Spent Coffee Grounds (SCGs) as a Soil Amendment: The Effects of Composting Time on Early Sunflower Development

The unsustainability of current and future agricultural use has led to increased interest in using organic waste products, such as spent coffee grounds (SCGs) and compost, as alternative nutrient supplements. While SCGs are initially phytotoxic, upon composting, they can be utilised as a soil amendment; however, the minimum length of composting time required is not well known. Two glasshouse experiments were conducted to identify the concentration where raw SCGs are toxic to sunflower seedling growth and to assess the age and concentration at which composted SCGs are most effective for crop soil addition. Both raw and composted SCG substrates demonstrated higher water-holding capacities and electrical conductivity levels than commercial soil mix, though differences were observed in pH, with raw SCGs being acidic and composted SCGs being neutral. Concentrations of raw SCGs ≥ 35% caused large reductions in germination, plant height, cotyledon and true leaf emergence. SCGs composted for 6 months were non-toxic for sunflower seedlings and most effective at ≥35%, which could potentially reduce composting times by 50%. The addition of raw or composted SCGs also reduced the root/shoot ratio, though the cause and effects are currently unknown.