Plant Tissue Testing for Phosphorus and Potassium in Corn
- 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.
- 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.
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.
- 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 at 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-V6 growth stage and in ear-leaf blades opposite and below the main ear at the R1 stage (silking).
- 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.
Results - Phosphorus
- 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-V6) and 0.25 to 0.31% P for ear leaves (R1).
- The proportion of variation explained (R2) 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.
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 R2 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 (R2) was very similar for the response models and tissue tests. Therefore, K tests based on young plants or leaves showed similar performance.
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 R2 values are shown.
- The reliability of tissue testing for P was significantly better for leaves at silking than for young plants at the V5-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.
Authors: Antonio P. Mallarino, Professor, Department of Agronomy, Iowa State University and Andrew Stammer, Graduate Student, Department of Agronomy, Iowa State University
Research funding provided by the Iowa Soybean Association and the DuPont Pioneer Crop Management Research Awards (CMRA) Program. The CMRA program provides funds for agronomic and precision farming studies by university and USDA cooperators throughout North America. The awards extend for up to four years and address crop management information needs of DuPont Pioneer agronomists, Pioneer sales professionals and customers.