Key Outputs From Reactive Phosphate Rock Research in New Zealand

摘要

Some key outputs from the last 20 years of research into direct application of reactive phosphate rocks (RPR) to New Zealand pastures are discussed. Combined research on the chemical reactivity and agronomic performance of phosphate rock (PR) has indicated that extraction with 2% formic acid, or a dissolution test performed in a simulated soil solution at a fixed pH, will provide improved measures of RPR quality compared to the currently used 2% citric acid. Currently PRs that have more than 30% of their total phosphate soluble in 2% citric acid have been classed as "reactive" and have been recommended on pasture soils with pH <6 and in annual rainfall regimes exceeding 800 mm. Field trials to evaluate the relative agronomic effectiveness (RAE) of RPR versus soluble-P fertilizers in adequate to marginally P-deficient pasture required long periods (3-6 years) of fertilizer withdrawal for pasture growth on some soils to become significantly responsive to applied P. Relative yield measurements from Presponsive trial sites were used to relate measurements of the extent of RPR dissolution in soils to their agronomic effectiveness. Three main modeling approaches, Kirk and Nye (1986a,b,c); Sinclair et al. (1993a); and Watkinson(1994b), have been used toexplain RPR dissolution in field trials. A combination of the Sinclair and Watkinson models gives a good explanation of the RAE of RPR compared with soluble-P fertilizer at five trial sites that vary in soil type and climate. Soils fertilized with RPR-Psources or soluble P were shown to require separate Olsen soil test P/yield response calibration curves. Tests involving mixed cation and anion resin membranes appear to be more appropriate for soils with unknown histories of soluble P and RPR use. Experiences gained from these research programmers in New Zealand should prove useful for others wishing to conduct further research on RPR as a direct application fertilizer.