Soybean Seed Potassium Concentration and Seed Yield

Potassium (K) deficiency in soil is known to result in reduced soybean seed yields, and yield increases from K fertilization of deficient soils are common. Soybean plants may not show visible K deficiency during the growing season, or deficiency symptoms may only be visible during the latter stages of reproductive development. Thus, yield losses due to K deficiency are unlikely to be avoided during the growing season of the deficiency. However, proper diagnosis of this deficiency is imperative so that correction can be made for a subsequent crop.

In a report published in Soil Sci. Soc. of America Journal 80:1231-1243 (2016), Arkansas scientists used data from 100 site-years of replicated field research that had been conducted in Arkansas, Indiana, Iowa, Missouri, Tennessee, Virginia, and Canada to determine soybean yield response to K fertilization. Their objective was to determine the relationships between seed- and soil-K concentrations and relative soybean yield, and use these findings to develop potential seed-K concentration thresholds that can be used to diagnose soil-K deficiency as a yield-limiting factor of soybean. This objective was based on the premise that the K concentration in mature soybean seed might provide a tool that can be used to explain lower-than-expected seed yields when K-deficiency symptoms are not obvious. Results from the Arkansas observations follow.

•    Average yield (irrigated) from sites that received no fertilizer K and sites that received fertilizer K averaged 58 bu/acre and 64 bu/acre, respectively.

•    Soybean seed-K concentration accounted for 66% of the variability in relative yield.

•    Relative yield increased linearly as seed-K concentrations increased, and plateaued when seed-K concentration reached 1.63%.

•    The critical range of seed-K concentration was determined to be 1.56% to 1.70%. Thus, seed-K concentrations <1.55% were considered deficient, and those >1.71% were considered sufficient.

•    93% of the sites that produced seed with K concentrations deemed deficient produced significant yield responses from added K fertilizer that averaged 9.1 bu/acre. Absolute yield difference attributed to fertilizer K was greatest at sites that produced seed with deficient seed-K and least for sites that produced seed with sufficient seed-K concentrations.

•    The difference in seed-K concentration decreased as soil-test K concentration increased, with seed-K difference plateauing when soil-test K concentration was equal to or greater than about 87 ppm (this equates to about 180 lb/acre K).

For the combined North American data, the following results were obtained.

•    Soybean seed-K concentration accounted for 60% of the variability in relative yield of unfertilized soybean, and the critical seed-K concentration was predicted as 1.71%, with a 95% confidence limit of 1.65% to 1.77%. Thus, using the entire data set provided results similar to those obtained from using only data from Arkansas.

•    77% of the sites that produced seed with a deficient seed-K level showed a significant yield benefit from fertilizer K that averaged 7.2 bu/acre. This compares to 93% of the Arkansas sites and a yield increase of 9.1 bu/acre.

•    Seed-K concentration of soybean receiving no fertilizer K increased linearly as soil-test K concentration increased to about 179 ppm, and seed-K concentration plateaued at 1.88%. Soil-test K concentration explained 40% of the variability in seed-K concentration of soybean that received no K fertilizer.

•    For soybean receiving K fertilizer, seed-K concentration increased linearly until soil-test K concentration reached about 170 ppm, and seed-K plateaued at 1.91%. Soil-test K concentration explained only 24% of seed-K variability when K fertilizer was applied.

•    The relationships between seed-K and soil-test K concentrations suggest that both fertilizer- and soil-K availability influence soybean seed-K concentration when soil-test K concentrations are less than about 170 to 179 ppm (this equates to about 340 to 360 lb/acre K).

The above relationships led the authors to the following conclusions.

•    The proposed deficient seed-K concentration of <1.65% correctly identified fields that responded to fertilizer K 77% of the time. Thus, soybean seed-K concentration can be used to diagnose soil-K deficiency for soybean production.

•    When soil-K availability is less than about 180 ppm, both fertilizer- and soil-K availability influence soybean seed-K concentration.

•    Seed analysis cannot be used to correct K deficiency during the growing season, but can be used as a post-harvest tool for diagnosing reasons for low seed yield and for correcting soil K deficiency for a subsequent crop.

As stated in a Tissue Testing White Paper posted on this website, tissue testing can be used to assess the in-season sufficiency of P and K in soybean, but is of doubtful value as a tool for correcting P and K deficiencies in a current soybean crop. Rather, it should be used to complement soil testing for making fertilization decisions for a soybean crop. Its primary value is likely for confirming P and K deficiencies in a soybean crop that has performed poorly. This same conclusion can be applied to the seed-K results from the above cited study.

In that same White Paper, results from another study are cited that showed that K concentrations of soybean leaves and petioles can be used to assess K deficiency or sufficiency in a soybean crop. The study authors proposed critical leaf- and petiole-K concentrations that can be used to diagnose K deficiency or sufficiency in plants in order to diagnose possible K deficiency as a yield limiting factor.

All of the above cited results indicate that tools are now available that can be used to definitively determine if K deficiency is a contributor to lower-than-expected yields from a particular production site. Also, these tools can be used to determine if K fertilizer recommendations resulting from a properly conducted soil test were indeed sufficient to ensure adequate soil K fertility for an intended or expected soybean yield.

Composed by Larry G. Heatherly, Jan. 2017, larryheatherly@bellsouth.net