Exploring the Feasibility of Integrating Weed and Nitrogen Management with Seed Meals in Organic Vegetables

Abstract

Corn gluten meal (CGM) and soybean meal (SBM) have demonstrated value as bioherbicides and organic fertilizers, but suggested application rates usually target either weed suppression or crop nutrition, not both. The objective of this study was to explore the feasibility of integrating weed and nitrogen management by evaluating effects of increasing seed meal rates within planting holes of plastic mulch film on weed density, soil nitrogen availability, and crop yield in tomato (Solanum lycopersicum) and broccoli (Brassica oleracea). CGM (10% N) or SBM (7% N) were applied at rates of 0.5, 1, 2, 3.5, or 5 g planting hole−1 N (depending on crop and year) after crops were transplanted, and 40 weed seeds per planting hole were seeded. Weed density decreased with increasing seed meal rate, regardless of type, and velvetleaf (Abutilon theophrasti) was more susceptible than the grass weeds tested. Velvetleaf suppression at the 5 g planting hole−1 N rate ranged from 66% to 97%, relative to the weedy control. Soil nitrogen availability increased with the application rate, but ammonium mineralized from seed meals applied at the highest rates were likely phytotoxic to weeds and crops. Seed meals never increased the crop yield and reduced the tomato yield in 2018 by 39% to 64%, relative to the weed-free control. The results suggest that integrating the management of weeds and nitrogen with seed meals in plastic mulch planting holes is not feasible because application rates required for consistent weed suppression are also toxic to crops.