Evaluation of a Rapid Method for Assessing Soybean Potassium Nutritional Status.

Potassium deficiency of soybean [Glycine max (Merr.) L.] is common in Arkansas. Visual symptoms of K deficiency may be absent or subtle during early and mid-reproductive growth stages. A rapid and economical method of assessing the K nutritional status of soybean would allow growers to monitor K during the season. The research goal was to assess the utility of petiole sap to monitor soybean K nutritional status to identify critical sap-K concentrations during reproductive growth stages for aiding K fertilizer management decisions. Specific objectives were to examine how petiole sap-K concentration i) compares to trifoliolate leaflfet- and petiole-K concentrations via traditional plant analysis, ii) responds across time during reproductive growth, and iii) varies among fertilizer-K rates. Twenty petioles and trifoliolate leaflets were collected weekly for 6 wk from selected fertilizer-K rates in three K fertilization trials, Sap-K was extracted from 10 petioles and the other petioles and leaflets were dried, ground and digested in concentrated HNO₃ for analysis. Sap-K concentration was determined using the Horiba LAQUA Twin Potassium Compact Ion Meter (HKIM, Horiba, Kyota, Japan). Sap-K concentration (n = 162, data from all site-years and sample times) was positively and linearly related to trifoliolate leaflet- (R² = 0.72) and petiole-K (R² = 0.65) concentrations. The relationship between petiole- and leaflet-K concentrations was also positive and linear but had a numerically greater R² value of 0.92. For the individual site-years, petiole sap-K concentration was a negative linear or quadratic function of time (days after the R1 stage) that depended on fertilizer-K rate with R² values ranging from 0.63 to 0.90. These preliminary results suggest sap-K concentration is a good rapid indicator of soybean K nutritional status but requires additional research. Research to characterize soybean sap nutrient concentrations will continue in 2017.