Post-Season Diagnosis of Potassium Deficiency in Soybean Using Seed Potassium Concentration.

Potassium deficiency of soybean [Glycine max (L.) Merr.] may result in yield loss without foliar symptoms (hidden hunger). Seed nutrient concentrations may be useful for identifying reasons for lower than expected yields (e.g., K deficiency) or perhaps for estimating seed quality parameters. Our objective was to develop a relationship between seed-K concentration and relative soybean yield. Seed-K concentration and yield data across multiple fertilizer-K rates were collected from research conducted at 33 site-years in Arkansas and from published research conducted in Illinois (1 site-year), Iowa (33), Missouri (1), Tennessee (6), Australia (4), and Canada (24). We calculated relative yield for each site-year separately by dividing the untreated control yield by the highest K-fertilized yield and multiplying by 100. The seed-K concentration at which relative yield plateaued or reached 95% of maximum was considered as critical. The relative yield of soybean receiving no fertilizer-K ranged from 40-110% and seed-K concentrations ranged from 10.0-23.5 g K kg^-1 across all site-years. Seed-K concentration accounted for 76-78% of the variation in relative yield. The relationships between relative yield and seed-K concentration increased to 85-87% for 30 site-years that had soil-test K <91 mg kg^-1 and yield >3 Mg ha^-1. Seed-K concentrations from 15.9-17.1 g K ha^-1 were predicted critical for all site-years. For irrigated soybean in Arkansas, seed-K concentration explained 62-63% of the variability in relative yield for all site-years and 76-78% for 19 site-years that yielded >4 Mg ha^-1. The critical seed-K concentrations for Arkansas ranged from 15.6-16.2 g K kg^-1. For non-irrigated soybean in Iowa and Canada, seed-K concentration accounted for 51 and 83%, respectively, of the variation in relative yield and the critical seed-K concentrations varied from 15.4-19.0 g K kg^-1. Across all site-years, the seed-K concentration of 15.9-17.1 g K kg^-1 should be considered critical for over 95% relative yield.