Effect of Soil Organic Matter on the Phytotoxicity of Thirteens-Triazine Herbicides

Abstract

The influence of soil organic matter on the initial and residual phytotoxicity of thirteens-triazine herbicides was investigated in greenhouse experiments using three Horotiu sandy loam soils with organic matter levels of 9.8, 15.5, and 20.6%. The amount of herbicide required to reduce the growth of oats (Avena sativaL. ‘Mapua’) by 50% (GR50) when compared with the control was determined for each herbicide and each organic matter level. Results showed that the GR50values for all herbicides were highly and positively correlated with the soil organic matter. In general, the phytotoxicity of compounds of high water solubility was less influenced by soil organic matter than those having low water solubility. The chloro-triazines persisted longer in soil than did the methoxy- or methylthio-triazines. Simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] were the most persistent of the chloro-triazines.