Foliar Fertilization of Soybeans
During the growing season, nutrient deficiencies in soybean are sometimes observed. Thus, the question that is often asked is “Can an in-season nutrient deficiency be remedied by the foliar application of the deficient nutrient”? The obvious next question is “Will a foliar application of the deficient nutrient increase yield, or rather will it increase yield enough to pay for the application”?
Information that appeared in the article “Research lacking to back claims for foliar-applied fertilizers” in the June 22, 2016 issue of Delta Farm Press should be considered. Key points from that article that was authored by a group of Univ. of Arkansas scientists/specialists follow.
• Producers who are considering the application of foliar-applied fertilizers should ask these questions. 1) What is the frequency of crop response? 2) What is the expected average yield increase, and will this increase cover or exceed the expense for the applied product? The answers to these questions should be based on valid, verifiable research.
• Recommendations to producers for foliar application of nutrients are usually based on tissue analysis. The tissue values that are used to define nutrient sufficiency or deficiency may have the following shortcomings. 1) They are not always based on research. 2) There is inadequate published information that defines the relationship between crop yield increase and the tissue values for many nutrients. 3) There is inadequate information to define the exact nutrient concentration that is needed to verify a yield response to foliar feeding. 4) Many of the accepted critical nutrient concentrations are specific to a particular plant part and growth stage–thus correct sampling is important. 5) There is not usually a single tissue nutrient concentration that can be used to define nutrient sufficiency or deficiency for the duration of the growing season. (Click here for a White Paper on this website titled “Soybean Seed/Tissue Analysis to Determine Plant Nutrient Status” that provides details about how correct analyses of soybean plant tissue can be a tool to detect/confirm nutrient deficiency in plants during the growing season and in seed following harvest.)
• The authors’ statement “While we would encourage tissue analysis at the appropriate critical growth stage, mid- to late-season foliar-feeding based on tissue analysis results we believe are largely unwarranted” is arguably the best summation of whether or not foliar fertilization of soybeans should be considered.
Results from a 2019 multi-state project (20 sites in 13 states, including Arkansas and Mississippi) confirm the above assessment of foliar fertilization of soybeans by the Univ. of Arkansas personnel. Pertinent details of and results from that project follow. 1) Six products that each contained some of the nutrients required by soybean were applied at labeled rates at growth stage R3 at each of the 20 locations. 2) Only 1 site (Fond du Lac, Wisc.) out of 20 showed a yield benefit associated with foliar fertilizer application. 3) The 19 no-response results were across all sites regardless of yield level that averaged from 20 to 80 bu/acre.
The above resources present a convincing case for not using foliar fertilization to increase soybean yield across a wide range of environments. Even though nutrient deficiencies may sometimes occur during the growing season, the best remedy is to apply the correct amount of fertilizer based on soil test results before planting the next crop so that the deficient nutrient becomes available for uptake by the plant’s root system during the following growing season. Application of deficient nutrients on the plant’s foliage during the growing season will not likely result in sufficient uptake of those nutrients in an amount that will provide a profitable response.
A few studies have shown small yield increases resulting from the application of some foliar fertilizers to soybeans. However, the yield increases were either not economical or their profitability was not determined. Thus, application of a foliar-applied fertilizer product should be considered only to correct a confirmed in-season nutrient deficiency. Even then, the certainty of a yield increase that will be profitable at the current commodity price is not guaranteed; this is especially so with depressed commodity prices.
Composed by Larry G. Heatherly, Aug. 2020, larryheatherly@bellsouth.net