Plant tissue analysis to assess phosphorus and potassium nutritional status of corn and soybean in Iowa

Interest in re-evaluating the value of tissue testing to assess P and K status in corn (Zea mays L.) and soybean (Glycine max L. Merr.) is increasing. The objective of this research was to study relationships between grain yield response to P and K and the concentration of these nutrients in plant tissue. Single-year and multi-year response trials were conducted in Iowa at 30 sites for P (32 sites-years with corn and 34 with soybean) and at 53 for K (67 sites years with corn and 52 with soybean) that encompassed 17 soil series. We sampled above-ground plant parts at the V5-V6 growth stage, corn ear-leaf blades at the R1 stage, and uppermost trifoliolate soybean leaves at R2-R3 stage. Critical concentration ranges were defined using linear-plateau and quadratic-plateau models. All models fit significantly (P ≤ 0.01) and R2 values were 0.31-0.45 for corn plants P, soybean plants and leaf P, and soybean plant K; 0.51-0.53 for K in corn plants and leaves and soybean leaves K; and 0.62-64 for corn leaf P. Critical concentration ranges were 4.8-5.5 and 2.5-3.1 g P kg-1 and 18.8-25.4 and 10.6-14.2 g K kg-1 for corn plants and leaves; and 3.3-4.1 and 3.5-4.7 g P kg-1 and 18.9-22.7 and 15.6-19.9 g K kg-1 for soybean plants and leaves. We conclude that P testing of corn ear-leaves at R1 was better than of young plants at V5-V6 but either tissue provided similar K assessments. Testing soybean plants for P at V5-V6 was better than testing leaves at the R2-R3 stage but K testing of leaves was better than K testing of plants.