Herbicide Reactivity of Soil Organic Matter Fractions in No‐Tilled and Tilled Cotton

Abstract

AbstractBecause of the increased use of herbicides in no‐tilled soils and the difference in organic C distribution between tilled and no‐tilled soils, it is important to compare herbicide activity in different tillage systems. Soil humus fractions and characteristics were determined on surface Lexington silt loam soil (fine, silty, mixed, thermic Typic Paleudalf) samples of no‐tilled and tilled cotton field plots in West Tennessee. Soil humic and fulvic acid were extracted by standard methods. Total C ranged from 9.9 g kg−1 in the tilled soil to 13.5, 16.6 and 23.5 g kg−1 in the no‐tilled soils, with rye (Secale cereale L.), vetch (Vicia villosa Roth) and crimson clover (Trifolium incarnatum L.) cover crops, respectively. The tilled soil had the largest fraction of extractable C and fulvic acid, relative to total soil C. Humic acid was characterized by 13C‐NMR (nuclear magnetic resonance spectroscopy), titration of functional groups, and elemental analysis. Carboxyl groups determined by 13C‐NMR ranged from 6.1 to 7.7 mol kg−1 humic acid and compared more closely with total acidity than with carboxyl‐group determinations by titration. A greenhouse herbicide bioassay was conducted and sorghum [Sorghum bicolor (L.) Moench] growth was measured to indicate soil organic‐matter effects on the phytotoxicity of two herbicides. Herbicide activity was inversely related to soil C, extractable C, carboxyl groups of humic acid, and fulvic‐acid C. Fulvic‐acid C correlated best with herbicide activity. Decreased herbicide activity in tilled (no cover crop) soil was largely attributed to the C in tilled soil being in a more reactive state than C in no‐till (cover cropped) soil.