Recycled nutrients supply phosphorus for organically-managed wheat and forage crops

Recycling urban nutrients onto farmland has the potential to improve phosphorus (P) supply and yields on P-depleted organic farms, reduce global reliance on non-renewable mined phosphate rock, and reduce environmental contamination caused by excess P. Three recycled nutrient sources (struvite from municipal wastewater, frass from insect-processed food waste, and anaerobic digestate of food waste) were compared with common conventional (mono-ammonium phosphate, MAP) and organic (livestock compost) P sources in field experiments in spring wheat and forage hay on a high-pH (8.1–8.3) soil. Experiments were conducted on a low-P site (3 mg kg−1 Olsen P) over 2 years with conventional- and organically-selected wheat genotypes. In wheat, P uptake increased by 172%, 130%, 92%, and 43% compared to an unfertilized treatment for frass, MAP, compost, and digestate, respectively. Frass, MAP, compost and digestate increased grain yield by 41%, 40%, 29%, and 20%, respectively. Struvite did not significantly increase yields or P uptake. For both P uptake and grain yield, frass and compost treatments were similar to the “conventional” MAP treatment, while digestate and struvite treatments were lower than MAP. An organically-selected wheat genotype produced greater biomass but similar yield compared to a conventional genotype, demonstrating some genotype advantage. In the forage-hay crop, cumulative P uptake increased by 124%, 99%, 86%, 73%, and 65% compared to an unfertilized treatment for frass, MAP, digestate, struvite, and compost, respectively. Yield increased only in the second year of the trial, by 136%, 125%, 112%, 94%, and 79% for frass, struvite, MAP, digestate, and compost, respectively. The recycled nutrient sources varied in their efficacy in the two crops and relative to other nutrient sources, but all showed some potential to supply P and improve yields on a P-depleted soil.