PHOSPHORUS BIOAVAILABILITY OF FERTILIZERS - A PREDICTIVE LABORATORY METHOD FOR ITS EVALUATION

A laboratory method and a laboratory index is proposed to estimate the phosphorus taken up by plants that is derived from fertilizers (Pdff). Pdff values were measured using greenhouse experiments and P-32 labelling technics. The laboratory index estimates the proportion of PO4-ions derived from the fertilizer in the soil solution and is measured by means of an isotopic exchange of (PO4)-P-32-ion procedure. This indicator was named JCF. Two typical soil-fertilizer conditions were studied. One concerned measurement of Pdff and JCF values for freshly-applied phosphorus as diammonium phosphate (DAP) at levels of 15, 30, 45, 60 and 90 mg P kg-1 soil. The other concerned measures of Pdff and JCF values for two types of P residues previously applied in soils as concentrated superphosphate (CSP) or Gafsa rock phosphate (GRP) applied at 0 and 43.7 kg.ha-1 each year over a 15 yr period. For freshly-applied DAP a linear relationship between Pdff and JCF values was obtained over the range of 0 to 90 mg P (kg soil)-1 levels of application: JCF = 1.16 Pdff + 1.78, (r2 = 0.98). For the P residues, JCF and Pdff values were not significantly different for a given residual treatment. However JCF and Pdff pair data for CSP treatments (56.0, 65.9) were about tenfold superior to those for GRP treatments (5.3, 4.6)). Consequently the nearly 1:1 ratio between JCF and Pdff values that was obtained for the two different soil-fertilizer conditions suggests that the proposed laboratory method can be used to predict availability of P fertilizers to plants. Thus it deserves to be considered in helping to estimate P fertilizer applications.