Estimating Fertilizer Substitution Values from the Dissolution Rates of Reactive Phosphate Rocks in New Zealand Pastoral Soils

A relationship is developed between the substitution value of reactive phosphate rock (RPR) relative to triple superphosphate (TSP), both applied annually to pasture, and the dissolution rate of the RPR. The residual effect of dissolved phosphate is included. Residual RPR is calculated from the dissolution rate of the RPR in the soil, while residual phosphate from TSP or dissolved RPR is taken as a constant fraction of the value in the previous year. Calculations including the residual effect of dissolved fertiliser produce smaller substitution values and longer lag times for RPR than when the effect is omitted. The relationship derived from RPR dissolution rate constants was consistent with available experimental data from dry matler.