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

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.