Varying plant population within a corn field means you also must vary your nitrogen rate. Deciding how much nitrogen to apply is challenging—multiple applications, including sidedressing, are required.
Almost overnight, it seems, variable-rate application technology (VRT) for seed and fertilizer has arrived. The new equipment brings the potential for higher yield and less pollution of natural resources. The equipment is ready, but are you?
"Variable-rate application of seed and fertilizer is a systems approach to farming," says Farm Journal Field Agronomist Ken Ferrie. "To make the equipment pay for itself, you have to think the system through and put all aspects of it into play."
What is variable-rate? "An environmentally sound variable-rate program usually involves moving seed and nitrogen from areas of excess to areas where you’re not applying enough," Ferrie says. "You’re not changing the total amount applied as much as you are moving resources around within the field itself."
Effectively juggling those resources requires excellent management. "If you’re overapplying nitrogen in part of a field by 90 lb. per acre and you move that fertilizer to places where you’re underapplying, the return can be big in terms of reduced nitrogen loss and increased yield," Ferrie says. "But if heavy rain causes you to lose 100 lb. of nitrogen, the areas you moved the nitrogen from are going to suffer. The areas you moved it to are going to win; they may still suffer, but not as much."
In other words, the purpose of variable-rate application is to manage inputs more precisely.
Nitrogen management. "Deciding how much nitrogen to apply is challenging because nitrogen is controlled microbially in the soil," Ferrie says. "In 2010, many growers applied more than 200 lb. of nitrogen per acre, but they still had yellow corn. It’s not a matter of applying a certain amount—it’s knowing when and where to apply it on a particular field or part of a field."
Microbial activity determines how much nitrogen your soil provides, Ferrie explains. That can range from 30% to 80% of a corn crop’s requirement.
"As they decompose the old crop, microbes immobilize nitrogen in the soil and make it temporarily unavailable to plants," Ferrie says. "Later in the season that nitrogen is mineralized, making it available once again.
"This process is tied to the amount and source of carbon in the old-crop residue [soybean residue breaks down more easily than corn] and the nitrogen reserve in your soil. Understanding this nitrogen cycle helps you know how much nitrogen you need to apply and when to apply it, based on soil type and population."
Each plant needs a certain amount of nitrogen. "There’s going to have to be a certain amount of nitrogen taken up by each plant," Ferrie explains.
"Increasing plant population is like putting more cattle in a pen—you have to put out more feed. So a higher population requires more nitrogen, which must come either from fertilizer or from the soil.
"In a silage study, 29,000 plants per acre produced 21 tons of silage, and 33,000 plants per acre produced 27 tons of silage," Ferrie says. "Silage analysis showed the 33,000 plants removed
60 more pounds of nitrogen from that field—the uptake into the plant itself.
"The rate of nitrogen per bushel leaving the field may be the same, but if you have 36,000 plants and 34,000 plants per acre, both producing 190 bu. of corn, the amount of nitrogen per plant has changed. And nitrogen drives the whole process of growth."
In a second study, 30,000 plants per acre showed no nitrogen deficiency; but 40,000 plants per acre, less than 50' away—same soil, same hybrid, same nitrogen rate—did. "Whenever you increase population, you must provide more nitrogen," Ferrie emphasizes.
Nitrogen fertilization is always an educated guess, rather than an exact science. Besides considering microbial activity, you must estimate the risk of nitrogen loss. "On one site where we got 12½" of rain, soil nitrate levels fell from 80 parts per million (ppm) to 7 ppm in three weeks," Ferrie says.
Tools you can use. If you understand the risk of loss, you can be prepared to make a late-season nitrogen application if conditions require it.
Start by developing a "comfort zone" rate for various soil types. "Put out nitrogen rate plots," Ferrie advises. "During the growing season, scout the plots and see when nitrogen deficiencies show up. Watch the effect of weather. Monitor yield response throughout the entire field, not pass by pass. Observe the yield response on various soils.
"At the end of the season, determine if you applied enough nitrogen to keep the corn green. Compare your nitrogen observations to potassium and fertility levels.
"With on-farm test plots, always check the results on a scale," Ferrie adds. "If test weight and moisture changes because of different nitrogen rates, it may throw off your yield monitor results."
In-season nitrate testing can tell you where you stand with soil nitrogen supplies. That can help you react in time to prevent a shortage, Ferrie says. However, it can’t tell you whether
soil nitrate supplies are headed up or down. That’s why you must understand the cycle of nitrogen immobilization and mineralization.
ISNT testing. The Illinois Soil Nitrogen Test (ISNT), also called the amino sugar test, can help predict how much nitrogen your soil may supply, if weather permits. The test requires some knowledge to interpret the results. It does not work well in acid or compacted soils (which are signs that you’re not ready for VRT).
Even with high ISNT values, you still must manage your nitrogen applications to pay the carbon penalty (the immobilization that occurs while microbes decompose old-crop residue) and to make up for losses caused by weather, Ferrie points out.
Because of all these variables, if you insist on applying all your nitrogen in the fall, you’re not ready for variable-rate application, Ferrie says. Instead of applying all your nitrogen in the fall, plan to make multiple applications, so corn plants never run short.
