EFFECTS OF PEST AND SOIL-MANAGEMENT SYSTEMS ON WEED DYNAMICS IN POTATO

Results from the ''Potato Ecosystem Project,'' a cropping systems stud y in northern Maine, were used to test the hypothesis that greater rel iance on organic nutrient sources and less reliance on synthetic ferti lizer sources can benefit weed management efforts. 'Atlantic' potato w as grown in a 2-yr rotation within a factorial arrangement of three pe st management systems, two soil management systems, and both rotation entry points. Weed control in the conventional (CONV) pest management system relied on full rates of herbicides, whereas the biointensive (B IO) system relied exclusively on cultivation. The reduced input (RI) p est management system relied on cultivation in 1991 and 1992 and on 50 % of standard herbicide rates plus cultivation from 1993 to 1995. The two soil management systems, unamended (barley/red clover rotation cro p; 1x synthetic fertilizer for potato) and amended (pea/oat/hairy vetc h green manure rotation crop; manure, compost, and 0.5x synthetic fert ilizer for potato) contrasted practices typical for the region to thos e designed to achieve rapid improvements in soil quality. Midseason we ed biomass in potato was dominated by common lambsquarters. In 1991 an d 1992, weed biomass in potato was least in the CONV system and did no t differ between the RI and BIO systems. In 1993, weeds in both RI and CONV potatoes were effectively suppressed below the level measured in the BIO system. Soil management had no effect on weed biomass from 19 91 to 1993 but became an important factor affecting weeds in the BIO s ystem in 1994 and 1995. Weed biomass was 77% lower in 1994 and 72% low er in 1995 in the amended soil management system than in the unamended system. No significant yield loss due to weeds was detected in the 19 94 BIO system, but in 1995 yield loss due to weeds was 37% in the unam ended system compared to 12% in the amended system. Soil management ef fects on weeds in the 1994 BIO pest management system carried through to the following season's germinable seed bank. Density of germinable common lambsquarters seed (0 to 10 cm soil depth) in the 1995 BIO syst em was 4,082 m(-2) in the unamended soil management system compared to 1,280 m(-2) in the amended soil management system. We suggest that or ganic amendments and green manure promote a potato crop better able to compete with weeds and that these inputs be considered as potentially important components of integrated weed management systems that have minimal reliance on herbicides.