Effect of incorporation depth and soil organic matter content on herbicidal activity

The effect of herbicide incorporation depth on the relationship between soil organic carbon and application rates of 3 pre‐emergence herbicides was studied under glasshouse conditions. The herbicides studied were alachlor, bromacil, and metribuzin, which are important for many horticultural crops. They were incorporated in the top 1, 2, or 6 cm of 4 Horotiu sandy loam soils varying in organic carbon content from 6.1 to 11.9%. Oats ( Avena saliva L.) were used as the test species, and the seeds were planted at a depth of 1.5 cm. Incorporation depth did not alter the effect of soil organic carbon content on the herbicidal activity of alachlor. However, increased percent organic carbon reduced bromacil and metribuzin activity more when these herbicides were incorporated in the top 1 cm compared to the top 6 cm of soil These results are consistent with the relative importance of shoot or root zone uptake of these herbicides. They also demonstrate that incorporation depth can alter organic matter‐herbicide relationships, and provide another criterion which must be considered in improving the predictability of soil‐applied herbicide activity.