What makes a good formulation for foliar feeding soybeans?

There has been a lot of interest in foliar feeding soybeans since the runup in soybean prices a decade ago. Growers have appreciated that the technology has increased soybean yields and are a bit reluctant to go back to the old ways of not investing in soybeans. However, there is no single perfect product out there. With multiple brands and products, and many look-alike products available from different suppliers, the market is crowded and often confusing.

One new practice is foliar feeding soybeans in season. We know that soybeans require plenty of nitrogen (N), phosphorus (P), potassium (K) and sulfur (S). Much of that comes from the soil or, in the case of N, from N fixation. The soil is the source of most of the nutrients needed by the plant. However, foliar nutrients can provide an edge in-season, particularly when deficiencies occur or as root growth wanes when reproduction commences.

Researchers found that foliar feeding maintains a better overall nutrient balance within the plant and across a field, which due to inherent variability may not be achieved through soil uptake alone. Nutrient stratification, root distribution, soil temperature, available soil moisture, soil-nutrient imbalance and other factors limit nutrient absorption through the roots alone.

University studies have demonstrated that foliar-fed nutrients can move both upward and downward from the leaf surfaces where they are applied. This helps move concentrations of nutrients to the most critical parts of the plants—buds, young leaves and growing roots. However, university studies have also confirmed little difference in yield when foliar applying macronutrients to corn or soybeans.

Some agronomists and growers follow a strategy of foliar feeding soybeans as a routine practice. Some believe the best bang would be N and potash at R3 with fungicide when the pods are beginning to fill. Other believe that micronutrients are the best candidate, particularly boron (B), manganese (Mn) and iron (Fe). Others believe that making two or three passes of a balanced foliar is important.

Because small amounts of nutrients enter plant leaves, foliar feeding cannot be a primary source of macronutrients like N, P, K and S. Foliar feeding is not sufficient to supply enough of these nutrients to meet all the plant’s requirements.

Micronutrients such as boron (B), copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn) can prove beneficial as a foliar feed, if deficiency symptoms exist. This is where early and frequent tissue testing comes into play to track nutrient concentrations. Since plants require small amounts of micronutrients, foliar application is sufficient to improve plant health. However, once a field is identified as deficient in micronutrients, growth and yield have already been impacted.

To learn more about tissue test and nutrient recommendations listen to the “Optimizing crop nutrition with tissue testing,” webinar at http://bit.ly/2t9KJQ2, presented by Jason Haegele, agronomy manager/regional agronomist with WinField United.

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.