University of Illinois scientists dug deep into the mysteries of how corn plants use fertilizer, and discovered that many of the long-held beliefs about nutrient movement in the crop are wrong. A new understanding of how today’s high-performing hybrids take up and transport nutrients could provide insight into fine-tuning fertilizer applications and rates, says Dr. Fred Below, professor of crop physiology at the University of Illinois.
It has been decades since scientists took a detailed look at how corn plants use nutrients. Average yields have risen by more than 50 bushels per acre since 1980, Below points out. 200-bushel crops barely raise eyebrows in many regions, and the vast majority of hybrids feature built-in protection from corn rootworm.
The researcher team followed a painstaking protocol of harvesting different parts of the plant separately over the course of the season and analyzing the nutrient makeup of each type of tissue. That allowed them to determine how much of each nutrient was taken up by the plant, where it was used, and how much movement – or remobilization – occurred as the crop matured.
Not surprisingly, notes Below, “Not all nutrients are acquired at the same rate or used in the same way.” What turned out to be a big surprise was the pattern in which some key nutrients were acquired and utilized.
Dr. Below points to sulfur as a prime example. For generations, plant physiologists assumed sulfur and nitrogen behaved pretty much the same way in the crop. After all, both are relatively soluble nutrients, and both are vital in the development of proteins and chlorophyll in the plant. As a result, deficiency symptoms of both nitrogen and sulfur appear early in the season as yellowed plants with low vigor, and the impacts on yield can be devastating.
The research team found that corn plants take up about 75 percent of their total nitrogen need by the time they flower. Later, when the plant needs nitrogen to help fill grain, it simply pulls some out of other tissue and transports it to the ear.
Sulfur does not work that way. Nearly half of the corn plant’s sulfur is drawn straight from the soil into the grain, so it must be taken up after flowering – leaves or stalks cannot offer a reserve for the plant to redirect to the ear when it is needed.
“Sulfur is relatively immobile in the plant – there’s relatively little redistribution of sulfur out of the leaf,” says Below. “That means we’d better have sulfate sulfur available for root uptake during grain fill.”
High Harvest Index
The sheer volume of sulfur used by the corn plant to fill its ear was also a surprise, Below notes – about 25 pounds per acre of sulfur for a 230-bushel-per-acre crop. Like nitrogen, phosphorus, and zinc, sulfur is considered a “high harvest index” nutrient, reflecting that the plant applies a high percentage of its total supply to produce grain.
“The ones that have the highest harvest index are the ones you’ve got to manage the most,” Below says. “It’s a complex term, but from a biology standpoint, it just means, ‘hmmm, this much goes into the grain, so we’d better make sure the crop gets enough.’”
That is why maintaining adequate levels of sulfur in the soil is vital to good yields, says Below. A sidedress application of sulfur in its immediately available sulfate form, such as the form in ammonium sulfate, can help ensure that the crop has access to sulfur when it needs it to fill grain.
“Nitrogen and potassium get taken up by the corn plant at a screaming rate earlier in the season,” Below says. “Sulfur’s not taken up at a gee-whiz volume like that, but it’s still a rapid rate, and half of it has to be taken up after flowering.”
For generations, farmers in many parts of the U.S. have taken free sources of sulfur for granted, including deposits from windblown sulfur released from industrial plants and other fertilizers, such as superphosphate, Below notes.
“Now we have cleaner air and people don’t use superphosphate,” he says. “That sort of tells us why, among other things, growers are seeing more and more response from sulfur fertilizer.”
Mercedes Gearhart, agronomy manager for Honeywell – which manufactures Sulf-N® ammonium sulfate – points out that in the wake of the Clean Air Act, a downward trend in the atmospheric deposition of sulfur is causing sulfur deficiencies in new areas. According to the U.S. Environmental Protection Agency’s latest report, sulfur deposition east of the Mississippi has dropped from a 1990-1992 mean of 11.5 pounds per acre per year to a 2010-2012 mean of 4.1 pounds per acre per year.
“That represents a decrease of 64 percent,” Gearhart points out. “With less sulfur provided by the air and increased removal with high grain yields, sulfur is being mined from the soil at a very rapid and increasing rate. Ammonium sulfate provides sulfur in the sulfate form, which is immediately available to plants. Today’s high-clearance, high-capacity spreaders allow for it to also be applied late in the season to address post-flowering needs. The more we understand about how much sulfur crops need and when they need it, the better we can do at providing 4R nutrient stewardship – the right fertilizer source, at the right rate, at the right time, with the right placement.”
Dr. Below hopes the new nutrient insight helps farmers rethink their fertilizer programs to take full advantage of the tremendous yield potential of today’s corn hybrids.
“There’s a huge gap between what plants can do and what farmers fertilize for,” he says.