Potassium deficiency on this corn crop is exhibited by yellowing and necrosis of the leaf edges in the lower part of the canopy. Image credit: Jeff Graybill
Scouting for nutrient deficiencies is a useful practice to identify soil fertility issues that may need to be addressed with a rescue treatment, if practical, or to guide fertility management decisions for future crops. Scouting fields for nutrient deficiencies can involve a combination diagnostic tools, including visual inspections of crop growth, tissue testing to measure nutrient concentrations, soil testing, and examining satellite or aerial imagery.
Routine visual inspection of crops often provides the first indicator that a growing crop may be suffering from a nutrient deficiency. Each nutrient will express a deficiency with different symptoms on the plant, so it’s worthwhile to be familiar with these symptoms ahead of time to help guide a diagnosis. If you see a symptom that looks like a nutrient deficiency, the next step is to look for patterns in the field. Is the symptom related to natural soil boundaries, such as changes in soil type or drainage class? If so, the deficiency symptom may appear worse in certain patches corresponding to these natural factors. Or is the symptom related to a management factor, such as skips in fertilizer application passes or malfunctioning spreading equipment? If so, the symptom may exhibit itself with clear straight boundaries throughout the field.
If a visual deficiency symptom is found in a field, the next step is to see if there is a recent soil test from the field that might help confirm the deficiency. If the recent soil test indicates a low nutrient level that aligns with the deficiency that is observed, you have some confirmation of the problem and can proceed with corrective actions. Without a recent soil test, new tissue testing and soil sampling is warranted.
Soil samples will give you an idea of whether the quantity of available nutrients in the soil is a limiting factor, while tissue testing will tell you what the plant is accessing from the soil and if there is a potential issue with nutrient uptake by the plant due to soil compaction or root stress. Soil samples and tissue samples collected from areas exhibiting the deficiency and those not exhibiting the deficiency can be compared to determine what nutrient is deficient.
If tissue tests from good and bad areas of a field can’t be compared, then you can use standard nutrient sufficiency ranges to interpret the tissue test results. When using these sufficiency ranges, it is absolutely critical to sample the correct plant part for a given growth stage, because the tissue test sufficiency ranges are calibrated for specific growth stages and plant parts. The lab you are submitting your tissue test samples to should have guidelines available for which plant part to sample at which growth stage. For instance, the Penn State Agricultural Analytical Services Lab provides detailed sampling instructions.
One tool that can be useful to help in scouting for potential soil fertility issues is satellite imagery. Many companies now offer subscription services that will deliver weekly satellite images of fields you have registered with their service. These satellite images are usually color coded to correspond with the crop canopy density and greenness. Areas with lower canopy density and greenness within a field may warrant a visual inspection to see if there is a soil nutrient issue. The image here is a satellite vegetation map of an experiment at the Penn State Agronomy Research Farm. The green areas in the border of the field were planted to corn two weeks ahead of the central area of experimental plots, so they appear with a greater vegetation values (greener in color). The lower half of the experimental plots were comparing manure application in the fall versus no manure application in the fall. Areas that appear yellow are strips that had received fall manure. Areas that were reddish-brown were strips that did not receive manure. A visual inspection of the plots that did not receive manure in the fall indicated classic potassium deficiency symptoms, yellowing and necrosis of the leaf edges in the lower part of the canopy (see photo above). Soil tests of these areas confirmed a potassium (K) deficiency, with Mehlich 3 K levels in the 50 to 70 ppm range (optimum is 100-150 ppm K). Strips where manure was applied in the fall had soil test levels in the optimum range, owing to the 240 lbs K2O/ac that were added with the fall manure application. In the upper half of the experimental area of the field, variation in the vegetation map corresponded with natural variation in soil test levels, which ranged from 50 ppm to 90 ppm in those plots.
Scouting for nutrient deficiencies does require an investment of time, and potentially analytical costs if deficiency symptoms are encountered. But it is a valuable endeavor to ensure that crops are growing without nutrient deficiencies so that you will get the best return on your investment of other inputs, like seed, labor, and pest protection.
A vegetation map derived from satellite imagery, provided by the Climate Fieldview Pro service. Brownish areas correspond to potassium deficient zones of the field. Screenshot from the Climate Fieldview Pro service by Charlie White