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Tissue Testing for P and K Deficiency in Soybean

An article in Iowa State University’s ICM News entitled Value of Soybean Tissue Testing for Phosphorus and Potassium authored by Dr. Antonio Mallarino sheds new light on using tissue testing as a tool for making phosphorus (P) and potassium (K) fertilization decisions for soybeans.

In an extensive study that included 86 site-years (34 for P and 52 for K) on 17 soil series in Iowa, tissue tests were used to evaluate P and K concentrations in aboveground soybean plants at the V5-V6 growth stage, and in upper fully mature trifoliate leaves at the R2-R3 stage. Relative soybean seed yield for each site-year was calculated for as a percentage of the maximum observed yield.

Initial soil test levels of P and K across site-years ranged from very low to very high, and seed yield ranged from 22 to 73 bu/acre. Statistically significant yield increases occurred in 22 of the 34 P site-years and in 22 of the 52 K site-years.

Yield increased with increasing P and K tissue concentrations, but the relationships were not statistically high. The relatively poor relationships between P and K tissue concentrations and yield indicate that tissue testing for these two elements is not an acceptable way to predict the magnitude of yield response that can be expected.

The author concluded that tissue tests can be used to assess the in-season sufficiency of P and K in soybean, but the results also indicate that tissue testing is not better than soil testing as a diagnostic tool. He also concluded that tissue testing is of doubtful value as a tool for correcting P and K deficiencies in a soybean crop.

Finally, he concludes that tissue testing should be used to complement but not substitute for soil testing as the primary method for making fertilization decisions for a soybean crop. Its primary value is likely for confirming soil P and K deficiencies in field areas that exhibit poor performance.

The results from this body of work underline the importance of soil testing to determine P and K fertility needs for soybeans.

Composed by Larry G. Heatherly, June 2016,