The determination of total phosphorus improves the accuracy of the bicarbonate extraction as an availability index

The efficient use of phosphorus (P) in agriculture should rely on accurate soil P tests (SPT). Organic P contributes to P supply to plants; however, it is not usually taken into account in assessing P fertilizer requirements. We hypothesized that there would be an increased accuracy of bicarbonate extraction as SPT in predicting P uptake by plants if total P (TP) in this soil extract is taken into account. We conducted a soil P depletion experiment with 36 soils involving four consecutive crops in pots. Molybdate-reactive P (MRP) and total P were determined in extracts centrifuged at 19,000 g (Bic-MRPC and Bic-TPC) or not (Bic-MRP and Bic-TP). MRP in extracts explained <47% of the variance in the cumulative P uptake, while total P (centrifuged at 19,000 g or not) provided the most accurate estimation of P uptake (59% with Bic-TP) and threshold values for fertilizer response (R-2 = 0.58 with Bic-TPc). When soils were separated in two groups according to their Ca carbonate equivalent to clay ratio, the variance in the cumulative P uptake explained by Bic-MRP was above 63%, and that explained by Bic-TP was above 73%. This separation also enabled more realistic estimation of the threshold values for fertilizer response. It can be concluded that the use of total P instead of MRP in bicarbonate extraction was promising in terms of improving its accuracy in assessing P fertilizer requirements.