Starter fertilizers are often part of a successful nutrient management strategy. As with any fertility program, implementation of starters requires attention to detail.
Avoid Fertilizer Burn
Fertilizer burn may occur when fertilizers are applied with or near seed at planting. Many fertilizers are salts that dissolve into the respective ions in the soil water when applied.
Think of table salt, NaCl, that dissolves into the respective positive and negative ions Na+ and Cl- in water. This salt dissolution creates a pressure difference that causes water to move from inside the plant roots to the surrounding soil. Plants may wilt, become blackened and die from the lack of water. This is called seed burn or fertilizer burn and may result in substantial stand loss.
Proximity is the key to susceptibility. Issues are rarely seen with broadcast applications because the fertilizer is dispersed over a wide area. Likewise, starter fertilizer banded 2 inches over and 2 inches below is a method developed to avoid seedling contact, which rarely creates problems.
However, low rates are also important to control issues with in-furrow or pop-up applications. No more than 10 pounds per acre of nitrogen and K2O combined should be applied in furrow. Highly soluble fertilizers with low salt indices should be used for this placement, such as ammonium polyphosphate (10-34-0) or orthophosphates. Retailers and consultants should be familiar with appropriate materials for these applications.
Avoid Ammonia Toxicity
Additional injury risk from some nitrogen fertilizers exists more so than is anticipated by salt content alone if ammonia is released when soil applied. It is toxic and can enter plant cells freely. Urea, UAN, ammonium thiosulfate and DAP present more ammonia-based problems than MAP, ammonium sulfate or ammonium nitrate. Ammonia production can be accelerated by higher soil pH, either in the bulk soil or via reactions in the vicinity of applied fertilizer.
Soil And Weather Conditions
Soil conditions are important for determining why injury may occur some years and not others. Crop injury is most likely when seedlings in sandy, low organic matter soils are directly exposed to fertilizer. Dry weather will intensify injury potential. Fertilizer salts are diluted by diffusion away from the band in moist soils, but little diffusion occurs in dry soils. The concentrated fertilizer leads to more burn risk.
Low cation exchange capacity soils, those with coarse textures and low organic matter content, react less with the fertilizer than higher CEC soils (fine textured). Soil temperature is also part of the issue as roots grow slower in cold soils exposing them to the higher concentration of fertilizer longer.
Article by Mississippi State University Extension soil specialist Larry Oldham and grain crops Extension state specialist Erick Larson.
Small amounts of nitrogen and phosphorus are often used as a starter or “pop-up” fertilizer. The main advantage of starter fertilizer is better early season growth. Corn planted in early spring is exposed to cool soil temperatures, which may reduce phosphate uptake.
Banding a starter fertilizer 2 inches to the side and 2 inches below the seed increases the chance of roots penetrating the fertilizer band and taking up needed nitrogen and phosphorus.
Deduct the amount of nitrogen and phosphorus used in a starter fertilizer from the total nitrogen and phosphorus needed for the season. However, total phosphate requirements of the corn crop can often be supplied in the starter fertilizer. Since nutrients applied in starter fertilizers are a part of the total fertilizer program, using this recommended practice is not that costly.
Currently, the most popular starter fertilizer is ammonium polyphosphate (10-34-0). Monoammonium and diammonium phophates are equally effective. There is generally no advantage in using a complete fertilizer (N-P-K) as a starter since applying phosphate is the primary objective.
There is an advantage to using additional N such as 28-0-0-5 particularly in sandy soils to encourage growth as soils warm. Depending on your needs, a typical pop-up application is 6 to 7 gallons each of 10-34-0 and 28-0-0-5.
Source: University of Georgia Extension soil specialist Glen Harris from UGA’s Corn Production In Georgia guide.