"Our studies in central Illinois show you should include at least 60 lb. per acre of nitrogen following soybeans, or 100 lb. per acre following corn, near the surface, to feed soil microbes as they decompose crop residue in the late fall and early spring," Ferrie says. "The exact rate and timing depends on your weather conditions and latitude. Also consider applying some nitrogen as starter to make sure young plants don’t go hungry."
Finally, plan to sidedress part of your nitrogen, so you can apply more where it is needed and less where it isn’t. As a rule of thumb, gear your equipment so you can sidedress corn in one less day than it takes to plant it.
Be prepared to make a late-season application if you need to replace nitrogen lost to excessive rainfall. "Nitrogen is king," Ferrie says. "A good manager—one who is capable of using variable-rate technology—will do what it takes to get nitrogen right."
How much seed where? Once you understand nitrogen management and have your soil pH and fertility at appropriate levels, you’re ready to think about varying plant population. How many plants an area can support is determined by several factors.
The first questions to answer are how much rooting capacity and waterholding capacity you have to work with. Both require good soil structure. If your soil has dense or compacted layers resulting from horizontal tillage or heavy harvesting equipment, they must be removed.
"Digging up plant roots will show you whether roots are penetrating downward, where they can reach subsoil moisture and fertility, or whether they are shallow and growing sideways along a compacted layer or a tillage pan," Ferrie says.
Compaction and density changes not only inhibit root growth, but also water infiltration. To illustrate that, Ferrie drove barrel sections into the ground and poured in 3" of water. Where soil density problems had been corrected by vertical tillage, the water infiltrated in 1½ hours. Where two tillage pans were created by a moldboard plow and a soil finisher, infiltration took 22 hours.
"In the real world, the water not only would have failed to infiltrate, but it would have run to a lower area and ponded," Ferrie explains. "The water would have crowded air out of the soil, and nitrogen would have been lost through denitrification."
After dense layers are removed, plant population depends on soil depth, its water-holding capacity, your water supply (whether from rainfall or irrigation) and your evaporation rate.
You can determine your soil’s water-holding capacity and other characteristics from soil surveys available on the Internet. "Water-holding capacity is based on soil texture," Ferrie says. "If you have been farming a piece of land long enough, you already may know which soils hold more water and which hold less.
"If your soils vary a great deal, you could be sacrificing yield in both high-yielding and low-yielding areas if you plant the same population across the entire field," Ferrie notes. "If your soil can hold 2" of usable water per foot and you have 3' of work zone to work with, that’s a place where you can push plant population."
Management zones. To use VRT, you’ll need to divide fields into management zones. Besides your own personal observations, there are a number of tools that can help you accomplish this.
"Start with a soil map," Ferrie says. "If you have good yield maps, lay that data over the soil map and yield zones will begin to emerge. Look for yield zones that show up year after year. Use only maps made with well-calibrated yield monitors."
Normalized Difference Vegetation Index (NDVI) maps, made from aerial photos, are even better than yield maps because they eliminate the problem of uncalibrated yield monitors, Ferrie says. They also provide much higher resolution, with more detail.
The management zones you designate will tell you where to apply more or less seed and fertilizer. After you designate the zones, test soil by management zone rather than by grid.
"Your observations will help you refine your management zones," Ferrie says. "If you see drought stress, down corn or poor stalk health, the population may be too high. Double ears that do not abort before pollination, suckers that tassel and ears filled completely to the tip suggest the population may not be high enough."
VRT can boost your yields, which will boost your bottom line, Ferrie concludes, if you use a systems approach. Otherwise, this marvelous technology could turn into nothing more than expensive decorations for your tractor cab.
What Rate to Plant Where?
You’ve mastered nitrogen management, removed compaction and dense soil layers, figured out your water-holding capacity and mapped out management zones. So what rate should you actually plant?
That requires knowing the potential of your hybrids, says Farm Journal Field Agronomist Ken Ferrie. "Even if you’re not yet ready to vary your planting rate, start working with your hybrids to see how they respond," he advises.
"Plant various rates of each hybrid on various soil types," Ferrie suggests. "If you’re planting a flex-ear hybrid, find out how much flex it really has. Some hybrids may surprise you at low populations."
In Ferrie’s studies, pushing the population with a flex-ear hybrid and failing to supply enough water and nitrogen sometimes caused yield to not just level off but to decline. "On the other hand, if we push determinant-ear hybrids but don’t provide enough water and nitrogen, the yields tend to flatline," he explains. "You need to learn how far to push the population level with each hybrid.
"Hybrid performance also varies by row width," Ferrie continues. "Our studies indicate that narrow-row corn responds to higher populations. These are all things that you can learn by studying the hybrids on your farm."
Nitrogen management tools:
- Visual inspection
- Tissue testing
- Stalk testing
- Soil nitrate testing
- Organic nitrogen testing, using the Illinois Soil Nitrogen Test (amino sugar test)
Management zone tools:
- Soil map
- Electroconductivity map
- Topography map
- Yield maps—must be calibrated
- NDVI maps
- February 2011