Wet Soils and N Loss: How Much of the Applied Nitrogen has Undergone Nitrification?

With the excessive and continual wet weather during most of May, many parts of eastern and central Kansas are faced with the potential for leaching and/or denitrification losses of valuable nitrogen (N) from fields already planted or intended for corn and sorghum. The potential for N loss is much greater when the weather is warm than when it is cool.

 

The leaching and denitrification processes are quite different, and normally occur on different types of soils and under different situations. However, it is important to keep in mind that both processes involve the nitrate form (NO3-) of nitrogen. The nitrate-N present in fertilizers such as urea ammonium nitrate (UAN) solution (25% nitrate), is immediately susceptible to leaching or denitrification loss. Other forms of N have to be converted in the soil to nitrate-N before leaching or denitrification would become a problem. This microbial conversion process is called nitrification and requires oxygen in the soil (an oxidation process). Before estimating how much N may have been lost in wet soils, producers should first try to get some idea of how much of the applied N may have undergone nitrification into nitrate-N at this point in the season.

 

 

Several factors influence how quickly ammonium-N converts to nitrate-N in soil. These factors include:

Nitrification is an aerobic microbial process and requires high levels of soil oxygen. Conditions that reduce oxygen supplies, such as saturated soils, will inhibit nitrification and keep N in the ammonium form (Figures 1 and 2). Optimum soil temperatures for nitrification are in the range of 75-80 degrees F. 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. Conversely, nitrate-N has a negative charge and is repelled by the soil, thus remaining in the soil water to potentially leach out of the root zone.

 

 

Denitrification is the conversion of nitrate-N to gaseous N by soil microbes in anaerobic (waterlogged) soils. Denitrification loss is a problem normally associated with medium- to fine-textured soils under wet and warm soil conditions. Criteria that must be met for denitrification in soils to occur include:

Table 1. Effect of soil temperature and duration of waterlogged soil conditions on denitrification. 

 

 

Table 1 presents some data regarding the potential for denitrification N loss at several soil temperatures and provides some general guidelines for potential N loss. Loss was minimal with soil temperatures of 55-60 degrees F, but severe at soil temperatures of 75-80 degrees. The average soil temperatures at the 2-inch depth for selected Kansas Mesonet stations are shown in Figure 1. Note that Table 1 represents the amount of nitrate-N potentially lost via denitrification – not necessarily the portion of total N applied. The portion of soil N in the form of ammonium-N is not subject to denitrification.

 

Figure 1. Average 2-inch soil temperatures in degrees F (KS Mesonet).

 

Figure 2. Corn field affected by saturated conditions in NE Kansas in early June 2015. Soil samples in saturated areas (yellow corn) and dry areas (green corn) reveal similar total N amounts, however the NH4 fraction was higher in the saturated areas (lower nitrification rate due to lack of oxygen). The yellow corn in this case is due to lack of oxygen in the soil rather than a lack of nitrogen.

 

 

Thus far this year, the saturated soil conditions have been accompanied by low soil temperatures (Figure 1), therefore we can expect that a good amount of the N applied in late fall or early spring to still be in the ammonium form. This would suggest a lower risk for N loss at this point in the season. However, producers that applied all their N very early in the fall should be in position to apply an additional 30 to 50 lbs of additional N if needed. Keep in mind that from the N uptake standpoint, we have a big window for any “supplemental” N application. Recent work at K-State has shown that N applied close to tasseling can be used effectively by corn. That will require dribbling the N on between the rows with high-clearance application equipment. If your corn “runs out of gas” later in the season, it gives you an option to correct the problem.

 

All corn that appears yellow at this time will not 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 the soil and the corn is probably yellow due to “wet feet” and will green up when things dry out and oxygen gets back into the soil. Thus, no additional N will be needed.

 

Trying to sort out exactly how much N loss has occurred in a specific field is difficult, if not impossible. Producers can 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. These areas can serve as a point of reference for evaluating your crop.