Nitrogen loss potential in wet soils
Some areas of Kansas are faced with the potential for leaching or denitrification loss of nitrogen in fields planted or intended for corn or sorghum due to recent persistent rains and very wet soils.
The leaching and denitrification processes are quite different, and normally occur on different types of soils and under different situations. But both involve the loss of nitrate 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. The residual soil nitrate-N measured by a soil test is also 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. This conversion is a biological process, and requires conditions appropriate for the activity of the bacteria involved.
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
Since nitrification is a biological process, 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, some characteristics of the fertilizer, and perhaps most importantly, how long the N has been in the soil. 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 75-85 degrees. But nitrification occurs any time the soil temperature is above freezing, just at a slower rate. As a result, the timing of N application is critical for estimating the amount of N that may be present as nitrate. For example, winter applications of urea are much more likely to have been converted to nitrate by this time of year than a preplant application of urea made in late April.
Another key factor impacting nitrification rate is how the fertilizer was applied. When urea or UAN are broadcast, nitrification will occur more rapidly than when those materials are banded. Broadcast fertilizer is in contact with more soil containing the bacteria responsible for nitrification, so the nitrification process occurs more rapidly. Banded UAN or urea reduces fertilizer-soil contact, and has fewer potential microorganisms in contact with the fertilizer, thus slowing the conversion rate.
The nitrification rate of anhydrous ammonia is even slower, due to the toxic effect of the ammonia on the organisms in the application band. Under the warm soil temperatures found in early fall or late spring, it can take 2-3 weeks for nitrification to begin where ammonia has been applied. Cooler soil temperatures, such as often found in late fall or very early spring applications, cause the conversions of ammonia to ammonium, and the recolonization of the ammonia band area by the organisms to be much slower. In addition, the wider the fertilizer spacing and higher the rate, the slower nitrification will proceed. This is why many people refer to ammonia as a self-inhibiting product. The addition of a nitrification inhibitor, especially with banded ammonia, will slow the process of nitrification even further. This has been shown to be is an especially effective tool on dark colored, poorly drained, heavier-textured soils.
Leaching involves the physical 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. Fortunately, many of our sandy soils contain lenses or layers of heavy-textured soil below the surface which can slow water movement and reduce the rate of leaching. This can significantly reduce the loss of nitrate from the root zone by leaching. Unlike nitrate-N, ammonium-N is not rapidly 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. These organisms are always present in the soil, but are capable of utilizing the oxygen from nitrate to support their respiration when free oxygen is not present in the soil. Denitrification loss is a problem normally associated with medium- to fine-textured soils under wet conditions, when the soil pores fill with water and oxygen is depleted. There are several conditions that must be met for denitrification to occur. These include:
- Lack of soil oxygen. Denitrification only occurs 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 75-85 degree range.
What should you do now?
While it has been a cool spring, it has been warm enough that a large part of the N applied early, especially the fall- and winter-applied N, has likely been nitrified. Where recent heavy rainfall resulted in several days of saturation, some significant denitrification loss likely has or will occur. Not all of the N has been or will be lost, but producers who applied all their N in the fall, winter 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 the N application was made in late April or early May, especially where ammonia was applied, the majority of the N is likely still present as ammonium and the corn is likely yellow due to the effect of soil saturation. In this case, the corn will green up when conditions dry out and oxygen gets back into the soil. No additional N may be needed at all. Watching several fields in northern Riley County over the last week to 10 days, the green-up of the corn has been noticable.
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. One idea to help you assess your situation is to create some reference strips in the field by adding some additional N when conditions become a little drier. Adding 1 pound of urea to an area 4 rows wide by 25 feet long would be equivalent to adding around 80 pounds of additional N. Observing the differences between the “reference strip” and the balance of the field can provide a good idea of the degreee of N loss which has occurred.
One important thing to remember is that corn doesn’t take up a lot of N until it reaches the 6 or 7 leaf stage, when the primary or nodal root system becomes well established. At that point vegetative growth and nutrient uptake explode. Also keep in mind that all of the N you have applied is not going to be lost. So there will be N present to support early growth. If N deficiency is going to occur, it likely won’t be noticable or measurable until after the corn is at least waist-high. This may be too late for normal N application techniques such as sidedressing or over the top applications of urea with ground equipment. But dribble application of N solutions with a high clearance sprayer can be an effective way of applying N later in the season. Recent work at K-State has shown that N applied by dribbling N on between the rows to minimize leaf damage with high-clearance application equipment as late as the 16-leaf stage can be used effectively and efficiently by both dryland and irrigated corn.
If you don’t have access to a chlorophyl meter, counting fired leaves at the base of the plant is another simple way to assess N loss.
If the wet weather continues your corn may “run out of gas” in a few weeks. But this gives you some ideas of options available to both assess and correct the problem. Establishing some reference strips now can help you better understand the situation in your fields later.
Dave Mengel, Soil Fertility Specialist
Dorivar Ruiz Diaz, Nutrient Management Specialist