Evaluation of Mined Humates As a Source of Iron for Soybean [Glycine max. (L.) Merr.] As Influenced By Variety and Soil pH.

Evaluation of Mined Humates as a Source of Iron for Soybean [Glycine max. (L.) Merr.] as influenced by variety and soil pH. A.P. Elvir and J. J. Varco Soybean is the second most planted field crop and the main oilseed in the U.S. with approximately 34 million hectares cultivated. Soybean fertilization is not a routine practice unless a specific nutrient is low in the soil or deficiencies are identified during the growing season. Soybean Fe deficiency is only observed in calcareous soils due to an alkaline pH and free carbonates. Known commonly as lime-induced chlorosis, the resulting Fe deficiency causes stunting and poor nodulation which greatly impact yield. Different management options have been identified in order to solve this problem, including selection of Fe efficient varieties and chelated Fe fertilizer sources. Chelated Fe fertilization has proved to effectively supply the nutrient to the plant and correct Fe deficiency, resulting increased biomass production and yields under different soil conditions. The most common chelates used in agriculture are EDTA, DTPA, and EDDHA. Additionally, carbonaceous substances or humates have demonstrated yield improvements. Naturally occurring mined humates present an interesting possibility of enhancing Fe availability considering its contents of pyritic forms of Fe and its chelating property which allows maintaining its solubility. This study compared FeEDDHA, FeEDTA and a commercially mined pyritic lignite source under greenhouse conditions. Two soils, a Leeper (pH 8.1) and a Marietta (pH 6.1) and two soybeans varieties where one is classified as Fe efficient and the other as iron inefficient were evaluated across Fe sources. Active Fe, plant total Fe, SPAD readings, dry matter yield, and plant height were evaluated. Sources are being evaluated for their efficacy in supplying Fe, especially under a calcareous soil scenario where Fe deficiency severely impacts soybean production.