Variable rainfall and soil temperatures across Iowa since April have resulted in variable planting dates and early crop growth. As of today, June 12, upper soil moisture ranges from deficient to about normal. Some producers and consultants are wondering if the dry conditions are affecting the normal crop uptake of soil nutrients and plant tissue-test results. Tissue testing is an in-season diagnostic tool that can be useful to assess plant nutrient sufficiency but is affected by deficient or excessive soil moisture.
Sample the right plant part at the right growth stage
Even with ideal conditions, plant nutrient concentrations vary greatly with the crop growth stage and plant part sampled. Samples should be collected at the same growth stage and from the same plant part that were used for the calibration of test results based on yield response. Iowa State University Extension and Outreach has research-based interpretations for in-season tissue testing only for phosphorus (P) and potassium (K) in corn and soybean and for sulfur (S) in alfalfa.
Recommended growth stages and plant parts for corn are the aboveground plant parts at the V5-V6 growth stage and ear-leaf blades at the R1 stage whereas for soybean are the aboveground plant parts at the V5-V6 growth stage and trifoliate leaves including petioles at the R2-R3 stage (see ISU Extension and Outreach publication CROP 3153, Phosphorus and Potassium Tissue Testing in Corn and Soybean). For S in alfalfa, sample the top 6 inches or plants at the early bud stage (see publication CROP 3072, Sulfur Management for Iowa Crop Production). There are no in-season tissue test interpretations for other nutrients or crops due to insufficient research, infrequent deficiency that precludes meaningful test calibration, or research showed it is an unreliable diagnostic tool. There are interpretations and guidelines for using the end-of-season cornstalk nitrate test but apply only from about a week before black layer until harvest (see publication CROP 3154, Use of the End of Season Corn Stalk Nitrate Test in Iowa Corn Production).
Comparisons of tissue test results between apparently adequate or deficient conditions from different fields are unreliable to assess sufficiency for a particular crop and field. These comparisons often suggest concentrations much higher than adequate and may encourage unnecessary fertilization for the current or future crops that reduce the profitability of production. This risk has been shown in the publications mentioned above and for micronutrients in publication CROP 3135, Micronutrients for Soybean Production in the North Central Region.
Precautions when interpreting test results with droughty conditions
Extra thought and caution is needed for interpreting tissue test results this year due to the prevalent drought conditions in much of Iowa. Effects of drought or plant diseases on plant growth and nutrient uptake often result in nutrient accumulation (increase) or dilution in dry matter (decrease) in tested plant material.
Potassium deficiencies symptoms in corn and soybean are being seen in droughty areas of the state, mainly in older leaves. A K deficiency is first seen as a yellowing and necrosis of the leaf margins (see IPM 42, Nutrient Deficiencies and Application Injuries in Field Crops). These symptoms may indicate a deficient soil K level, but they may also be drought-induced deficiency in the plant since K uptake is very sensitive to limited soil moisture. Potassium fertilization to the soil or to the foliage during drought will not improve K uptake or alleviate these symptoms. With rain, however, and if soil-test K is optimum or higher and the induced deficiency was severe and prolonged, deficiency symptoms will not appear in newer leaves and yield may not be greatly affected. Be aware, however, that soybean K deficiency also can be seen later in the season in the top leaves after the R2 or R3 (see this ICM News article).
Soybean iron deficiency chlorosis (IDC) is commonly observed in high-pH calcareous soils and sometimes in field borders along gravel roads probably due to windblown limestone. Iowa research has shown that tissue testing and in-season management practices such as foliar fertilization with iron products may green-up leaves a bit but seldom will alleviate the deficiency and the yield loss and that using soybean varieties tolerant to soybean cyst nematode and IDC is the most cost-effective way to minimize yield losses due to IDC. Further information is available in publication CROP 3135.
Pale green or yellow corn, soybean, or alfalfa leaves sometimes are observed with excess soil moisture in soils with moderately poor or poor drainage. These symptoms likely are due to poor soil aeration and are easily confounded with nutrient deficiency symptoms. Tissue sampling and testing in these conditions may give misleading results, even when sampling the right plant parts at the right growth stage. See further useful information in IPM 42, Nutrient Deficiencies and Application Injuries in Field Crops, Nitrogen Deficient Soybean with Excess Soil Moisture, What's with all the yellow soybean?, and Yellow Corn Plants.
Sample both soil and plant tissue from "normal" and "affected" areas
The best way of using tissue testing and interpreting results correctly is when there are areas within a field with normal growth and areas with poor growth and/or nutrient deficiency symptoms. In these situations, collect and analyze soil and plant tissue samples from both normal and poor areas and compare results from both areas and to published information about soil and plant tissue interpretations. This is useful not only due to reasons mentioned above but also because pests or disease infestation and plant injury due to herbicides or other factors often mimic nutrient deficiency symptoms. Sampling both plants and soil from these field areas also may help when the tissue samples are not collected at the suggested growth stage and plant parts or for nutrients for which there are no yield-response based tissue test calibrations. Antonio Mallarino Professor of Soil Fertility and Nutrient Management, Extension Specialist
See additional online resources in the various nutrient topics of the Iowa State University Extension and Outreach Soil Fertility website.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 12, 2023. The information contained within may not be the most current and accurate depending on when it is accessed.