Plant availability of phosphorus in recycled fertilizers from wastewater treatment - effect of grain size

Due to the depletion of global phosphorus deposits and stricter legal regulations regarding phosphorus loads in wastewater, the recovery of phosphorus from wastewater is gaining more and more interest. There are plenty of techniques to recover phosphorus from wastewater and sewage sludge. Depending on the recovery technique and the subsequent processing, the availability of phosphorus to plants can differ widely. However, beside the chemical characteristics also grain size of the fertilizer, as is well known from rock phosphates, might be essential for plant availability. Thus, the plant availability of phosphorus in three wastewater-based fertilizers - sewage sludge ash (SSA), Mephrec and struvite - in relation to their grain size was evaluated in a pot trial with marigold. Recycled P fertilizers were applied to a peat based growing medium on basis of 30 mg P per pot (1) as received and (2) after grinding to a fine powder with a disc mill. For comparison, rock phosphate (Hyperphos) was treated in the same way and water soluble mono calcium phosphate (CaP) was used as benchmark. Marigold seedlings were pricked in 10-cm pots and cultivated for seven weeks according to common horticultural practice in a glass-sheltered greenhouse. With exception for finely ground struvite, P uptake of plants from the recycled fertilizers was significantly less than from CaP. However, P uptake from coarse struvite and powdered SSA was at least 50% of P uptake from CaP, whereas it was less than 5% for the remaining treatments. For all three recycled fertilizers as well as for the used rock phosphate grinding significantly increased P uptake, but only for SSA the effect was of practical relevance. The results reveal that grinding can improve the plant availability of phosphorus in recycled fertilizers, but the effect strongly depends on the chemical characteristic of the product.