Residual value of superphosphate and rock phosphate on an acid soil II. Soil analysis and greenhouse experiments

1. Analyses and a greenhouse experiment on soils taken from a field experiment to estimate residual values of phosphates were used to show changes in the amount and distribution of residual phosphate in the soil.

2. Total and NaHCO2-soluble P were large in the top 0–3 in. of soil after applying superphosphate. Total P was large, but NaHCO3-soluble P much less, where rock phosphate had been applied in the field. Ploughing distributed total and NaHCO3-soluble P more uniformly through the 0–6 in. layer of soil.

3. Soil analysis and crop yields in the greenhouse showed that the residual value of equal amounts of P2O5 as rock phosphate and superphosphate, three years after application, were very similar.

4. At the end of the field experiment the sand fraction of the soil (2000–75 μ) contained about 80% of the phosphate that was applied as rock phosphate in this particle-size range.