Better Together: Water Treatment Residual and Poor‐Quality Compost Improves Sandy Soil Fertility

Water treatment residual (WTR) is an underused clean water industry byproduct, generally disposed to landfill. This study assesses the benefits and risks of ferric‐WTR as a soil amendment or co‐amendment for plant growth in a nutrient‐poor sandy soil. A 12‐wk pot trial tested the efficacy of WTR and a locally available, low‐quality, municipal compost as single (1, 5, and 12.5% dry mass) and co‐amended treatments (1:1 WTR/compost ratio, at 2, 10, and 25%) on wheat ( Triticum aestivum L.) growth in a sandy soil. The low total N content of the compost and low WTR P and K contents resulted in significantly lower (up to 50% lower, p < 0.05) plant biomass in single amendments compared with the control, whereas the highest co‐amendment produced significantly higher plant biomass (33% higher, p < 0.05) than the control. This positive co‐amendment effect on plant growth is attributed to balanced nutrient provision, with P and K from the compost and N from the WTR. Foliar micronutrient and Al levels showed no toxic accumulation, and co‐amended foliar Mn levels increased from near deficient (20 mg kg −1 ) to sufficient (50 mg kg −1 ). Total WTR metals were well below maximum land application concentrations (USDA). Trace element bioavailability remained the same (Ni, Cu, and Hg) or significantly decreased (B, Al, Cr, Mn, Fe, Zn, As, and Cd; p < 0.05) during the pot trial. These results suggest, within this context, that a WTR–compost co‐amendment is a promising soil improvement technology for increasing crop yields in sandy soils. Core Ideas Iron (Fe) water treatment residual (WTR) is a byproduct of potable water production. Wheat biomass increased by 33% when poor‐quality compost and Fe‐WTR were combined. Combined amendments and control outperformed single amendments of compost and WTR. Bioavailable metals measured after the pot trial did not increase. Co‐application of compost with WTR may counter the negative effects of P sorption.