Combined compost and clay amendments improve water and nutrient retention in sandy soil

Context Low organic matter and clay content in sands result in poor water and nutrient retention, which limit the crop and pasture productivity on such soils throughout the world. Compost amendments to sands can increase the reactive surface area and improve nutrient and water holding capacity. However, the added nutrients in composts can be leached rapidly in sandy soils especially when they decompose quickly. Aims It is hypothesised that clay addition with compost could increase nutrient retention and reduce the nutrients leached in sands. Methods A leaching experiment was conducted in columns to investigate the effect of compost and clay amendments on water and nutrient retention under two different irrigation/rainfall regimes at two fertiliser rates. Results Compost addition at both 10 and 30 t/ha had positive effects on initial water retention in sand. Clay added at 6% under high irrigation conditions also significantly increased initial water retention compared to unamended sand. The clay and compost combinations reduced ammonium and phosphorus leaching by 30% and 30–50%, respectively, compared to unamended sands. The addition of clay at 6% with compost 30 t/ha reduced potassium leaching by 25% compared to compost only. Conclusions Compost at 30 t/ha ± clay retained significantly higher levels of phosphorus, potassium, sulfur, calcium, magnesium, zinc, boron and manganese than unamended sand and clay-only treatments in final soils under both irrigation and fertiliser rates. Implications Combined compost and clay amendments, compared to single applications of either compost or clay-only, improved water retention which could potentially support plant growth in sands during dry periods, and retention of nutrients that otherwise are prone to leaching on sands.