Evaluating Soil Micronutrient Bioavailability of Manganese In Glyphosate-Resistant Soybeans.

Laboratory experiments and greenhouse studies were performed to determine the potential cause of manganese (Mn) deficiency in glyphosate-resistant crops. The goal was to examine whether the herbicide caused binding of Mn, thus rendering it nonbioavailable for plant root uptake, or if soil matrix conditions (i.e., soil pH, electrical conductivity, cation exchange capacity, bioavailable micronutrients, and/or sorption capabilities) were involved in the response of Mn in the soil-plant environment. Soil used for this study was an eastern Wyoming Mollisol (fine-​​loamy, mixed, mesic Fluventic Haplustoll) that was collected from the Sustainable Agriculture and Research Extension Center (SAREC) in Lingle, Wyoming. The greenhouse experiment utilized three varieties of glyphosate-resistant soybeans, amending the soil with five rates of Mn prior to planting, and adding two rates of Roundup® Weathermax herbicide plus untreated blanks of all factors. After a 12-week growth period, biomass productivity, chlorophyll production, and total Mn concentrations in the soil and plants were evaluated. Several laboratory experiments were conducted to determine soil solution sorption kinetics of Mn, glyphosate, and glyphosate-Mn interactions. In addition, experimentation of water-soluble Mn, bioavailable Mn fractions, and total Mn concentrations were determined.