Starter fertilizer will often result in increased early-season growth, but this early-season growth does not always translate to increased yield. Although it is difficult to predict exactly where yield responses will occur, it is easy to provide conditions that improve the potential for response. These include the following:|
a. Low phosphorus availability. Corn plants need a readily available supply of phosphorus early in the growing season. It is especially important that the plants have phosphorus near the row on low-testing soils (soils testing less than 20 pounds P per acre are considered low). However, even on medium- to higher-testing soils, there are data to show that starter will increase yield if the soil remains cool for several weeks after plant emergence. High-pH soils (pH greater than 7.3) often respond well to starter fertilizer, because high pH reduces phosphorus availability.
b. Cool soil temperatures. Reduced tillage systems increase the amount of residue left on the soil surface and, as a result, reduce soil temperature early in the growing season. This reduced temperature slows root growth and thus reduces nutrient uptake unless the nutrients are near the seed. Low temperatures also reduce the rate of microbial release of nitrogen from soil organic matter. Therefore it is important to have some nitrogen near the seed. Recent Illinois experiments showed a significant corn yield increase to starter fertilizer 90 percent of the time in no-till experiments but less than 30 percent of the time when conservation (chisel plow) tillage was used. Nitrogen accounted for most of the response, but including phosphorus along with the nitrogen gave an even better response. This differential in response to starter fertilizer in these two tillage systems is most likely due to differences in soil temperature early in the season.
Where should I place starter fertilizer?
Research has shown that the best place for starter fertilizer is 2 inches below and 2 inches to the side of the seed. No other placement location has been shown to be as consistent in giving an early-season starter response. Placement of starter with the seed "pop-up placement" is often done because it is easy and the equipment is relatively inexpensive. However, there is a risk of seedling injury, particularly in years when there is not adequate rainfall. The typical recommendation is to limit the total amount of fertilizer to no more than 10 pounds of nitrogen plus potassium in seed-placed programs. In dry springs, even this rate may be too high. If you are using pop-up fertilizer, be sure to calibrate each row to make sure that one row is not over applying at the expense of another. Do not include a urea-containing fertilizer--either dry urea or urea-ammonium nitrate solutions--in a pop-up fertilizer program. Urea converts to ammonia, which is very toxic to corn seedlings. In addition, most urea contains a small amount of biuret, an impurity that is very toxic to corn seedlings.
Can I apply all of my nitrogen through the corn planter?
The answer is yes, if you are careful to make sure that the fertilizer does not get near the seed. If the full nitrogen program is being applied through the corn planter, the fertilizer attachment should be adjusted to deposit the fertilizer at least 4 inches to the side and 2 inches below the seed. There are no data to suggest that application of all of the nitrogen through the planter will result in any better yield than sidedressing soon after planting. In fact, on most soils, application through the planter will be no better than a preplant application a few days before planting.--Robert G. Hoeft