Late Season Nutrient Diagnostics

It is common for nutrient issues that went unnoticed during the growing season to become apparent as harvest approaches. With the growing season coming to an end, what can be done to diagnose potential nutrient issues as corn and soybean harvest approach? The options become increasingly limited as the plant gets closer to harvest maturity.

While tissue tests are effective for identifying nutrient deficiencies during the growing season, the remobilization of nutrients within the plant, combined with the loss of dry matter due to tissue senescence, can artificially alter nutrient concentrations in the plant sample. This can occur after R3/R4 in soybeans or R3 in corn. Beyond these points, plant senescence will lead to nutrients leaving the plant and alterations in biological processes that will not be reflected in reference data. Dead and/or yellowing tissue will not produce valid tissue test results. Corn that maintains a healthy ear leaf is more likely to produce meaningful tissue test results.

Near these growth stages, “good” vs. “bad,” or perhaps more accurately “bad” vs. “worse,” tissue test comparisons can yield insight into potential deficiencies. These results can also be highly subjective and should be carefully considered. A high tissue test level at this time is not necessarily a good thing. Rather, it is often the result of an associated deficiency in which that nutrient was not efficiently used and remains in excess.

Soil tests can often be more useful at this point in the growing season. Just before harvest often reflects the lowest point of nutrient availability in the soil. Don’t forget to test for micronutrients as well. Sample the negatively impacted area directly, as it may be small enough that it is not reflected in routine soil testing. Again, comparing a “good” vs. “bad” soil test is a good practice in this situation. This may provide additional information to further define the issue, especially when more than one nutrient interaction is involved.

In corn, a stalk nitrate test can provide additional insight into the conditions leading up to the visual symptoms. Late-season issues are often identified by changes in yield parameters, such as the number or size of ears, and nitrogen often plays a role. If the “bad” corn has an excessive stalk nitrate level, the issue likely arose late in the season and limited the plant’s ability to achieve a higher yield goal. If the “bad” corn has a very low stalk nitrate level, either the nitrogen program was limiting, nitrate loss was excessive, or the root system was not developed enough to access nitrogen. Additional data can be evaluated, or additional observations can be made, to confirm or rule out these possibilities.

Digging roots and evaluating stalks should be done before harvest. Limited root development can also be associated with low phosphorus levels. Evidence of a limited root system can provide insight into late-season tissue samples or interactions with limited soil fertility.

Notice that may, may be, should, and could are used frequently in this article. Late-season diagnostic laboratory analysis can be used to help define the cause of an issue; however, a critical evaluation of the data is required. Seasonal environmental conditions and crop physiology can significantly impact laboratory results, and those effects are not predictable.