The Correlation of Herbicidal Activity with Soil and Climatic Factors

Abstract

The phytotoxicity from preemergence applications of the herbicides 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), 3- (3,4-dichlorophenyl)-l,l-dimethylurea (diuron), isopropyl N -(3- chlorophenyl) carbamate (CIPC), 2-chloroallyl diethyldithiocarbam- ate (CDEC) and 2-chloro-N,N-diallylacetamide (CDAA) was measured at 17 field locations in the coastal plain of North Carolina over three years. Site selections were made with specific regard for soil organic matter, which varied from 0.7 to 48.8 percent. Phyto- toxicity responses determined at four to five weeks after treatment were the amounts of herbicide required to cause a 50 percent reduction in (1) soybean dry shoot weight, (2) cotton dry shoot weight and (3) number of weedy annual grasses. Fourteen factors were measured at each location to characterize soil, climatic and biotic factors, and correlated with plant responses obtained. For simazine, diuron and CIPC the factors measured accounted for 60 percent of the plant response variation. The highest simple corre- lation was usually between soil organic matter and plant response. Phytotoxicity of the aromatic herbicides was inversely correlated with soil organic matter level. CDAA and CDEC usually gave reli- able and repeatable performance at constant rates on soils containing between 3 and 40 percent soil organic matter. Below 3 percent soil organic matter, extreme variation in CDAA and CDEC toxicity was encountered.