Date of Graduation

5-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Crop, Soil & Environmental Sciences (PhD)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Trenton Roberts

Committee Member

Larry Purcell

Second Committee Member

W. Jeremy Ross

Third Committee Member

Nathan Slaton

Fourth Committee Member

Md. Rasel Parjev

Fifth Committee Member

John Rupe

Keywords

Plant nutrition; Potassium; Soybean

Abstract

Potassium (K) deficiency is one of the most important yield limiting factors in Arkansas soybean (Glycine max (L.) Merr.) production, although there are recently improved diagnostic and management opportunities available. However, several questions regarding the successful implementation of in-season K management in soybean production remain. Therefore, the research objectives were to (a) delineate the relationship between drought stress and K deficiency on soybean vigor, (b) identify the spatial dependencies of leaf-K concentrations at the production scale, (c) develop a leaf sampling protocol, (d) calibrate the fertilizer-K rate needed to correct varying levels of K deficiency at 15, 30, and 45 days after first flower (DAR1), and (e) evaluate the economics of in-season K management in soybean. The research encompassed complementary rain-out shelter and field experiments at the small plot scale as well as commercial production fields. Drought stress significantly (P < 0.05) reduced the total K uptake (TKU), aboveground biomass production, and yield, with greater reductions when drought stress was imposed during reproductive growth. Across producer managed fields, no consistent spatial dependencies were found in the leaf-K concentrations indicating that a soybean tissue-K grid sampling protocol cannot be generalized to a specific area size. Therefore, one composite sample consisting of at least 18 of the upper-most fully expanded trifoliolate leaves from each management zone is appropriate to capture the area’s crop nutritional K status. Additional small plot research was conducted on silt loam soils to quantify the level of deficiency with trifoliolate leaf samples and assess the yield response to corrective applications of 0 to 149 kg K ha-1 at 15, 30 and 45 DAR1. As expected, higher levels of deficiency required higher rates of fertilization to reach the yield goal. Linear plateau models were considered for each timing, using the join point as the threshold determining whether a corrective application is warranted. The parameters of the significant (P < 0.05) individual models predicting the 15 and 30 DAR1 rate recommendations were not statistically different (P = 0.902). Therefore, data were combined and a new model fit, providing a predicted site-specific fertilizer-K rate recommendation from R2 to 30 DAR1. At both the 15 and 30 DAR1 times, economic ramifications associated with corrective applications were quantified in a payoff matrix by calculating yield averages, partial returns (PR), and regret, each assuming 5-year average prices. At both times, large increases in PR and differences in regret were reported when in-season K deficiencies occurred. The 45 DAR1 results failed to provide a reasonable calibration curve (P = 0.401), suggesting that this is too late to correct a K deficiency in soybean. Overall, the ability to follow a leaf sampling protocol to appropriately capture a soybean field’s K status and use the results to determine a site-specific, calibrated fertilizer-K rate will enable producers to diagnose and correct deficiencies in-season and maximize both yield and profit. These research findings are the first to provide site-specific fertilizer-K rate recommendations for in-season applications and accompany the information with economic payoff matrices to facilitate informed management decisions.

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