SOME EFFECTS OF MOISTURE AND TEMPERATURE ON TRANSFORMATION OF MONOCALCIUM PHOSPHATE IN SOIL

Pre-weighed monocalcium phosphate pellets, containing about 15 milligrams of P, were placed in 200 grams of soil and stored for 2 weeks at four moisture tensions and three temperatures. Pellet residues were then removed and the amount of phosphorus remaining was determined. Small cores containing pellet residues and the surrounding soil contacted by fertilizer solution were removed for determination of water-soluble and total inorganic P. Phosphate phases present at the granule sites and the surrounding soil were identified by their optical properties.The mean amount of phosphorus remaining at the granule sites was 20.2 per cent. Although both moisture tension and temperature significantly affected the quantity of phosphorus retained, no consistent trend was apparent. Residues remaining at the site of application were found to be mixtures of anhydrous and dihydrated dicalcium phosphate, with the latter predominating. Moisture tension and temperature did not greatly alter the proportion of the two phases.Periodic precipitates or Liesegang rings of dicalcium phosphate were formed in the soil surrounding monocalcium phosphate pellets. Dicalcium phosphate dihydrate was the predominant phase. The proportion of dihydrated to anhydrous dicalcium phosphate increased as the temperature decreased and as the moisture tension increased.Water-soluble P increased significantly with increased moisture tension and was significantly greater at 5 °C. than at either 16 or 27 °C. The mean of all treatments was 5.6 per cent. Increased amounts of dicalcium phosphate dihydrate in the surrounding soil seemed to be responsible for the increase in water solubility.Between 89.5 and 99.2 per cent of the added phosphorus was recovered in the water and acid extracts of soil cores containing about 1.4 cm.3 of soil.