Maximize fertilizer return and protect water supplies by understanding your soil types
Nitrogen’s elusive behavior makes it challenging to produce top yields while minimizing the escape of nitrate. But understanding how soil influences the presence or absence of nitrogen during the growing season will make your fertilizer program more cost-effective while safeguarding water quality. The good news is you already possess much of that knowledge from farming fields for many years.
“Managing nitrogen means supplying the difference between what the crop needs and the amount the soil supplies, and doing it at the right time,” explains Farm Journal Field Agronomist Ken Ferrie.
Let’s start by thinking about what the crop needs, which can get a little confusing. “We often talk in terms of application rates from 0.8 lb. to 1.2 lb. of nitrogen per bushel of corn produced. But that is only the amount of nitrogen that you apply,” Ferrie says.
Soil holds thousands of pounds of organic nitrogen per acre. Only a small percentage of that nitrogen mineralizes into inorganic nitrogen that plants can use. Farmers must apply the rest.
“A corn crop consumes up to 1.6 lb. of nitrogen per bushel,” Ferrie adds. “So a 200-bu. crop needs 320 lb. of nitrogen per acre. It’s common to apply 180 lb. of nitrogen per acre for a 200-bu. goal. If we achieve the 200-bu. goal, the soil supplied 140 lb. of nitrogen (converted from the organic to inorganic form by soil microbes).”
Figuring out what nitrogen rate to apply requires understanding each soil type you farm. “In our studies comparing fertilized and unfertilized strips, we find soils supplying from 70 lb. or 80 lb. of nitrogen per acre to more than 250 lb.,” Ferrie says. “We learned this by determining the potential for mineralizable nitrogen using the Illinois Soil Nitrate Test, and then testing for nitrate in the soil throughout the growing season.”
Weather complicates the matter by impacting the amount of nitrogen the soil will supply. “A dry summer slows mineralization,” Ferrie explains. “A warm, wet fall can speed it back up. In that case nitrogen becomes available at the wrong time.”
Soil’s ability to mineralize nitrogen is tied partly to soil health—factors such as compaction, infiltration and soil pH. “Considering all these factors, we can determine a range of low, medium and high nitrogen-supplying potential,” Ferrie says. “The higher the nitrogen-supplying power of a soil, the more latitude you have on nitrogen recommendations. With high nitrogen-supplying power and soil that’s in good shape, we might be able to apply only 0.5 lb. of nitrogen per bushel and be confident we won’t jeopardize yield.”
Organic matter content provides a clue to a soil’s nitrogen-supplying power. “A 4% organic matter soil has more nitrogen-mineralizing ability than soil with 1% organic matter,” Ferrie says. “A common rule of thumb is each percent of organic matter in a soil is worth 15 lb. of nitrogen per acre.”
Here’s where knowledge of your fields becomes important. Various soil testing laboratories might obtain different organic matter readings because they use different extraction methods. It’s not unusual for one lab to report 2% organic matter and another 4% organic matter from the same soil sample. If you have multiple years of soil test results from the same lab, you’ll have a feel for how your soils compare on organic matter content.
Your soil’s nitrogen-supplying capability also depends on your knowledge of field history. “Over 15 years of research, we have discovered while organic matter plays a role it doesn’t always correlate with the amount of mineralizable nitrogen,” Ferrie says. “We have found situations where a soil with 2% organic matter has the ability to supply more than 200 lb. of nitrogen per acre; but across the road or at the end of the field the same soil might supply only 90 lb.”
The mineralizing power of soil is influenced by past farming practices. “Soils where pastures were located 30 years ago often have higher levels of mineralizable nitrogen, although the organic matter content differs very little from the rest of the farm,” Ferrie explains. “Sometimes we find high mineralization rates that can be explained by manure and crop rotations. Other times we find high-mineralization zones we can’t explain because we don’t know the farm’s history.”
