Abstract Glasshouse experiments were conducted to determine the influence of soil organic matter on the phytotoxicity of five soil-applied herbicides using six Horotiu sandy loam soils with organic matter contents between 8.0 and 22.1%. Herbicides were incorporated in the top 5 cm of soil in the pots, and test plants grown therein for 5 weeks. A range of rates enabled a GR50 value to be determined (amount of herbicide required for 50% growth reduction) for each herbicide and each soil organic matter level. Results showed that the GR50 values were highly and positively correlated with the soil organic matter. From 2 to 4 times more herbicide was required for a GR50 at 22.1% soil organic matter than at 8%, depending on the herbicide. Soil organic matter affected herbicide performance as follows: terbacil < atrazine < alachlor < linuron < trifluralin. In general, the herbicides of relatively high water solubility would be expected to be more effective in higher organic matter soils than chemicals of low wate...
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