The effect of soil Ph on olsen bicarbonate phosphate values

The effect of increases in soil pH by liming on Olsen bicarbonate phosphate (P) test values was investigated using two contrasting New Zealand soils: a highly P retentive Egmont soil and a Tokomaru soil with a low P retention capacity. After incubation with three rates of Ca(OH)2, soil pH was increased, and a reduction in the Olsen P values was observed. The amounts of 32P‐exchangeable P, however, increased with increasing soil pH, and there was very little change in water‐extractable P. The addition of lime had no significant effect on either the yield or uptake of P by ryegrass grown on the same soils in the glasshouse. It appears that the decline in Olsen P values in soils following liming is a result of an artefact in the Olsen procedure. High concentrations of Ca are involved in the decrease in Olsen P values in limed soils. It is suggested that the precipitation of calcium phosphates in the Olsen extracts of limed soils is responsible for the decrease in the Olsen P values. In a field experiment, increasing soil pH by lime addition also resulted in a decrease in the Olsen P values.