How effective are Calciphos and Phospal?

Data from 194 published pot and field experiments were used to calculate the fertilizer effectiveness of two commercially available calcined iron-aluminium rock phosphate fertilizers— Calciphos and Phospal. A very wide range of effectiveness values (RE) relative to superphosphate have been reported for freshly applied fertilizers ( < 0.1 to 3.0). These differences are primarily not due to differences in citrate soluble P, soil pH, plant species and mean annual rainfall. For both fertilizers most of the variation in published values is due to the use of poorly responsive soils and to incorrect methods of fertilizer assessment. Lower RE values (< 0.1 to 0.5) were derived for experiments on highly P responsive soils and when several levels of P were applied to provide complete response curves. For these experiments high RE values (about 1.0) for Calciphos only occurred for sandy soils in which water-soluble P was rapidly leached. The residual value of Calciphos and Phospal remained low relative to freshly applied superphosphate.