Phosphorus and Potassium Tissue Testing to Guide Fertilization of Corn and Soybean.

Interest in tissue testing as a method of guiding fertilization and detecting nutrient deficiencies in corn and soybean in the upper Midwest is increasing. No current yield-response based calibrations for tissue test results are available in Iowa. Calibrations that are available are old or are from outside the Midwest. Plant-tissue samples were collected from 226 crop response trials conducted during the 1990s, 2000s and in 2013 across a wide range of soil series, tillage management, and climate conditions.  Corn tissue samples were collected at the V6 growth stage (six exposed leaf collars) and the R1 stage (silking). Soybean samples were collected at V6 stage (six unfolded trifoliate leaves) and the R2-R3 stage (flowering to beginning pod). The aboveground portions of plants were collected at the V6 stage while top mature leaves with petioles were collected at R2-R3. Samples were analyzed for total tissue concentrations of P and K. Various curvilinear response models were fit to relationships across site-years between the relative grain yield response to fertilization and the tissue P or K concentration. There were statistically significant (P ≤ 0.05) relationships for all crops and tissue sampling dates. Determined critical concentration ranges based on linear- and quadratic-plateau models for corn were 3.7-4.0 and 2.2-2.6 g P kg^-1 and 25-33 and 9.5-11 g K kg^-1 for young plants and leaves, respectively (R² were 0.27 to 0.40). Critical concentrations for P in soybean tissue could not be determined (R² were only 0.02 to 0.03) and for K were 19-24 and 15-19 g K kg^-1 for young plants and leaves, respectively (R² were 0.24 to 0.60). Identified critical concentrations were lower or within the lower range of previously reported sufficiency ranges for other regions. We concluded that tissue testing for P and K in corn and soybean is not a better diagnostic tool than soil testing.