Soil testing is a useful and commonly used diagnostic tool for making phosphorus (P) and potassium (K) fertilization decisions for crops. Tissue testing has been suggested for decades as another tool to use to test for these and other nutrients.
However, tissue testing has not been widely used for P or K in Iowa or the north-central region of the U.S. because of inconclusive results from limited field calibration research that has associated yield response. ISU has no P or K tissue test interpretations for any crop. Thus, research was conducted the past few years by ISU to study the value of tissue testing for P and K in corn. ISU professor of agronomy Antonio Mallarino, an ISU Extension specialist in soil fertility and nutrient management, heads this research team.
ISU will use results to develop the first tissue-test guidelines
What has Mallarino and his team learned so far? Commenting on use of tissue testing to measure the need for more P and K, he says: "Although our research results will be used to develop the first tissue-test guidelines for P and K in Iowa, the value of tissue testing is not better than soil testing as a diagnostic tool. Tissue testing is of doubtful value to correct deficiencies for the sampled crop. Thus, tissue testing should be used to complement— and not substitute for— widespread use of soil testing and making fertilization decisions.” Following is Mallarino’s report on the research results and what he and his research team have learned:
Procedures for evaluating P and K concentrations
Several P or K fertilizer application rates were applied in 73 response trials in 2013 and 2014, and these data were combined with data from 20 K trials and six P trials that had been conducted previously since 2003. The study included 99 site-years, 32 for P and at 67 for K distributed across Iowa and encompassed 17 soil series. The sites were managed with no-till or chisel-plow/disk tillage.
The tissue tests evaluated the corn plants for P and K concentrations in the aboveground corn plants at the V5 to 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 for each treatment as a percentage of the statistically maximum observed yield.
Critical nutrient concentrations in soil or plant tissue distinguish between conditions of nutrient deficiency with likely crop response to fertilization— from conditions with adequate levels of nutrients and an unlikely response. A critical concentration range was determined for each nutrient and tissue sampled by using two response models commonly used for this purpose.
ISU tests show the corn yield responses to fertilization
Initial soil-test P and K results from across the sites ranged from very low to very high according to ISU interpretations. ISU fertilizer recommendations based on the test results are outlined in ISU publication PM-1688 “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa.” Average grain yield across treatments and trials ranged from 78 to 238 bushels per acre.
There were statistically significant yield increases in 27 of the 32 P site-years (up to 77 bushels per acre) and in 37 of the 67 K site-years (up to 111 bushels per acre). No corn yield response to P and K fertilization was expected in several sites because soil-test P or K was in the high or very high interpretation categories.
Using and interpreting tissue tests for phosphorus
The research results (crops.extension.iastate.edu/cropnews/2016/06/research-shows-role-phosphorus-and-potassium-tissue-testing-corn) show the yield response to P decreased (so the relative yield increased) with increasing tissue P concentrations. The critical concentration ranges were 0.48% to 0.55% P for young plants and 0.25% to 0.31% P for leaves. The models R2 values indicate the proportion of variation explained and were very similar for both models but were higher for the sampled corn leaves than for the young plants. Also, the capacity of the tissue test values to predict the magnitude of response below the critical concentration range was poorer for young plants than for leaves at silking. So, a P tissue test for leaves at silking was better than for young plants.
Using and interpreting tissue tests for potassium
The research (crops.extension.iastate.edu/cropnews/2016/06/research-shows-role-phosphorus-and-potassium-tissue-testing-corn) shows that the yield response to K also increased with increasing tissue K concentrations. The critical concentration ranges were 1.88% to 2.53% K for young plants and 1.06% to 1.42% K for leaves. The R2 values of the relationships were approximately similar for response models and tissue tests. Therefore, K tests based on young plants or leaves showed similar performance.
Conclusions from using tissue tests for P and K on corn
“This ISU study demonstrated that tissue tests can be used as in-season tools to assess the P and K sufficiency in corn,” says Mallarino. The estimated critical concentration ranges for P and K are lower or in the lower portion of sufficiency levels suggested in various literature sources, which were developed mostly from older research or for other regions.
“Although these results will be used to develop the first tissue-test guidelines for P and K in Iowa, the value of tissue testing is not better than soil testing as a diagnostic tool, says Mallarino. “And, tissue testing is of doubtful value to correct the P and K deficiencies for the sampled crop. Thus, tissue testing should be used to complement and not substitute widespread use of soil testing for making fertilization decisions.”
Acknowledgement: Funding for this study was provided by the Iowa Soybean Association and the DuPont-Pioneer Crop Management Research Awards Program.