Richard Taylor, Extension Agronomist; rtaylor@udel.edu
In the past couple of days, I’ve had several questions concerning the use of starter fertilizer for corn. A number of years ago, Dr. Bill Mitchell showed that on dryland corn there is a boost in yield when starter fertilizer is used. The improvement in yield is at least in part due to an increase in rooting, especially around the banded starter fertilizer, although supplying essential nutrients during the initial establishment of the corn plant is also part of the equation. Many growers take the opportunity to supply essential micronutrients along with the major nutrients [nitrogen (N), phosphorus (P), and sometimes potassium (K)]. The starter band tends to acidify a small area of soil around the band and this lower pH soil also improves the availability of micronutrients such as zinc and manganese above that added to the starter fertilizer. Another benefit of starter fertilizer placed in a band two inches below the seed and two inches to the side of the seed is that it limits the contact surface area between P and the soil. This helps slow the reversion of soluble P into insoluble, non-available P.
Another value to starter fertilizer was demonstrated last year when the heavy rainfall we received delayed sidedress N applications. Although many growers wished they had applied even more starter N last year, what they did apply helped keep the corn crop going until they were finally able to get the next shot of N on the corn. This year, following the heavy rainfall and very cold winter weather, the amount of residual N that might be in the soil will be very minimal. Mineralization of organic matter or amendments has not proceeded to any great degree since the soil temperatures are still very low. It will be even more important to supply N at planting either along with your herbicide program or as a banded starter. In answer to one question I received, I did suggest that growers might want to err on the side of supplying more N at planting rather than less. One thing that you do not want to happen is for the young corn plants to become deficient in N around the time they begin to form the future ear which occurs between the 4th and 6th leaf stage or around typical sidedress time. Nitrogen deficiency at this time will reduce ear size and result in lower yield potential regardless of how good growing conditions are later in the growing season.
Dr. Mitchell did come up with guidelines as well for how much N plus K should be applied in the banded starter based on the soil type. On heavier soils such as the silt loam soils in New Castle County, the total pounds per acre of N plus potash (K2O) in the starter should be less than 120 lbs/acre. If the soil is dry at the time the corn is planted, I would suggest keeping the limit at 90 lbs/acre rather than the 120. For sandy soils, such as those in Sussex County, the limit of N plus K2O was 75 lbs/acre. If these rates are exceeded, the banded fertilizer can end up drawing moisture away from the seed causing the seed to fail to germinate; or, if it had imbibed (absorbed) water and begun to germinate, the seed could be killed.
Another consideration for starter fertilizer is the tendency in our soils to show sulfur (S) deficiency early in the growing season due to reduced atmospheric deposition as a result of the lower S emissions from diesel cars and trucks as well as from power plants and other factories. In the past few years, we’ve seen more and more evidence of S deficiency on young corn plants across the state. I would suggest that growers should consider supplying at least some S along with the starter fertilizer by using ammonium sulfate (liquid or granular depending on equipment availability).
Lastly, I was also asked about the advisability of supplying some potash in with the typical N plus P and now S starter. If your soil test K level is in the high optimum or higher area, including K2O in with the starter is probably not going to help with corn growth. However if you soil test level is in the medium to low optimum area and especially if you’re growing irrigated corn and shooting for a high yield goal, you should consider adding at least some potash to the starter mix you use or applying enough potash broadcast to raise your soil test level to the mid-point of optimum or higher. Growers should remember that they will not see any dramatic growth response to added potash since it doesn’t produce the visual impacts seen with N and P. However, K helps the crop in many ways, from strengthening stalks to reduce lodging to tolerating all kinds of stresses such as drought, heat, insect, and disease. Remember the limitation of pounds of N plus K2O that can be in a starter and don’t exceed those values for your soil type or you may cause germination problems.