Raising Soybean Yield Potential in Dry Seasons: Increased Rooting Depth and Greater Soil Water Extraction with Deeper Depth to the Fragipan

The primary goal of this research is to raise soybean yield potential, in dry seasons, on fragipan soils. Fragipan soils occupy a significant portion of Kentucky's crop land. The most significant limitations associated with soybean production on fragipan soils are excessive wetness in the spring and increased probability of crop moisture stress in the summer. Both limitations are due to the presence of the fragipan and both limitations become more severe as the depth to the fragipan lessens (becomes shallower). Field work at the Princeton Research and Education Center suggested that sodium (Na) salt amendment might cause fragipan depth to increase. The initial field work must be verified, and a valid mechanism/explanation found. If the fragipan depth is lowered, is the fragipan merely dissolving (fragipans contain a cementing agent) or is the fragipan dissolving and reforming, at a deeper depth? Additionally, the subsoil above the fragipan is often very acid (pH values of 4.0 to 5.0). Work in other regions indicates that gypsum application can significantly reduce the impact of subsoil acidity on root growth and soil moisture extraction by corn. No such comparable information exists for soybean. There is interest in determining whether application of poultry litter and/or gypsum would enhance soybean root development, soil moisture availability, and raise soybean yield potential in dry summer seasons. Recently, work by Karathanasis, Matocha and Murdock suggests that excess sodium and potassium (K) could assist with dispersion and break-up of the fragipan. This caused these investigators to add a NaCl (road salt) treatment in the spring of 2014, and we wish to consider an added potassium treatment (at a rate well above that needed for soybean K nutrition) to evaluate any field response. As potassium chloride (KCl), muriate of potash, is the cheapest form of soil-mobile K, this source will be added in this field trial in the spring of 2015. The major goal of this work is to determine whether any of these amendments (poultry litter, gypsum, NaCl, KCl), or any combination of these treatments, will result in greater soybean yield potential on fragipan soils - and then to determine the mechanism behind that increased yield potential so as to extend the results of the experiment to other soils. In 2012, 2013 and 2014, which were very dry, very moist, and moist, respectively, poultry litter has resulted in a positive soybean yield response, with the largest benefit in 2014, the latest year of the study (Table 1, below). Gypsum had not benefitted soybean yield until 2014, when a small yield benefit was observed (Table 1c, below). The road salt (NaCl) application reduced soybean yield, except in the presence of poultry litter, where the NaCl had no impact on yield. This proposal is to fund an extension of our study so that the impact of these other amendments might be thoroughly investigated.