate. Maximum P(i) concentration was observed every year about 6-10 months after PR application. Application of SSP at 60 kg P ha(-1) or greater rate resulted in enhanced solution P(i) up to 75 mm depth of soil sampling. Application of PR at equivalent rates of P did not significantly increase solution P(i) below 20 mm depth. It was concluded that for permanent pastures PR/S is as effective as SSP from the first year of application whereas PR from the fourth to sixth year.