Reevaluation of Soil-Test-Based Potassium Fertilizer Recommendations for Louisiana Soybean
Applied Research
Md Rasel Parvej
Soil Fertility Specialist
LSU AgCenter
Winnsboro
Abstract
Most land-grant universities have developed fertilizer-K recommendations based on soil-test K concentration from 0-4, 0-6, or 0-8-inch soil depth. Although topsoil-K concentration is a good indicator of fertilizer-K need for soybean [Glycine max (L.) Merr.], it sometimes gives false positive error i.e., soil-K concentration indicates a positive yield response to fertilizer-K but in fact it does not occur. This can partly be due to K leaching down to the soil profile (6-12-inch or deeper), which soybean plants can easily access resulting in no yield response to added K. We evaluated soybean yield response to five different fertilizer-K rates (0, 40, 80, 120, and 160 lb K2O ac-1) and two different soil-K concentrations from 0-6 and 0-12-inch depths across 32 sites in Louisiana from 2020 to 2021. Based on 2-yr results, fertilizer-K increased soybean yield at 14 of 32 sites. Soil-K concentration explained 74% of relative soybean yield variability for 0-6-inch depth and 69% for 0-12-inch depth. The critical soil-K concentrations were similar across all soil types for both soil depths (123 and 126 ppm for 0-6-inch and 0-12-inch depths, respectively) and were 100% accurate in predicting K deficient sites. Our critical soil-K concentration does not support the current critical soil-K concentrations for Louisiana soybean, which vary with soil types (98-141 ppm for silt-loam and 142-274 ppm for clayey soils at 0-6-inch depth). Tissue K and crop sensing results at the R2 stage showed that both leaflet- and petiole-K concentrations and visible atmospherically resistant index (VARI) were strongly correlated with relative soybean yield. However, plant VARI was better in identifying K deficient sites and correlating with soil-K concentrations at both depths than tissue K concentrations. Our 2-yr data suggest that it may be possible to develop only one critical soil-K concentration (110-135 ppm K) across all soil types for soybean production in Louisiana. Plant VARI can be alternative tool in diagnosing in-season K deficiency and help in rescuing soybean yield loss. Our calibration model suggests that soybean requires more K to maximize yield than currently recommended rate under low and very low soil-K levels. This study will be continued in 2022.
Poster has NOT been presented at any previous NACAA AM/PIC
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Authors: Md Parvej
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Parvej, M. Soil Fertility Specialist, Louisiana State University Agricultural Center, Louisiana, 71295