Phosphorus availability to maize from partially acidulated phosphate rocks and phosphate rocks compacted with triple superphosphate

Two phosphate rocks (PR), moderately reactive Hila PR from Colombia and slightly reactive Capinota PR from Bolivia, were compacted with triple superphosphate (TSP) such that 50% of total P in the mixture was in water-soluble form. The effectiveness of these materials as phosphorus sources was compared with that of partially acidulated phosphate rocks (PAPR) at 50% acidulation with sulfuric acid and TSP in Hartsells silt loam (pH 4.5) with maize as the test crop. Huila PAPR and Huila PR compacted with TSP were as effective as TSP as phosphorus sources. Huila PR was only half as effective. The slightly reactive Capinota PR with 8.8% Al₂O₃ + Fe₂O₃ content was not suited for direct application, and Capinota PAPR was only half as effective as TSP. Capinota PR compacted with TSP, however, was as effective as TSP. PR compacted with TSP, urea, and KCl was no more effective as phosphorus source than PR compacted with TSP alone.