Partitioning the nutrient and nonnutrient contributions of compost to dryland-organic wheat

Organic materials supply nutrients to plants but may also have other, nonnutrient-related benefits which are more difficult to quantify. This study partitioned the winter wheat (Triticum aestivum L.) yield response from compost applications into nutrient and nonnutrient fractions. Composted dairy manure and wheat straw bedding was applied at five rates (0, 10, 25, 50, and 75 Mg dry weight ha(-1)) to dry-land wheat in an organic wheat-fallow farming system. The compost was fall-applied and incorporated prior to planting at adjacent sites in sequential years. Maximum grain yield increases from compost application ranged from 2,139 kg ha(-1) in a year with 186% of average annual precipitation to 1,324 kg ha(-1) in a year with 87% of average annual precipitation. The Mitscherlich equation was used to describe the yield response to compost rate. The nutrient and nonnutrient contributions of compost to grain yield were partitioned by solving the Mitscherlich equation for compost rates where applied nutrients were in surplus (greater than or equal to25 Mg ha(-1)), calculating a nonnutrient Mitscherlich response function, and subtracting the nonnutrient response function from the full response to determine the nutrient contribution across compost rates. At the 10 Mg ha(-1) compost rate, the nonnutrient to nutrient yield response ratio varied from 0.25:1 for the year with above average precipitation to 2.2:1 for the year with below average precipitation. Compost significantly increases dryland wheat yields. These results suggest that nonnutrient benefits of compost applications may be significant and exceed nutrient benefits under dryland production in low rainfall years.