A laboratory study of the effect of time and temperature on the decline in Olsen P following phosphate addition to calcareous soils

The effects of time and temperature on the changes in Olsen P after phosphate application were studied in 13 calcareous soils from Pakistan, an Oxisol from Colombia and an Inceptisol from England. The phosphate sorption reactions were monitored in two stages. The short-term reaction (30 min shaking with added phosphate in the presence of the Olsen bicarbonate solution) showed that over this time the nature of the sorbing material and number of available sites for P adsorption were important but temperature was not. The extent of the short-term sorption was not related to the amount of calcium carbonate. In the long-term reaction (incubating the soils with phosphate at 10, 25 and 45 degreesC for one year) the amount of Olsen P decreased with time following a power relationship. Increased temperature increased the rate of reaction, following the Arrhenius principle i.e. Q(10) approximate to 3 (activation energy 83 kJ mol(-1)). The effects of time and temperature were well described by a modified power equation Y = a (1 + f(T)t)(-b), where Y is the amount of Olsen P extracted after time t, a is the Olsen P value after the short-term reaction (the initial value), f(T) is the ratio of the rate constants at any two temperatures and b is a coefficient which represents the loss in extractability with time. On the basis of the initial Olsen P values and subsequent Olsen P values at different times and temperatures a unified decay curve Y/a = (1 + t)(-0.20) was developed where the initial Olsen P values are normalized to 1. The parameters of this equation allow, with limitations, the prediction of changes in Olsen P in these soils if the initial Olsen P value of the soil is known.