Is In-Season Soybean Fertilization Effective?

Oftentimes questions surrounding emergency or catch-up fertilization for soybeans arise in June, as highlighted in a recent e-newsletter from the Iowa Soybean Association. We’ve seen a lot of conversations on Farmers for the Future and New Ag Talk surrounding this question, as well. Most producers don’t have to worry about this since adequate amounts of phosphorous (P) and potassium (K) are usually applied before planting. In a year like this, however, excessively wet conditions alter the crop nutrient uptake and affect fertilization.

Is applying dry, granulated fertilizer a viable post-emergence option?

The short answer is “probably not” for two main reasons:  First, both P and K (but especially P) are needed at early growth stages to enhance plant cell multiplication when the number of nodes, leaves, and potential seed numbers are largely determined. Second, the application of fertilizer to the soil surface or banded/injected between the rows will be of low efficiency, mainly due to the common low, or infrequent summer rainfall (which hasn’t been the case for many areas this season). There is one situation in which an in-season application of granulated P and K fertilizer might be considered, and that’s when soil tests are very low, and for certain the producer will have to apply a high fertilizer rate for the following year’s corn crop to assure adequate P and K levels.

What about foliar fertilization?

Some producers ask if foliar fertilization could help improve soybean growth and grain yield. While it’s a possibility, the chances are low for fields that have been well fertilized or where growth is limited by factors other than nutrient supply.

Thus, the article concluded in-season fertilizer application for soybean will seldom prove cost-effective in Iowa production systems. The exception might be when soil samples confirm that the soil tests very low or low and there was insufficient pre-plant fertilization. A large application of granulated fertilizer to soil during the very early growth stages may result in some yield increase and will begin to build up soil test levels that will have to be increased for the next crop anyway, but the economic benefit for this year’s soybean is very doubtful. The probability of an economic response to foliar fertilization is small, but this practice may be justified when nutrient deficiency symptoms are obvious, with confirmed deficient-testing soil, or when soil or climactic factors (other than drought) limit nutrient uptake in early summer.

Source: Iowa Soybean Association, Gold Standard