Amy Shober, Extension Nutrient Management and Environmental Quality Specialist, ashober@udel.edu; Jarrod O. Miller, Extension Agronomist, jarrod@udel.edu; Phillip Sylvester, Kent County Extension Agent, phillip@udel.edu; Cory Whaley, Sussex County Extension Agent, whaley@udel.edu
Barring any major weather delays, the lion’s share of the corn crop will be planted in the next three to four weeks. Applying starter fertilizer for corn at planting has become standard practice, yet we regularly receive questions regarding the benefits and what nutrients should be included in a starter fertilizer blend. Before we dive into the proverbial weeds, we would like to point out the difference between “pop-up” and “2×2” starter fertilizer setups. Pop-up fertilizers are placed directly in the seed trench with the seed, and because of potential salt-injury, often contain low-salt formulations of fertilizer and are used at low rates. Emergence may be delayed if too much fertilizer is used due to burning of the roots by the fertilizer. More common is the 2×2 setup, which places fertilizer in a band 2” off to the side and 2” below the seed, because it is less likely to injure roots. Therefore, more fertilizer can be used in a 2×2 setup and has greater utility for supplying nutrients to young corn. While pop-up fertilizer setups might be useful, especially for applying pesticides directly in the furrow, the focus of the article will be on 2×2 setups.
Now on to what to include in a starter fertilizer mix for corn. While dry fertilizers were the standard years ago, most growers have switched to liquid formulations for ease of handling and blending with other nutrients. Applications of nitrogen (N) and sulfur (S) are common in starter fertilizers, as they have been shown to be quite effective at increasing early season growth for corn planted in Delaware’s sandy, low organic matter soils. Increased N-use efficiency has been observed when N is applied at planting (15 to 25%) and again when plants are 12 to 15 inches tall. Commonly used N containing liquid starter fertilizer products include UAN (30 or 32%), ammonium polyphosphate (10-34-0 or 11-37-0), and ammonium sulfate (8-0-0-9S). Sulfur should be included in a starter fertilizer, and at rates high enough to sustain the corn crop through V5. High yielding, irrigated corn may require 30 to 40 lbs/ac of sulfur, which can be split applied at planting and again at sidedress. As mentioned above, ammonium sulfate will supply both N and S and should be used instead of ammonium thiosulfate (S not immediately available to the plant). Ammonium sulfate has a strong acidifying effect, which can make certain micronutrients more available in high pH fields, averting deficiencies.
In addition to N and S, you may be wondering if applications of other nutrients, mainly phosphorus (P) or micronutrients, such as zinc (Zn), manganese (Mn), or boron (B), would be beneficial. Here are some guidelines to help you decide on what nutrients to include in your starter fertilizer. Researchers typically agree that application of starter P can be beneficial and improve crop growth when soil test P concentrations are below the agronomic critical level (<50 FIV). When soil test P is <100 FIV, we recommend application of 20 to 40 lb P2O5 in the starter band, with higher rates reserved for soils testing low or medium in soil test P. Even so, the efficacy of starter P is variable when soil test P is within the agronomic optimum range (50 to 100 FIV). Based on 62 site years of data from starter P strip trials, Cornell researchers recommended that applications of >25 lb P2O5 could be beneficial for soils with no manure history. However, when soil test P is in the optimum range and the field has a history of manure application, Cornell researchers suggest you skip the starter P. Cornell researchers also do not recommend application of starter P when soil test P concentrations are in the excessive range (>101 FIV), regardless of manure history. We concur with this recommendation based on results of recent small plot studies (conducted by University of Maryland in conjunction with University of Delaware) in Maryland on soils with excessive soil test P, even when those soils received no additional P for 15 years. In the MD small plots, we saw an early season response (visual) to starter P, but that response did not translate into a yield response.
What about micronutrients? The potential for Mn deficiency is predicted based on the Mehlich 3 Mn concentration and soil pH. Similarly, potential for Zn deficiency is predicted based on Mehlich 3 Zn concentration, soil pH, and soil test P concentration. Detailed instructions for determining the Mn and Zn availability index is available online as part of the University of Delaware fertility recommendation for grain corn: (Link). If Mn or Zn deficiency is predicted by a soil test, or you have documentation of Mn or Zn deficiency in your field, you may benefit from application of these nutrients in the starter band. When banding Mn or Zn, lower rates can be applied, particularly if using a chelated micronutrient. University of Wisconsin recommendations are 0.5 lb chelated Mn/acre or 0.5 to 1 lb chelated Zn/acre. Broadcasting micronutrients aren’t recommended when soil tests are low, as they are likely to be tied up in the soil. Boron deficiency has been noted occasionally in intensively managed, irrigated corn, but we currently do not recommend starter applications of B.
A couple final thoughts on starter fertilizer applications; as you may have already figured out, it is nearly impossible to squeeze everything you want in a starter fertilizer mix because of volume and equipment limitations. Even if you could, there are limitations due to potential fertilizer salt injury next to the seed. The standard recommendation is no more than 75 to 100 lbs of N+K in a 2×2 setup, with the lower limit for sandy soils. Therefore, think about what is most likely to give you a return on investment. In Delaware, leachable nutrients like N and S should be at the top of your list. How are you going to supply enough of these nutrients early season, particularly S? Starter fertilizer blends can provide enough N and S (and micros) to get the plant off to a good start and through V5, but you may not be able to include much P in the mix. When deciding on a starter fertilizer blend, think about return on investment, and what makes the most sense given the limitations.