Making Herbicide Rate Recommendations Based on Soil Tests

Percent soil organic matter content, as determined by standard chromic acid oxidations, was highly (r = 0.89) correlated with soil humic matter content, as determined by NaOH/DTPA (diethylenetriaminepenta-acetic acid)/alcohol extraction of 201 U.S. soils. Humic matter content of the soils was equally or better correlated (r = 0.89 to 0.97) with herbicide bioactivity, as measured in field experiments, than was percent organic matter content (r = 0.87 to 0.92). Regression equations provided allow herbicide rate recommendations for 80% weed control to be calculated based on soil humic matter or soil organic matter levels.