Richard Taylor, Extension Agronomist;rtaylor@udel.edu
I received a question through one of our county agricultural agents just a week ago about the usefulness of starter fertilizer for wide-row (30-inch) soybean plantings. I thought it would be worthwhile to discuss my answer in case other growers have the same question in mind.
Soybeans on Delmarva are generally planted after corn planting is nearly complete and the soil temperature has risen to at least 60 or 70°F. Soybeans are very efficient scavengers of soil nutrients. This characteristic, along with the warmer soil conditions we generally have when planting soybeans means that under most circumstances starter fertilizer is of little to no value in boosting soybean yield potential.
If you read articles in the popular press, you could have the impression that starter fertilizer does boost soybean yield as long as the fertilizer is applied in a band two inches to the side and two inches below the seed. Even the popular press articles stress the danger of applying any fertilizer directly in with the soybean seed as well as stressing the futility of expecting yield increases using broadcast surface applications of phosphorus (P) and potassium (K). If you look at the source of the articles, you’ll notice that the research showing small increases comes from very northern locations or from areas where soils do not contain very much available P or K. Thus, the exceptions to whether starter fertilizer can boost soybean production are planting situations dealing with cold soils (planting in the northern tier of states) and soils low in either P or K, but especially P. Since here on Delmarva, we have few fields that will test in the low category for P and, for the most part, we plant soybeans into warm soil, starter fertilizer will not be a cost effective addition to a soybean production system.
On Delmarva, we do have some fields that are low in potassium. The research indicates that there can be a yield response to starters containing P and K if soil potassium is low although the research I’ve seen suggests that the response is not enough to cover the cost of potash added to the starter. In our region where we plant no-till on a high percentage of our acres and have begun using turbo-till and other equipment that allows some soil disturbance while maintaining a lot of crop residue on the surface, I feel a broadcast application of potash will be more effective over the long-term to raise the soil K levels for not only the soybean crop but for future crops in the rotation.
Another aspect to the question I received was whether the starter could supply manganese (Mn) if there was a history of Mn deficiency in the field. Yes, if supplemented with Mn, starter fertilizer could supply the needed Mn. Since most soybeans are herbicide resistant at this time and a post-emergence herbicide application will be used, foliar Mn at 0.5 to 1.0 lb Mn/acre can be applied at the same time as the herbicide. In fields with severe Mn deficiency, a second foliar application at the same rate as the first could be needed and should be applied just prior to when the soybean crop begins to flower.
My last comment on this issue concerns the use of wide rows in soybeans. I’ve conducted trials which involved row spacing many times over the past years. I’ve never had a situation where soybean planted on 30 inch rows yielded more than soybeans planted on narrower rows. There have been studies in which a 15-inch row spacing yielded as good, or better than drilled beans although long-term it appears that there is a slight yield advantage to the drilled beans especially when seed spacing in-row is relatively uniform.