Comparing the agronomic performance of soluble and slow release phosphate fertilisers: the experimental basis for RPR recommendations

Abstract

Recent results from field trials comparing the agronomic effectiveness of water soluble fertilisers (single superphosphate (SSP), triple superphosphate (TSP)) and fertilisers of low water solubility ('slow release', reactivephosphaterock (RPR) fertilisers) are reviewed. It is shown that the pasture production data from the 'National Series' of trials are consistent with, and can be described by, a model for the dissolution of RPR in soil. Applying both the pasture production data and the dissolution model the term 'lag time' associated with 'slow release' RPR is defined and quantified for New Zealand conditions. Results show that on average the rate of release of P from RPR is about 30% within the year of application, 23% in year 2 and progressively less in subsequent years. It follows that when RPR is applied annually, the amount of Preleased annually, from the current application and from the residues of previous annual applications, is 30,53,70,82, 9 1,96% as a fraction of the total P applied annually. Consequently about 3.5 times the amount of RPRP is required to achieve the same yield as soluble P in year 1, about 2 times in year 2 and 1.5 in year 3. The lag time is defined as the time required to accumulate sufficient RPR residues in the soil from applications such that the annual amount of P dissolved from RPR each year is equal to or greater than 90% of the amount of total RPR-P applied annually. The lag time associated with RPR use is about 4-6 years depending on the site. The agronomic performance of RPR based on the National Series data was not associated with soil pH (5.1 to 6.3), annual rainfall (700 to 1800 mm) or soil phosphate retention (13-98%). This probably reflects the narrow range and confounding effects of the soil and climate factors. The experimental basis for the current soil pH and rainfall boundary conditions are briefly discussed. Available evidence suggests that the P dissolved from RPR has the same agronomic effectiveness as P from soluble fertiliscrs. The agronomic implications of these results on P fertilisers of intermediate solubility (i.e. PAPR and Longlife) are discussed in relation to field results. Keywords agronomy, comparison, dissolution, fertilisers, Longlife, PAPR,phosphorus, RPR, slow release, soluble P, single superphosphate, triple superphosphate