Effect of soil organic matter on the phytotoxicity of soil‐applied herbicides — field studies

Abstract

Field trials were conducted over 3 years to study the phytotoxicity of 5 soil‐applied herbicides on 13 Horotiu sandy loam soils with 8.0–19.3% organic matter. Oats and soya beans were grown as indicator plants to test the activity of herbicides. Phytotoxicity of all five herbicides was highly and negatively correlated with the soil organic matter content. Two to three times as much herbicide was required for similar weed control in the highest organic matter soil compared with the soil lowest in organic matter. In general, there was an inverse relationship between herbicide water solubility and its inactivation by organic matter. The activity of trifluralin was most affected by changes in the organic matter content and terbacil was least sensitive to such variations. Regression equations for the amount of herbicide required for 80% weed control in oats and soya beans were computed for each herbicide. The actual rate of herbicide required at various organic matter levels varied depending on the test species, but the relationship between herbicide activity and soil organic matter content was fairly similar for the two species.