Poorly soluble phosphates in Australian superphosphate: their nature and availability to plants

Abstract

Australian superphosphates contain approximately 20 % of their phosphorus content as residual apatite and two water-insoluble phosphates, Ca(Fe0.12Al0.88)H(HPO4)2F2.2H20 and (Fe1.88Al1.12) (K0.28Na0.72)H8(PO4)6.6H20 that result from the use of impure rock phosphates. These compounds resemble phosphates occurring in American and New Zealand fertilizers, and comprise the acid and citrate soluble phosphorus fractions that are determined by statutory chemical tests. Glasshouse trials with wheat indicate that they are less effective phosphorus sources than monocalcium phosphate and will probably reduce the agronomic effectiveness of superphosphate.