Soybean Response to Potassium and Phosphorus Fertilization in Mississippi.

Soybean (Glycine max (L.) Merr.) is grown on the greatest hectares of any row-crop produced in Mississippi.  However only 13 and 26% of the total hectares receive P and K fertilization annually.  Limited recent research exists in Mississippi regarding correlation of soil test indices to plant nutrient concentration and/or yield.  With the current shift in hectares from cotton (Gossypium hirsutum L.) to grain based systems, more producers are seeding soybean on hectares that has greater potential to become deficient in both P and K unless supplemental fertilization is supplied.  Beginning in 2012 research was established at the Delta Research and Extension Center and at off-station producer sites to evaluate soybean yield response to both P and K fertilization rate.  The main goal of the project is to correlate both Lancaster and Mehlich-3 soil test P and K with soybean tissue concentration and relative grain yield.  Research was initially established during 2011 at the Delta Research and Extension Center, Stoneville, MS and from 2012 to present on production farms within the Mississippi Delta.  Individual trials evaluating P or K response were complete randomized blocks with each treatment replicated five times. Prior to fertilizer application, approximately 6 0-15 cm soil cores were collected from the unfertilized control in each replicate for soil test analysis.  For each nutrient, five total P₂O₅ or K₂O rates were hand applied to individual plots.  Eleven total siteyears have been established for K response trials with three positive yield improvements (p < 0.10).  Soil test indices at the responsive sites ranged from 89 to 212 mg K kg^-1.  Eight siteyears evaluating P response have provided 4 responsive sites (p <0.10).  Average soybean yield improvement over the four sites was 470 kg ha^-1.  Soil test P measured from responsive sites ranged from 30 to 55 mg P kg^-1.