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PLANT TISSUE TESTING FOR PHOSPHORUS AND POTASSIUM IN CORN return to table of contents by Antonio P. Mallarino, Ph.D., Professor, Department of Agronomy, Iowa State University, and Andrew Stammer, Graduate Student, Department of Agronomy, Iowa State University BACKGROUND • Soil testing is a useful and commonly used diagnostic tool for phosphorus (P) and potassium (K) but involves some uncertainty because it estimates soil supply from a relatively small sample of surface soil taken before planting. • Tissue testing has long been suggested as a complementary tool. It has not been widely implemented in the North Central region, however, because of limited calibration research with yield response to P or K. • Iowa State University (ISU) has no P or K tissue test interpretations for any crop. Many states do not have them, or if they do, they are based on old research that may not apply to current hybrids and yield levels. OBJECTIVES • Field research was conducted as part of the DuPont Pioneer Crop Management Research Awards (CMRA) program. • The objective was to evaluate the value of tissue testing for P and K in corn to complement soil testing and guide fertilization. STUDY DESCRIPTION • Conventional corn yield response trials that included several P and K application rates were conducted at 73 sites in 2013 and 2014. The study also included results of 20 K trials and 6 P trials conducted from 2003 to 2010. The combined research encompassed a total of 99 site-years, 32 for P and 67 for K. • The trials were performed at fields of several ISU research farms distributed across the state that encompassed 17 soil series. Some trials were managed with no-till and others with chisel-plow/disk tillage. Hybrids, seeding dates and rates, and weed control practices were those normally used at each farm. • Initial soil-test P and K ranged from very low to very high according to published ISU interpretations. • Total P and K concentrations in aboveground corn plants were evaluated at the V5 to V6 growth stage and in ear-leaf blades opposite and below the main ear at the R1 stage (silking). Research to evaluate P and K tissue testing as a complement to soil testing was conducted across multiple years at 32 locations for P and 67 locations for K. • Relative grain yield response was calculated for each site-year by expressing the yield (15% moisture) for each treatment as a percentage of the statistically maximum observed yield with fertilization. • Critical concentrations distinguish between conditions of nutrient deficiency with likely response to fertilization from conditions with adequate levels and unlikely response. • A critical concentration range was determined for each nutrient, crop, and tissue with the linear-plateau (LP) and quadratic-plateau (QP) response models. • These models estimate critical concentrations at a 100% nutrient sufficiency level and have been used in many previously published studies. RESULTS – CORN YIELD RESPONSE • In the P trials, average yield across treatments and trials ranged from 135 to 238 bu/acre with statistically significant yield responses in 27 of the 32 site-years. The largest yield increase was 77 bu/acre. • In the K trials, yield ranged from 78 to 233 bu/acre with significant yield responses in 37 of the 67 K site-years. The largest yield increase was 111 bu/acre. 74

eturn to table of contents RESULTS – PHOSPHORUS Corn Relative Grain Yield (%) Corn Relative Grain Yield (%) • The yield response to P decreased and the relative yield increased with increasing tissue P concentrations (Figure 1). • The critical concentration ranges were 0.48 to 0.55% P for young plants (V5 to V6) and 0.25 to 0.31% P for ear leaves (R1). • The proportion of variation explained (R 2 ) was very similar for the LP and QP models but higher for the ear leaves than for the young plants. Therefore, a P test for ear leaves at silking was better than for young plants. 100 90 80 70 60 50 100 90 80 70 60 50 QP: 0.55, R 2 0.34 LP: 0.48, R 2 0.35 0.2 0.3 0.4 0.5 0.6 0.7 Corn Plant P at V5-V6 (%) QP: 0.31, R 2 0.62 LP: 0.25, R 2 0.64 0.1 0.2 0.3 0.4 0.5 Corn Leaf P at R1 (%) Figure 1. Relationship between yield response and P in plants at the V5-V6 stage and leaves at the R1 stage. Critical concentrations and models R 2 values are shown. RESULTS – POTASSIUM • The yield response to K also increased with increasing tissue K concentrations (Figure 2). • The critical concentration ranges were 1.88 to 2.53% K for young plants and 1.06 to 1.42% K for leaves. • The proportion of variation explained (R 2 ) was very similar for the response models and tissue tests. Therefore, K tests based on young plants or leaves showed similar performance. Corn Relative Grain Yield (%) Corn Relative Grain Yield (%) 100 90 80 70 60 50 QP: 2.53, R 2 0.53 40 LP: 1.88, R 2 0.51 30 0 1 2 3 4 5 6 7 Corn Plant K at V5-V6 (%) 100 90 80 70 60 50 40 QP: 1.42, R 2 0.53 LP: 1.06, R 2 0.53 30 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Corn Leaf K at R1 (%) Figure 2. Relationship between yield response and K in plants at the V5-V6 stage and leaves at the R1 stage. Critical concentrations and models R 2 values are shown. CONCLUSIONS • The reliability of tissue testing for P was significantly better for leaves at silking than for young plants at the V5 to V6 growth stage. • In contrast to results for P, the reliability of tissue testing for K was similar for young plants and leaves at the silking stage. • The identified critical concentration ranges were lower or in the lower portion of previously published sufficiency, which were developed mostly from older research or for other regions. • However, the results showed that tissue testing is not better than soil testing as a diagnostic tool and should be used to complement and not substitute widespread use of soil testing. 75

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