Improving crop growing conditions with water treatment residual and compost co‐amendments: Soil–water dynamics

Land application of water treatment residual (WTR) in combination with phosphate‐rich organic wastes, like compost or sewage sludge, in nutrient‐poor soils was previously shown to promote crop growth. This WTR diversion from landfill to agriculture supports local and international mandates for waste circularity. Although soil–water dynamics—like saturated hydraulic conductivity, water retention, and hydrophobicity—are well‐defined for compost and somewhat defined for WTR (except for hydrophobicity), the impacts of co‐amending sandy soils with both are not well‐defined. In laboratory analyses, co‐amendment had an intermediate effect between individual amendments on the hydrophobic sandy soils, increasing water retention by 27% (WTR and compost both increased water retention), decreasing hydrophobicity by increasing hydraulic conductivity twofold (WTR and compost both decreased hydrophobicity), and having no effect on saturated hydraulic conductivity (decreased by WTR and increased by compost). With two positive effects and one “no effect” on soil–water dynamics in laboratory trials, the co‐amendment was expected to buffer both crop water use efficiency (WUE) and nutrient availability under drought stress, for Swiss chard (Beta vulgaris L. var. cicla), co‐investigated in a multifactorial pot trial. Soil nutrients, particularly phosphate, were shown more critical than soil–water dynamics to improve crop WUE. Thus, co‐amended soils have significantly higher crop biomass and WUE than sandy soils. Phosphate‐rich organic co‐amendment is necessary for crop nutrient sufficiency and thus drought resilience in sandy soils amended with WTR. Thus, pairing wastes to soils for optimum fertility is a critical consideration in waste land application for both biomass and drought resilience. Core Ideas Water retention is increased in sandy soils by water treatment residual (WTR), compost, and their co‐amendment. Hydrophobicity is decreased in sandy soils by WTR, compost, and their co‐amendment. Saturated hydraulic conductivity is increased by compost, decreased by WTR, and co‐amendment has no effect. WUE strongly correlates with soil phosphate and crop biomass, more than soil–water parameters. WTR must be co‐applied with a nutrient‐rich waste, promoting crop biomass and consequently drought resilience. Plain Language Summary Sludge wastes from the clean drinking water treatment process (called water treatment residuals, WTR) can improve sandy soil, for growing crops. Sludges