Nitrogen loss potential in wet soils
Many parts of eastern and central Kansas are faced with the potential for leaching or denitrification loss of nitrogen from fields planted or intended for corn and sorghum due to recent heavy rains. The warmer the weather, the greater the potential for loss.
The leaching and denitrification processes are quite different, and normally occur on different types of soils and under different situations. But both involve the nitrate form of nitrogen. The nitrate-N present in fertilizers such as ammonium nitrate (50% nitrate) or UAN solution (25% nitrate), is immediately susceptible to leaching or denitrification loss. Other forms of nitrogen have to be converted in the soil to nitrate-N before leaching or denitrification would become a problem. Before estimating how much N may have been lost in wet soils from leaching or denitrification, producers should first try to get some idea of how much of the N they applied may have undergone nitrification into nitrate-N at this point in the season.
Factors affecting nitrification
How quickly ammonium-N in soil converts to nitrate-N is a function of soil oxygen content, soil temperature, pH, how the N is applied, and some characteristics of the fertilizer. Nitrification is an aerobic process and requires high levels of soil oxygen. Conditions that reduce oxygen supplies, such as wet soils, will inhibit nitrification and keep N in the ammonium form. Optimum soil temperatures for nitrification are in the range of 75-80 degrees. When urea or UAN are broadcast, nitrification will occur more rapidly than when those materials are banded. The nitrification rate of anhydrous ammonia is even slower, due to the impact of the ammonia on the organisms in the application band. The use of a nitrification inhibitor, especially with banded ammonia, will slow the process of nitrification even further.
Leaching involves the movement of nitrate-N below the root zone with water. Leaching losses are primarily a concern on coarse-textured, sandy soils, where water moves quickly through the soil profile. Ammonium-N is not readily lost to leaching, even on coarse-textured soils. Ammonium-N has a positive charge, and is retained on the cation exchange capacity (CEC) sites of soils, while nitrate-N has a negative charge and is repelled by the soil and remains in the soil water.
Denitrification is the conversion of nitrate-N to gaseous N by soil microbes in anaerobic (low-oxygen, waterlogged) soils. Denitrification loss is a problem normally associated with medium- to fine-textured soils under wet weather conditions. There are several conditions that must be met for denitrification to occur. These include:
- Lack of soil oxygen. The specific soil microbes responsible for denitrification only function under anaerobic soil conditions. Poorly drained, compacted, and/or waterlogged soils have the highest potential for denitrification loss. Poorly drained soils in central and eastern Kansas, and the claypan soils of southeast Kansas, are normally the soils in the state with the most significant potential for denitrification. Well-drained soils normally pose little risk of significant denitrification loss.
- Nitrate-nitrogen. Denitrification only affects nitrate-N; it has no impact on ammonium-N. Maintaining N in an ammonium form is an effective strategy to avoid denitrification losses, and is the reason there are differences among N sources in denitrification potential.
- Warm soil temperatures with organic residue and/or organic matter. Denitrification is a microbial process, and ample food (organic materials) and warm soil temperatures are required for microbial activity. Like nitrification, the optimum temperatures for denitrification are in the 75-80 degree range.
It has been warm enough this spring that at least some of the N applied early, especially the fall-applied N, has likely been nitrified. Where heavy rainfall in early June resulted in several days of saturation, some denitrification loss likely has or will occur. Not all of the N will have been lost, but producers who applied all their N in the fall or very early spring should be in position to apply additional N if needed.
All corn that appears yellow at this time won’t be seriously N deficient. In fields where N application was delayed until late April or early May, especially where ammonia was applied, the majority of the N is likely still present. In this case, the corn is likely yellow due to the effect of soil saturation and will green up when things dry out and oxygen gets back into the soil. No additional N may be needed at all.
Trying to sort out exactly how much N loss has occurred in a specific field is difficult, if not impossible. One thing producers can do is to establish some reference strips in the field to serve as a base for comparison. Apply the equivalent of an additional 50 to 75 pounds of N per acre to 3 to 5 areas in a field. (Note: If your crop is in 30-inch rows, then applying 1.8 pounds of urea to 6 rows in a 40-foot-long section = 60 lbs N/acre.) These areas can serve as a point of reference for evaluating your crop.
If you have access to a chlorophyll meter or active crop sensor, you can use these instruments to make measurements of greeneness and growth, and make some fairly good estimates of the amount of N needed. But, even without these tools, the reference strips can help you visually evaluate whether the crop will respond to additional N. At the very least, it will allow you to make an “educated guess.”
Recent work at K-State has shown that N applied at the 16-leaf stage can be used effectively by dryland corn. That will require dribbling the N on between the rows to minimize leaf damage with high-clearance application equipment. But if your corn “runs out of gas” in a few weeks, it gives you an option to correct the problem.
Dorivar Ruiz Diaz, Nutrient Management Specialist
Dave Mengel, Soil Fertility Specialist