EFFECT OF SOIL PROPERTIES ON HYDROLYSIS OF AMMONIUM POLYPHOSPHATE ANDTETRAPOTASSIUM PYROPHOSPHATE IN SOME ARID AND SEMIARID SOILS

Hydrolysis of ammonium polyphosphate (APP) and tetrapotassium pyrophos phate (TPP) was studied in Typic Camborthids, Typic Torripsamments, an d Typic Ustochrepts, varying in organic carbon, calcium carbonate, and clay content. A surface (0-15 cm) sample of each soil was treated wit h 400 mg P kg(-1) soil and incubated at 25 +/- 2 degrees C at a moistu re level of 60% saturation. Subsamples were removed at completion of 1 , 3, 7, 14, 21, 28, 56, 84, and 112 days of incubation and were analyz ed for hydrolysis of condensed phosphates. The kinetics of hydrolysis was satisfactorily described by two parallel first-order rate equation s having relatively fast and slow rates. The first rate constant (k(1) ) was 9.71- to 33.33-fold of the second rate constant (k(2)). Highly s ignificant positive correlation between organic carbon content and rat e of hydrolysis of APP was obtained (r = 0.89, p = 0.01). The rate of hydrolysis of phosphates increased with clay content and decreased wit h CaCO3:organic carbon ratio only in soils not varying in other proper ties. The positive effect of clay on hydrolysis of condensed phosphate was due to positive correlation between organic carbon and clay. Hydr olysis of condensed phosphates to orthophosphate did not go to complet ion during the 112 days of the experiment. A rather stable fraction am ounting to 2.5-25.0% of the applied condensed phosphates tended to per sist in the soil in unhydrolyzed form.