Many organically managed farms in Europe have low levels of soil phosphorus (P). Arable farms that rely strongly on biological nitrogen fixation (BNF) have been shown to have rather low outputs and a tendency to deplete soil P and potassium (K) compared with arable farms that have a lower reliance on BNF and higher external inputs. Therefore, research focusing on providing a balanced input of nitrogen (N), P, K and sulphur (S) from alternative sources is of interest to organically managed farms in Europe.

The aim of this study was to quantify P availability from different organic wastes applied alone or in combination to improve the mixtures’ N:P:K:S ratio. P availability was measured by P uptake and recovery in ryegrass grown in pots. The isotope dilution approach was used in which a non-labelled fertiliser is added to a soil that has been pre-incubated and equilibrated with labelled 33P.

The P recovery of the different organic wastes varied significantly (10–20 %). Manure and anaerobically digested manure mixed with ash from straw had the lowest P recovery. All the organic waste treatments had higher plant growth and P uptake compared with the negative control, but none of them reached the values observed after application of mineral P. Mixing digested manure with ash increased soil pH at the end of the experiment, which may explain the lower P availability. The highest P recovery was found in digested products, either manure alone or mixed with municipal waste or the industrial waste product Fertigro®. However, the mixture of digested manure and Fertigro® led to lower dry matter production, whereas Fertigro® used alone resulted in high leaf P concentrations but depressed shoot and root growth, presumably due to salinity effects and a decrease in soil pH. Anaerobic digestion increased the availability of P, which may be explained by the lower immobilisation potential of the remaining organic matter in the digestate.

This study highlights the potential challenges when attempting to improve the N:P:K:S ratios of waste-based fertilisers through mixing due to material interactions. However, such effects are likely to be overexpressed in pot trials that have a limited soil volume. Field trials are therefore needed to quantify such effects in practice.

Beatriz Gómez-Muñoz, Dorette Müller-Stöver, Veronika Hansen,Lars Stoumann, Jensen Jakob Magid