Fine-tuning nitrogen rates requires putting fields into management zones based on soil type, texture and slope. “Then add details from yield maps, aerial images and other sources,” Ferrie says. “Over time, subdivide the field into additional management zones. For example, in an area with the same soil type we might find one half yields more than the other half. It’s not unusual to find 50 lb. to 180 lb. more mineralizable nitrogen per acre in higher-yielding areas of the same soil type.”
Besides knowing your fields’ history, take into account the risk of nitrogen loss by soil type. “It’s possible for the same soil to be high in mineralizing power and also have a high risk of nitrogen loss,” Ferrie says.
The place to start is with your state soil survey. It will tell you the soil’s natural drainage characteristics, subsoil texture and percentage of slope for each soil you farm. In the 1980s, University of Illinois agronomy professor John D. Alexander found these traits, along with the nature of the slope, subsoil texture and rainfall during the growing season, determine the soil’s potential to lose nitrogen (see “Determine Your Risk of Nitrogen Loss” below).
Alexander rated each of his state’s soils as high, medium or low based on its potential to lose nitrogen applied during the growing season. He refined his ratings to include the effect of average or above-average rainfall. He determined with average rainfall, a low-risk soil would lose 5% of its applied nitrogen; a medium-risk soil, 10%; and a high-risk soil, 20%. With above-average rainfall, the losses would be 10%, 20% and 40%, respectively.
“The idea is to consider the loss potential for each field and soil type, and treat each one accordingly,” Ferrie says. “We can do that using variable-rate application technology.”
Is it worth the effort? Imagine a 40-acre central Illinois field with five soil types, one of which has two different types of slopes. Applying Alexander’s ratings, three acres have low risk of nitrogen loss, 17 acres are medium risk and 20 acres are high risk. If the farmer applies 200 lb. of nitrogen per acre across the entire farm, the potential nitrogen loss with average rainfall is 1,170 lb. (calculated by multiplying the number of acres in each class times the rate times the percentage of potential loss). With above-average rainfall, the potential loss is 2,340 lb., which might find its way into water supplies.
If nitrogen costs 65¢ per pound (as it did when Ferrie conducted the study), with average rainfall, that’s a potential loss of $19 to $26 per acre. With above average rainfall, the cost of that lost nitrogen rises to $38 to $52 per acre.
“Avoiding these losses can provide savings to offset implementing a 4R [right product, right rate, right time, right placement] nutrient management program, such as the cost of sidedressing or applying a nitrogen stabilizer,” Ferrie notes.
Applying the correct nitrogen stabilizer can minimize the loss. “Based on our study, using the cost of stabilizer at that time, if you don’t protect nitrogen, you can plan on losing $19 worth of nitrogen per acre in an average year, and more in a wet year,” he says. “Or you can spend $10 to $40 per acre, depending on the product, to protect your nitrogen.”
Variable-rate technology lets us apply nitrogen based on soil-supplying power and potential for loss. Here are factors to consider in soil-by-soil nitrogen management:
- The easiest scenario to manage is when an entire field has a high nitrogen-supplying power and low risk of loss. “That means you can crank down your nitrogen recommendation and have a lot of options to apply,” Ferrie says.
- The toughest scenario is a field with low nitrogen-supplying power and high risk of loss. To reach your yield goal, you’ll probably have to apply more nitrogen than on your other soils. “Make multiple applications,” Ferrie says. “On irrigated land, consider making up to five applications.”
- Many fields contain soils that vary in their loss potential. “Put a risk factor to each soil,” Ferrie says. “Decide if you should apply a nitrogen inhibitor, and which one.”
- If you have a lot of high-risk soil, it’s even more important to follow a 4R nutrient management plan to maintain yield and reduce nitrogen loss.
- If you drain a wet area, you shift the risk of loss from denitrification to leaching, which might require a change to your nitrogen management program.
- If you farm high-risk soil, pay close attention to the soil temperature if you apply nitrogen in the fall. Consider shifting applications from fall to spring, and making multiple applications of smaller amounts.
- If nitrates carry over in the soil after harvest because of drought, consider planting cover crops on high-risk soils.
