Validation of Critical Soybean Tissue Potassium Concentrations during Reproductive Growth Stages.

Tissue potassium (K) concentration can be used to monitor the sufficiency of plant K nutrition but critical K concentration ranges for soybean [Glycine max (Merr.) L] and most other plants are available at only one or two growth stages. Our research objective was to refine and validate preliminary continuous critical soybean tissue K concentrations during reproductive growth. Petiole and trifoliolate leaflets were collected throughout reproductive growth from soybean receiving different fertilizer-K rates grown in soils having a range of K availabilities. The SoyMap program was used to predict the R1 growth stage and begin tissue sampling. Soybean relative yield means and tissue-K concentrations from nine cultivar-site-years of research were taken weekly (time expressed as days after R1 stage, DAR1) to identify the critical tissue K concentration ranges at multiple growth stages. Relative yield was regressed against tissue-K concentrations in 7 d intervals during reproductive growth (0-7 to 63-70 DAR1) to predict the critical K concentration required to produce near maximal yield. The defined critical K concentrations for production of 90 and 95% relative yield were then regressed across DAR1 to define a continuous critical tissue concentration range. There was no significant relationship for leaflet or petiole K concentration 0-7 DAR1 (n = 10). The other nine intervals each had 20 to 40 data points and R² values ranged from 0.58 to 0.97 for leaflets and 0.52 to 0.83 for petioles. Quadratic relationships were delineated for the tissue-K concentrations needed to produce 90 and 95% relative yield regressed across the midpoints of each DAR1 interval. Equations that describe the curves will be presented. These defined relationships allow growers to assess the K nutritional sufficiency of soybean at multiple growth stage and make management decisions regarding the need for supplemental, in-season K fertilization. This research is ongoing and additional data are being added annually.