Wet Soil Can Lead To Nitrogen Loss In Farm Fields

With the recent heavy rains across parts of Kansas, saturated soils have become common in many areas. This can potentially cause loss of nitrogen (N) from the soil, says Kansas State University (K-State) Agronomist Dave Mengel.

"There have been a number of questions concerning potential loss of fertilizer N applied last fall, this winter or earlier this spring, through leaching or denitrification. These two processes are quite different, and normally occur on different types of soils," says Mengel, who is a soil fertility specialist with K-State Research and Extension.

Denitrification loss is of much greater concern than leaching loss on wet, medium- to fine-textured soils, he says. Denitrification is the conversion of nitrate to gaseous N by soil microbes in low-oxygen, waterlogged soils. There are several conditions that must be met for denitrification to occur, including:

• Lack of soil oxygen. 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-N. Denitrification only affects N in the nitrate form. It has no impact on N in the ammonium form. Maintaining N in the ammonium form is an effective strategy to avoid denitrification losses.
• Warm soil temperatures with organic residue or organic matter. Denitrification is a microbial process. Organic materials and warm soil temperatures are required for microbial activity. Optimum temperatures for denitrification are in the 75-80° range.

So what does all this mean? In many areas of Kansas, the early fall was wet enough to cause N loss through denitrification on heavy, poorly drained soils. In some cases, the N loss has been great enough to cause deficiency symptoms in wheat.

Nitrogen applied in the winter on wheat has been taken up into the plant by now, and is safe from loss, Mengel says. However, some of the N applied early and intended for row crops is at risk of loss through denitrification, especially as soils warm.

"Producers who applied N in the fall for corn and sorghum should be thinking about how they will evaluate fields to determine if additional N may be needed later, and developing contingency plans in the event of a wet May or June," he says.

Leaching is also a possible source of N loss from wet weather, Mengel says. This involves the movement of nitrate 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.

"Leaching of nitrate is rarely a problem in medium- and fine-textured soils during the growing season in Kansas. Ammonium is not readily lost to leaching, even on coarse-textured soils," the agronomist says.