Humification of organic matter in soil affecting availability of phosphorus from its mineral compounds

Release of available P from insoluble phosphate compounds (basic slag and rock phosphate) during humification of clover hay in a clay alluvial soil (pH 7.9) was studied at 60% water‐holding capacity and at 20 ± 2°C for a period of 180 days. Content of total humified carbon in soil treated with insoluble phosphate compounds was higher than that in P‐untreated soil. Humic acid predominated over fulvic acid in all treated soils. Ratio of HAIFA was always >1.0, and it decreased following the application of organic material and of inorganic P compounds. Presence of basic slag diminished the HAIFA ratio more than did the rock phosphate. Humic‐substance associated P and available P contents increased with the addition of organic material and of insoluble phosphate compounds. The content of both forms of P in the humic substances was higher in the presence of basic slag than in the presence of rock phosphate.