SPRINGFIELD, Ill. – An agronomist with a major fertilizer manufacturer says fertility is becoming a growing concern as the ethanol-induced boost in corn prices encourages farmers to grow the crop continuously rather than in a multi-crop rotation.
“We’ve had a two to three year bad trend, where farmers were cutting back on (phosphorus and potassium) levels due to the high fertilizer costs and lower grain prices,” Dan Froehlich, who works for Mosaic in Plymouth, Minnesota, told the recent InfoAg 2007 Precision Farming conference in Springfield.
“That might have been OK because their soil levels may have been high enough and we had higher than expected and higher than average corn yields, but we can’t continue to do that and achieve those consistently high yields.”
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Froehlich said farmers who cut back on nutrients still knew they needed high nitrogen rates, so they would tend to keep their nitrogen rates up and cut back on their phosphorus.
“One of the disadvantages of doing that is you can increase the amount of nitrogen in nitrate form that ends up leaching out and washing away,” he said.
“It’s really crucial that when you adjust those rates up or down, that you do it in balance, so you don’t screw up the whole system.
“It becomes even more critical when in continuous corn. You’re going to have a shift in organic matter. A 180 bushel corn crop will produce 4.5 to five tons of residue. That’s not so bad with a soybean in rotation because there’s only another half ton to a ton of soybean residue, but with continuous corn, you’re dealing with a lot more residue.”
That extra organic matter will affect soil microorganisms, resulting in short-term nitrogen, phosphorus, sulfur and zinc deficiencies. Typically, farmers would need to increase nitrogen fertilizer levels by 30 to 50 pounds per acre to compensate for that.
“If you’re just switching to continuous corn from a corn-soybean rotation, you probably would be wise to adjust your yield goals down five to eight percent for three to five years,” he said.
“You also need to ensure you have more nitrogen available early in continuous corn. There may be plenty of organic nitrogen there, but it might not be there early enough for the corn to get a hold of.”
Froehlich said large amounts of residue and cooler and wetter soil could also result in phosphorus and potassium problems.
“If you band (phosphorus and potassium) pre-plant, it will be more advantageous with continuous corn, especially with higher pH soils. Starter applications will give it an extra pop, to get the corn up and growing faster,” he said.
“In continuous corn, there will be standability issues as plant populations are pushed higher. As we push yields higher, we won’t have enough (potassium) available soon enough to handle it. (Potassium) levels need to increase as corn yields increase to improve stalk strength and reduce crop lodging issues.”
With sulfur, about 90 percent comes from the organic matter residue. Sulfur is critical in the plant life cycle for amino acid, vitamin and chlorophyll production. Sulfur deficiencies are typically associated with coarse, well drained soil with low organic matter, but sulfur variability can exist across a field, making if difficult for producers to predict problems.
“It’s a mobile nutrient, so it’s good sulfur gets tied up in the organic matter so it sticks around, but as you have more residue, it may not release fast enough for the early crop and you may start to see deficiencies.”
Froehlich said newer formulations of phosphorus fertilizer have less gypsum in the granule than older versions. Also, less acid rain in many areas has reduced the amount of sulfur delivered to the soil.
He said many fields in the U.S. corn belt received 11 to 12 lb. per acre of sulfur per year from acid rain in 1985. That has dropped to three to four lb. per acre in 2005. Increased acres of canola, which is a heavy sulfur user, will also diminish soil sulfur levels.
“On the output side, it is mobile so you can have leaching losses,” he said.
“And from the high yields, organic sulfur levels will be drawn down. A 180 bu. corn yield will remove eight to 10 lb. more sulfur than a 130 to 150 bu. corn crop.”
Froehlich said corn doesn’t need large quantities of zinc: a 180 bu. corn crop might need half a pound. The problem is making sure the plants can find it.
“It does have a dramatic effect on early plant size,” he said.
“It’s required in cooler soils. In corn-soybean rotations, soybeans use virtually no zinc. It’s like a fallow situation, where soil zinc can be mineralized and become available for the following year’s corn. If you’re in continuous corn, after Year 2 or 3, you’ll likely start to see some zinc deficiencies start to show up.
“We’ve also found as zinc becomes more deficient, your plants become more susceptible to pathogens. We want to make sure we keep that level up to provide protection from insects and disease pressures.”
Froehlich said there’s a big interaction between phosphorus and zinc. Zinc deficiencies could result if phosphorus is added to soil that is already high in phosphorus, such as in regularly manured soil.
Mosaic has looked at fertilizer formulations to better distribute sulfur and zinc.
Traditionally, a blend of zinc sulfate would have a few scattered zinc granules in a ton of fertilizer.
“Most farmers that have determined they need zinc apply it in a blend with four to 10 lb. of zinc per acre. It’s not because the plant needs that much. It’s because you need that much to get the distribution and being able to blend it,” Froehlich said.
“We wanted to be sure we had sulfur and zinc there all the time. We designed a fertilizer that has ammonium sulfate and elemental sulfur, then incorporated micronutrients into the granule.”
The products, called microessential fertilizers, have layers of micronutrients incorporated in traditional fertilizer granules.
“By putting it in every granule, you need a lot less. We may be putting on a pound or a pound and a half depending on the requirements and you still get 18 to 20 times better coverage distribution wise, than you would with a blend.
“To a plant, we’ve been able to increase the (phosphorus) content in the leaves 10 to 30 percent, the sulfur content up to 42 percent and the zinc concentration up significantly. We’ve developed a bigger plant early that can tolerate stress better.”
