Hydrolysis of ammonium polyphosphate (APP) and tetrapotassium pyrophosphate (TPP) was studied in a Typic Ustochrept having 9.6, 32.8 and 48.0 exchangeable sodium percentage (ESP) at 25 ±2°C and 60 per cent of water holding capacity. There was differential hydrolysis of both condensed phosphate at different ESP levels. The kinetics of hydrolysis Was satisfactorily described by two parallel first order rate equations, having relatively fast and slow rates. The first rate constant, k1 was 16.82 to 33.30 fold of second rate constant k2. The k1 for TPP decreased progressively wiih increasing ESP. The k2 for hydrolysis of both phosphates increased with increasing ESP. Hydrolysis of condensed phosphates to orthophosphate was not complete during 112 days of experiment. A rather stable fraction which amounted to 10 to 15 per cent of the applied condensed phosphates tended to persist in soil.
Ammoni~m polyphosphate, tetrapotassium pyrophosphate hydrolysis, rate constants, ESP
