The Goldilocks Dilemma
Nitrogen Management Plays Important Role in Profitability
The 4Rs of nutrient management – applying fertilizer from the right source at the right time at the right rate in the right place – are at the core of any successful crop management program. Of all the essential plant nutrients, nitrogen (N) is the poster child for why the 4R principles are so important.
Nitrogen is one of the three major nutrients necessary for plant growth, and it is also the most likely to be deficient in soils due to volatilization, leaching and denitrification. Because it is quite mobile in the soil, any excess N not taken up by plants can be lost to the environment and cause problems, in addition to increasing unnecessary input costs. At the same time, a deficiency in N can result in a significant loss of yield, so applying the right amount at the right time and in the right place is vital to a profitable harvest.
Almost 80 percent of our air is made up of nitrogen gas (N2). Rain can pick up atmospheric N and deposit it in the soil, and some plants, most notably legumes, can “fix” nitrogen from the air and convert it into a form usable by plants. Soybeans and alfalfa, while being worthwhile crops on their own, are also nitrogen-fixing plants and play an important role in crop rotation. Several hundred pounds of N per acre per year can be deposited in the soil by an alfalfa crop.
Without going into too much chemistry, it is important to know plants can only use N in two forms – ammonium (NH4+) or nitrate (NO3-). Nitrogen from animal manure or other organic material must be converted to these forms over time as it decays. About 20 pounds of usable N in the form of ammonium per acre per year is released into the soil for each percent of organic matter. Commercial fertilizers can provide ammonium or nitrate directly to plants so it is immediately available. Hydrogen gas (H2) is combined with N2 from the atmosphere to make ammonia (NH3), the starting point for manufacturing commercial fertilizers.
Nitrogen can be lost from the soil through crop removal, soil erosion and runoff, and leaching. Surface-applied N containing urea can also be volatilized back into the atmosphere as ammonia gas, especially in warm weather and high-pH soils. When soil is waterlogged and warm, conditions are right for denitrification, a process where bacteria convert nitrate back into nitrogen gas that goes back into the atmosphere. Nitrogen protection technologies can prevent losses from volatilization, leaching and denitrification, protecting yield and the environment.
The Goldilocks Dilemma: How to Get the Amount Just Right
Because N fertilizer is a significant cost for growers, and excess N is detrimental to the environment, it’s important to have the right quantity – not too much, not too little, but just right. At some point, the yield gains become smaller and smaller as more N is applied; it isn’t a straight-line relationship, but curvilinear.
A study by Jim Camberato and Bob Nielsen at Purdue University evaluated corn yield response to N in 53 field trials over a period of 11 years to determine the average Agronomic Optimum N Rate (AONR) for corn on various soil types in Indiana. They define the AONR as the N rate that will produce maximum grain yield, no matter what the cost. They also calculated the Economic Optimum N Rate (EONR), which identifies the N rate that will result in the maximum ROI of the applied N. The EONR fluctuates depending on the cost of N and the price of corn.
Any calculations must include the amount of N already in the soil from organic matter, and this can vary greatly. Camberato and Nielsen saw differences of existing N providing as little as 25 percent of a crop’s needs or as much as 50 percent. The weather, crop health, N loss from crop removal and other processes, and soil type can also cause variations from year to year. In their words, “N use in corn is part of a complex biological system that interacts with everything under the sun, including the sun.” Their goal was to come up with an AONR for each region that will work to optimize profit over a period of several years; the “just right” amount.
The authors also made the following recommendations to optimize N efficiency and reduce loss:
- Inject N just prior to the beginning of the crop’s stage of most rapid N uptake: V6, which is six leaves with visible leaf collars, about 18 inches tall
- Use anhydrous ammonia in fall or early-spring applications, because it is the slowest to convert to the nitrate form that can be lost to leaching or denitrification
- Incorporate urea-containing fertilizers as well as manure to eliminate volatilization or use a urease inhibitor
- Consider using testing tools such as the various nitrate/nitrogen tests available to evaluate N content
Determining a rate of nitrogen that optimizes yield and profit can feel a bit like hitting a moving target, and it is. But smart management can make a big difference in terms of ROI and the environment.