July 3, 2018

The practice of conducting tissue testing for phosphorus and potassium in corn and soybean plants is a relatively new tool in Iowa crop production. While soil testing has been widely used for decades, proper tissue testing recommendations are still emerging.
A new publication from Iowa State University Extension discusses the results of tissue testing research and interpretation at Iowa State University. The publication Phosphorus and Potassium Tissue Testing in Corn and Soybean, CROP 3153, provides interpretations specific for Iowa’s growing conditions.
“Iowa research into tissue testing in previous decades has shown that either this was not a reliable practice, or the research wasn’t sufficient,” says Antonio Mallarino, professor and ISU Extension specialist in agronomy. “As interest in tissue testing has grown in the last few years, we saw a need for additional research to determine the value of tissue testing and develop guidelines on how to interpret test results.”
Tissue testing guidelines for Iowa
The resulting publication is the first interpretations for tissue testing specific to Iowa and is based on 66 site-years of research results for phosphorus and 119 site-years for potassium.
“Tissue testing isn’t perfect, but it can be used to monitor nutrient levels available to crops,” Mallarino says. “Now that the research has been completed, people can see the interpretations and results, with information backed by good science. Tissue testing can be helpful in assessing the available supply of phosphorus and potassium for corn and soybean crops when used as a complement to soil testing.”
The publication also provides information on how to sample plant tissue, including detailed photographs of the different growth stages of both corn and soybeans. The specific part of the plant to be sampled is detailed for the reader.
Test tissues and soil
Research has shown tissue testing can be used as an in-season tool to assess P and K deficiency in corn and soybean fields. The results also show, however, that variation in correlation between tissue test results and crop yield responses is similar or higher than for soil testing. In addition, tissue testing is of doubtful value to correct P and K deficiencies for that sampled crop.
Thus, tissue testing should be used to complement and not substitute for recommended soil testing in making fertilizer decisions, Mallarino says.
Interpret results carefully
A particularly useful approach for tissue testing is when normal and poor growth areas exist in a field that may be related to P or K nutrient supply, Mallarino says. In such situations, you should collect and analyze both soil and plant tissue from the areas to help diagnose deficiencies. Any stress, such as drought, excess moisture, pests or diseases, and deficiencies of other nutrients can influence plant growth and nutrient uptake and can increase or decrease nutrient concentrations.
Therefore, careful interpretation of tissue test results is always needed, but especially when test results are much lower or higher than normal. A comparison of tissue and soil test results along with consideration of field history information is helpful for detecting and understanding potential crop nutrition problems.
The new publication is written by Mallarino and John Sawyer, professors and Extension specialists in agronomy and soil fertility at Iowa State.
Source: Iowa State University
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