WOOSTER, Ohio -- Corn producers who use "pop-up" or in-furrow starter fertilizer at planting should be mindful of their application rates so as not to cause seed injury, resulting in significant plant stand and yield losses.
Robert Mullen, an Ohio State University Extension soil fertility specialist, said that he is receiving calls from farmers who are experiencing crop injury following pop-up starter applications of nitrogen, potassium and/or phosphorus. Mullen suspects that farmers might be adding too much fertilizer.
"Anytime you put pop-up materials near the seed you risk germination problems for two reasons. One is the effect from salt; it inhibits the seed's ability to take up water," said Mullen, who also holds an appointment with the Ohio Agricultural Research and Development Center. "The other is the zone of ammonia that is created when putting too much urea-based nitrogen too close to the seed. Ammonia is very toxic to a germinating seed, and its toxicity is a function of how wet or dry the soil is. The drier the soil, the larger that volume of soil is saturated with ammonia."
Based on five years of Ohio State University research, Mullen recommends that farmers apply no more than 8 pounds of pop-up starter, and if the soil is of a sandy, course-texture, the amount applied should be no more than 5 pounds.
"Eight pounds is the 'safe' number. If you push the envelope and apply more, you run the risk of seed injury," said Mullen. That injury can translate into significant stand reductions of "upwards of 50 percent to 60 percent," said Mullen. "You've cut the number of plants per acre in half and have made a significant impact on yield."
Mullen is a bigger fan of two-by-two starter, where fertilizer is applied 2 inches to the side and 2 inches below the crop, creating more of a buffer between the seed and the fertilizer.
"With starters, the greatest benefit we get is from the nitrogen. With a pop-up, the maximum amount of nitrogen recommended is 8 pounds. That's not really a whole lot to work with," said Mullen. "But with a two-by-two starter, a farmer can put upwards of 30 to 40 units of nitrogen without any risk to the seed."
Pop-ups are becoming more popular as starters, but it's more by equipment design than by choice, said Mullen.
"Not everybody has the same planter capabilities. With planters getting larger -- I've seen 24–row planters here in Ohio -- it can be a challenge to get two-by-two starter attachments on such large planters because of weight and how the implement folds," said Mullen. "But it's relatively easy to put a pop-up on a planter. It just goes right in the seed furrow."
Whether using a two-by-two or a pop-up, farmers generally apply starters at planting to give plants that extra boost of growth they need to get up and out of the cool, wet soils in the spring. Mullen's research has shown that starter fertilizer typically results in improved plant performance of at least one growth stage ahead of those plants that did not receive a starter application.
In addition, Mullen has found that starters are beneficial when farmers are delayed in sidedressing their crop.
"What it does for the producer is it buys him time. We have found that if you delay sidedress to V-8 growth stage or later and you don't have any starter material on that crop you'll take a yield hit. You've lost 20 bushels per acre on the spot," said Mullen. "In those situations, starter is sort of like an insurance policy."
However, researchers have found no significant yield advantages to using a starter if a farmer sidedresses corn in a timely manner (V-4 growth stage or earlier).
"Typically we observe early season increased vigor and growth compared to the control. But we come back to those same plots at harvest and yield levels are usually similar," said Mullen. "The field looks good, but applying the starter didn't necessarily translate to a yield advantage."
Mullen understands why farmers find value in applying pop-up starter fertilizer, but strongly encourages them to follow the recommended rates. He cautions that just because a farmer gets away with higher pop-up applications one year, doesn't mean that same rate will work to his or her advantage the following year. Too many environmental variations are involved for farmers to risk taking a significant hit on corn yields, said Mullen.