How much compost or mulch do you need? For mulching, spread 1-3 inches of compost on beds in fall or spring. As a soil amendment before planting new beds, use 1-3 inches of compost dug or tilled into the soil. (Use 3 inches to improve sandy soils, or 1-2 inches for heavy clay soils). How thick … Continue reading Compost & Mulch Calculator