- Understanding your soils makes it easier to “call an audible” during the growing season. “For example, if you get a lot of rain in June, you can sample your highest-risk soils for nitrogen loss,” Ferrie says. “If the nitrogen you applied is still present, the rest of your farm probably hasn’t lost much either. Or you might find only your highest-risk areas need a rescue application.”
Nitrogen management based on soil-supplying power and loss risk takes more effort. “But once you learn the characteristics of your soils, you will know how to manage them in any weather patterns,” Ferrie says.
Watch for more articles on how to mind water quality and keep nutrients in your fields where they belong.
Six Steps to Assess a Field’s Risk of Nitrogen Loss
In the 1980s, University of Illinois agronomist John D. Alexander devised a rating scale to determine a soil’s risk of nitrogen loss. It was based on the characteristics of natural soil drainage class, permeability, slope class, slope configuration, subsoil texture and the amount of warm-season rainfall. Research and observation by Farm Journal Field Agronomist Ken Ferrie suggest Alexander’s rating system is still valid today.
However, in modern soil surveys, some of the soil characteristics Alexander used are being reported in a different format. Thomas Zerebny of Crop-Tech Consulting has modified Alexander’s scale to fit the format of modern soil surveys. The modified scale can be used as a guideline to evaluate your soils’ potential for nitrogen loss.
1. Go to the Natural Resources Conservation Service Web Soil Survey at websoil survey.sc.egov.usda.gov. Click on the green “Start WSS” button.
2. Locate your area of interest by clicking on the map or using the “Quick Navigation” menu.
3. Highlight the area of interest using the “Define AOI by Rectangle” or “Define AOI by Polygon” button.
4. Click on the “Soil Map” tab for descriptions of the soils in your area of interest.
5. Under “Map Unit Legend,” click on the soil type to open a box containing the characteristics of that soil.
6. Find the down-slope shape, subsoil texture, slope, natural drainage and Ksat information for that specific soil type. Google average rainfall for your area of interest. Use the table below to record numerical ratings for each of the six characteristics.
Taking a Nitrogen Plan to the Field
Here’s how Farm Journal Field Agronomist Ken Ferrie applies soil-by-soil nitrogen management to a field with various soil types. With a 200-bu. per acre yield goal, the crop will require 340 lb. of nitrogen per acre.
1. Calculate how much nitrogen would be required in the absence of practices that minimize nitrogen loss. Study each soil’s characteristics in your state soil survey. “From the Illinois Soil Nitrogen Test, we learn some soils are capable of supplying only 110 lb. of nitrogen per acre,” Ferrie says. “On those soils we must apply 230 lb. of nitrogen per acre.”
With average rainfall, 5% of the applied nitrogen will be lost in low-risk areas and 20% in high-risk areas. “If we fail to manage nitrogen loss, we will need to apply 240 lb. of nitrogen per acre in the low-risk areas and 270 lb. per acre in the high-risk areas to reach our yield goal,” Ferrie says.
“But other soils in the field are capable of supplying 175 lb. of nitrogen per acre. There we need to apply 165 lb. of nitrogen per acre to meet our 200-bu. yield goal,” he adds. “But if we fail to manage nitrogen loss, we’ll have to apply from 173 lb. to 198 lb. of nitrogen per acre to make up for the 5% to 20% we’ll lose, depending on whether those soils have low, medium or high risk of loss.”
2. Implement nitrogen management practices that minimize the loss of applied nitrogen fertilizer, so the total rate applied will be as close as possible to the amount actually used by the crop. “We split our nitrogen fertilizer up into multiple applications, following the principles of the 4Rs—right product, right rate, right time, right placement,” Ferrie says.
If the year turns wet, he advises testing for nitrate in the soil, starting with the areas of high loss risk. “If necessary, we increase the nitrogen rate in our later applications to make up for losses caused by leaching,” Ferrie adds. “When appropriate, we use a nitrogen stabilizer to prevent or minimize nitrogen loss.”