Soil Surface Chemistry As Influenced By Long-Term Glyphosate-Resistant Corn and Soybean Production in the Central Great Plains.

Glyphosate-resistant (GR) corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) production could affect soil organic matter (SOM) storage and nutrient cycling due to changes in quantity and quality of crop residue return to the soil surface. We determined soil chemical properties after 14-yr of various herbicide treatments, continuous GR corn (CC) and GR corn-soybean rotation (CS) under no-tillage. Herbicide treatments were 1) glyphosate at 0.42 and 2) 0.84 kg a.e ha^-1 each applied twice in all years, 3) an alternating treatment of glyphosate at 0.84 kg ha^-1 applied twice followed the next year by a non-glyphosate treatment, and 4) a non-glyphosate treatment in all years. Results showed that crop rotation and soil sampling depth had a significant effect on pH, exchangeable Ca, K, Mg, and available P concentrations. Soil pH, exchangeable Ca and K concentrations at the soil surface were significantly reduced by CC production. In contrast, SOM, Fe, Mn, and soil P concentrations in the top 7.5 cm of the soil increased with CC production. Glyphosate application affected concentrations of P, SO₄-S and Fe but not pH, Ca, Mg, K, Mn or Zn. Findings of this long-term study showed that herbicide program and crop rotation can affect soil chemical properties, but crop type and cropping system seems to have more profound effect on soil chemistry than herbicide treatment.