Estimation of plant availability of different phosphates by the CaCl₂/DTPA and CAL method

Due to depletion of global phosphorus (P) resources, there is an increasing interest in P recycling products as fertilizer for growing media. However, in contrast to currently used fertilizers, which are almost completely based on calcium phosphates, at least in sewage sludge-based recycling products, remarkable amounts of P can be bound to iron or aluminum. In previous studies, the CaCl₂/DTPA (CAT) and calcium-acetate-lactate (CAL) method partly underestimated plant availability of such sewage sludge-based recycling products, which might be due to a low solubility of iron- and aluminum-bound P in these two extractants. Thus, a pot trial with marigold was undertaken where plants were fertilized with mono-calcium phosphate (MCaP), di-calcium phosphate (DCaP), tri-calcium phosphate (TCaP), aluminum phosphate (AlP) or iron phosphate (FeP) at growing media pH of 5.5 and 6.5, respectively. No effect on plant growth was observed for the three calcium phosphates, irrespectively of pH. However, plant fresh weight was reduced if P was applied as FeP and AlP. No effect of pH was found for AlP but for FeP. Against expectations, plant availability was poorer at the higher pH. Contrary to the trend for fresh weight, P uptake per plantdecreased from mono- to tri-calcium phosphate and with increasing pH. For AlP and FeP, results for P uptake were comparable to those reported for fresh weight. Whereas plant availability of the three calcium phosphates was well reflected by CAT and overestimated by CAL, P uptake from FeP was strongly underestimated by CAT, but matches CAL soluble P. Availability of AlP was strongly underestimated by both extractants alone as well as by the combination of CAT and CAL soluble P. This supports previous findings for various sewage sludge-based products and raises the question of the need of other extractants.