ALTERNATIVE PHOSPHATIC FERTILISERS. NORTHLAND FARMERS, WEIGH THE EVIDENCE

Abstract

Fertilisers based on or enriched by reactive phosphate rock (RPR) compare favourably in price with superphosphate under certain conditions. The ultimate choice of fertiliser depends on three factors:- the agronomic effectiveness of phosphorus in RPR in different soils; the sulphur requirements of pasture and the effect of this on price; and the effect of high proportions of potassium chloride on the physical properties of fertilisers. The rate of dissolution of RPR is reduced by high pH and high P and Ca concentrations in the ambient solution. The so-called gumland soils, recognisable by their bleached subsoIls, have higher P and Ca concentrations in the soils solution than other soils in Northland, pH and P status being equal. They also tend to lose more sulphate by leaching than other soils, so maintenance of requirements of their pastures for sulphur are very high. In addition many of the gumland soils need low rates of phosphorus but relatively high rates of potash. Pending further field evidence, therefore, RPR based or :einforced fertilisers at present are recommended for hill country and for soils derived from basic or semi-basic volcanic materials, provided the latter soils do not require a very high proportion of potassium chloride for pasture maintenance. Keywords: Soil pH, calcium, phosphorus, pasture, organic matter, sulphur, potassium, superphosphate, phosphate rocks.