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Are farmers missing the boat when it comes to soybeans by not applying a starter in furrow?

Soybeans remove significant amounts of phosphorus (P) and potassium (K) when harvested. Because of this, soybeans can respond to starter fertilizer applications when soil tests are low in P and K. There is even some current thinking that a little nitrogen (N) applied on soybeans at planting will stimulate growth before nitrogen fixation kicks in.

Historically corn has shown a better response to starter fertilizer than soybeans. Partly because they are planted earlier when soils are cooler, soil biological activity is low and nutrients like N, P and K haven’t been mineralized from organic matter. Starters were commonly used in corn until farmers moved to bigger planters to cover more acres and did not want the inconvenience of plumbing a planter for liquid—or even tendering liquid. However, starters were never commonly used on soybeans because of the risk to seedling burn, the threat of poor emergence patterns and the lack of probable response.

In soybeans one of the keys to high yield is planting earlier—the more time there is between planting/emergence and summer solstice, the more nodes and potential pod-bearing sites are formed. The earlier a crop is planted and the cooler the soil, the greater the probability for a response to starters. However, we have to keep in mind that starters (like many other technologies) do not work each and every year and on each and every acre.

Like many production practices, the use of starter fertilizer is partly a risk management tool that helps with early seedling vigor when planting early, when soils are cool and wet and when soil test levels are low for phosphate and potash. Starters may “even out” emergence across a field since every acre responds differently. And if you are applying a starter, either in-furrow or in a 2” x 2” band (2 inches deep and 2 inches to the side), you can also carry sulfur, micronutrients and one of today’s many biological inoculants.

With the introduction of the Clean Air Act in the early ’70s, atmospheric sulfur (S) deposition has declined from a range of 8 to 15 pounds per acre annually down to just 1 to 3 pounds per acre. So crops are more dependent on sulfur mineralized from organic matter or from supplemental fertilizer.

A study done by Michigan State University evaluated the application of starters on soybeans where starter fertilizer containing 17 percent sulfur was banded at planting in a 2” x 2” band at 2 quarts per acre and compared with an untreated check. Treated plots averaged 43.2 compared to 42.4 bushels per acre for untreated, not significantly different. Also S-tissue test levels were in the sufficient category and again not statistically different. However, they did not report the organic matter levels, so we don’t how much sulfur was being mineralized. When reading results from studies like this we have to be careful. A single result from one field, in one year cannot be used to make a general recommendation beyond that one field and set of conditions.

So what are the general rules for applying a starter on soybeans? If you apply it in-furrow, keep the rate low at 2 or maybe 3 gallons per acre. I like to dilute it with water up to 5 gallons per acre. If you apply in a 2” x 2” band, the risk of salt burn and seedling damage is much less. Stay with low-salt fertilizers that include P and keep N and K to a minimum. Conventional 10-34-0 at 2 to 2.5 gallons per acre is a good choice; however, it lacks potash. And if you plant in narrow rows, the volume applied per row should be reduced to further reduce the risk.

You will see a more consistent response on soybeans when soils are deficient in P and K, in very high-yield-potential fields, when soils have low-to-medium fertility levels, planting in lighter soils or when planting early. Just be aware of the analysis of what you are buying, how much you are applying and where you are placing it.

Are you using starter fertilizer on soybeans today, or have you used it in the past and would readopt this practice?

Dr. Daniel Davidson, agronomist, posts blogs on agronomy-related topics. Contact him at djdavidson@agrwrite.com.