Plant availability of secondary phosphates depending on pH in a peat-based growing medium

Phosphorus (P) fertilizer consumption will deplete global reserves of rock phosphate within the next few centuries. Several technologies have been developed within the last decade to recover phosphate from P-rich waste streams. Recycling P fertilizers (secondary phosphates) is gaining increasing interest but little is known about the possible use of such products in soilless plant production systems. In this study, a greenhouse experiment with Tagetes patula × T. erecta ‘F₁ Zenith Red’ as P sensitive model plant was conducted to determine the plant availability of three secondary phosphates: precipitated magnesia ammonium phosphate (Struvite), sewage sludge ash (SSA) and metallurgic granulate (Mephrec). Water soluble monocalcium phosphate (CaP) and rock phosphate (rock-P) were included as controls. The growing medium was peat limed to two pH levels (4.5-5.0, 5.5-6.0). Calcium chloride/DTPA (CAT) and calcium-acetate-lactate (CAL) soluble P was extracted at the beginning of the experiment to test if these solvents were suitable to predict plant availability of the P fertilizers. After 6 weeks of cultivation, dry matter and P uptake of the plants were determined. Ranking of dry weight at pH 4.5-5.5 was CaP = Struvite = SSA = rock-P ≫ Mephrec and at pH 5.5-6.0 CaP = Struvite > SSA > rock-P > Mephrec. Ranking of plant P uptake was Struvite > CaP > SSA=rock-P ≫ Mephrec at pH 4.5-5.5 and Struvite > CaP ≫ SSA=rock-P > Mephrec at pH 5.5-6.0. Among plants fertilized with SSA or rock-P, a significant increase in dry weight and P uptake was found by decreasing the pH from 5.5 to 6.0 to 4.5-5.5 in the growing medium. In comparison to CAL, CAT extracted a slightly lower amount of P from the growing medium. Neither CAT nor CAL was able to predict the increased P uptake from SSA and rock-P at pH 4.5-5.0.