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

Due to depletion of global phosphorus resources, there is an increasing interest in phosphorus 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 a remarkably amounts of phosphorus can be bound to iron or aluminum. In previous studies, the CaCl₂/DTPA (CAT) and CAL method partly underestimated plant availability of such sewage-sludge based recycling products, which might be due to a low solubility of iron- and aluminium-bound phosphorus in these two extractants. Thus, a pot trial with marigold was done: plants were fertilized with mono-calcium phosphate (MCaP), di-calcium phosphate (DCaP), tri-calcium phosphate (TCaP), aluminium 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, whereby no effect of pH was found for AlP but for FeP. Against expectations, plant availability was poorer at the higher pH. In contrary to fresh weight, P uptake of plants decreases from mono- to tri-calcium phosphate and with increasing pH. For aluminium and iron phosphates results for P uptake was 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 iron phosphate was strongly underestimated by CAT but matches CAL soluble P. Availability of aluminum phosphate was strongly underestimated by both extractants as well as by the combination of CAT and CAL soluble phosphorus. This supports previous findings for various sewage-sludge based products and raises the question of the need of other extractants.