A variety of microorganisms are known to solubilize different insoluble inorganic phosphates. These include fungi, bacteria, and actinomycetes. Many trials have been conducted with innoculation of soils, crops, and the phosphate itself with the various microbes.

The important mechanisms involved in the dissolving of insoluble phosphates in soil include formation of organic acids and chelating substances. During composting, there is concentrated microbial activity and significant amounts of organic acids and chelating substances are produced. Compost of plant material, cattle dung, old compost, and rock phosphate were mixed in a ratio of 6:1:0.5:2.5 on a dry weight basis. The rock phosphate was a carbonate apatite of sedimentary origin, containing about 67% apatite. The mixed material was allowed to decompose for 3 months in pits.

The water soluble phosphorus decreased with the addition of rock phosphate to the composted material. However, the phosphorous soluble in 2% citric acid increased dramatically after 90 days. This fraction is presumed to be available to plants.

Similarly, the response of pigeon peas (Cajanus cajanus) to addition of compost that included rock phosphate was dramatic. The P-enriched compost was comparable or better than single superphoshate in crop response and phosphorus uptake in neutral soils.

Adding rock phosphate to compost is one method farmers might use to better utilize the limited phosphorus available to them. Responses will depend on the composition of the rock phosphates and soil types.

Mishra, M. and K. Bangar. 1986 Biological Agriculture & Horticulture 3:331-340.

For more information:

M. Mishra
Dept. of Microbiology
Haryana Agricultural University
Hissar 125004, India