Plant Tissue Analysis to Assess Phosphorus and Potassium Nutritional Status of Corn and Soybean

Core Ideas There is a need for developing more reliable interpretations for tissue tests in corn and soybean. Tissue testing has no widespread use in many regions partly due to lack of reliable interpretations. We determined critical P and K concentrations at vegetative and reproductive growth stages. There is a need for reevaluating the value of tissue testing in corn ( Zea mays L.) and soybean [ Glycine max (L.) Merr.] for current yield levels and genotypes. The objective of this research was to determine tissue critical P and K concentrations for these crops at vegetative and reproductive growth stages. Response trials were conducted in Iowa at 30 sites for P (32 site‐years with corn and 34 with soybean) and at 53 sites for K (67 site‐years with corn and 52 with soybean). We sampled whole plants at the V5‐V6 stage, corn ear‐leaf blades at the R1 stage, and trifoliate soybean leaves at the R2‐R3 stage. Critical concentration ranges (CCRs) were defined using linear‐plateau and quadratic‐plateau models. All models fit significantly ( P ≤ 0.01). Model R 2 values were the lowest (0.31–0.45) for corn plant P, soybean plant and leaf P, and soybean plant K; intermediate (0.51–0.53) for corn plant and leaf K and soybean leaf K; and the highest (0.62–0.64) for corn leaf P. For corn plants and leaves, CCRs were 4.8 to 5.5 and 2.5 to 3.1 g P kg¯ 1 and 18.8 to 25.4 and 10.6 to 14.2 g K kg¯ 1 , respectively. For soybean plants and leaves, CCRs were 3.3 to 4.1 and 3.5 to 4.7 g P kg¯ 1 and 18.9 to 22.7 and 15.6 to 19.9 g K kg¯ 1 , respectively. Testing of corn ear‐leaves for P was more accurate than for plants but either tissue provided similar K assessments. Testing soybean plants for P was more accurate than for leaves, but for K testing of leaves was more accurate than for plants.