Nitrogen Loss from Wet Soils – Vegetable Crops Hotline

Nitrogen Loss from Wet Soils

​High rainfall amounts lead to loss of nitrogen from the soil. Sometimes the loss is great enough that a crop will benefit from additional nitrogen application. This article will describe how nitrogen is lost and factors to consider in deciding whether to apply extra nitrogen.

There are two main ways nitrogen is lost from wet soils. Nitrogen is lost to the air by denitrification. Denitrification occurs in saturated soils when there is little oxygen in the soil. In the denitrification process, nitrate is broken down by bacteria to form oxygen and volatile nitrogen compounds including nitrous oxide and nitrogen gas. These volatile compounds move into the air and nitrogen is lost from the soil. Denitrification is common on heavier soils. In Indiana, saturated soils lose 4% to 5% of their nitrate nitrogen for each day they are saturated.

Nitrogen is lost below the root zone of the crop by leaching. Nitrogen in the soil solution will be carried with water as it percolates down through the soil. Nitrate leaches very readily. Leaching losses are greater on light-textured soils and when rains fall over a period of time so that water has time to soak into the soil rather than run off the surface. An inch of rain may move nitrogen a foot deeper in a sandy soil; after 4 inches the nitrogen may be down 4 feet and below the root zone of many crops. Leaching losses from rainfall are much reduced when nitrogen is applied under plastic mulch.

For both denitrification and leaching, it is the nitrate form of nitrogen that is lost. Some fertilizers, such as calcium nitrate and potassium nitrate, contain all the nitrogen in the nitrate form and so are very susceptible to nitrogen loss as soon as they are applied. Other materials, such as urea or diammonium phosphate, break down initially to supply nitrogen in the form of ammonium, and then the ammonium is converted by bacteria into nitrate in the process called nitrification. The bacteria are not active in dry or cold soil; most nitrification occurs when soil temperatures are 60 to 86 degrees F. Under typical Indiana conditions, the ammonium will be completely converted to nitrate in a month. With these materials, the longer it has been since application, the more nitrogen will be in the nitrate form, and the greater the potential for nitrogen losses. By this time in the growing season, much of the preplant nitrogen for summer crops will have been converted to nitrate.

What about organic fertilizers, or nitrogen from a legume cover crop? Nitrogen from these sources also eventually gets converted to nitrate, and then is susceptible to loss as described above. How quickly the conversion occurs depends on the C:N ratio of the material as well as other factors. Once the nitrogen becomes available to the crop it can be converted to the nitrate form. As an example, last year at Pinney Purdue Ag Center on June 16 we measured 80 lb. nitrate-N/A in plots where a hairy vetch cover crop had been incorporated in early May, and 128 lb. nitrate-N/A in plots where an organic 13-0-0 fertilizer had been applied in addition to the hairy vetch. On our sandy loam soil that nitrate would have readily leached, and on a heavier soil, it could have denitrified.

To evaluate the potential for nitrogen loss in a particular field this year, consider the following questions:

  • How much nitrogen was in the nitrate form when it rained? Only the nitrate form is lost to leaching or denitrification. All kinds of fertilizer nitrogen eventually gets converted to nitrate. If ammonium or urea was applied, it would take about one month for all the nitrogen to be converted to the nitrate form.
  • How warm has the soil been since fertilizer application? The warmer the soil, the faster the conversion of ammonium to nitrate, which can then be leached or denitrified. With soil temperatures in the 40s or 50s conversion to nitrate will be slow.
  • Was plastic mulch used and when? Little leaching due to rains will occur under plastic mulch.
  • What is the soil type? On a sandy soil 5 inches of soaking rain can leach nitrate beyond the root zone of most vegetables. On a heavier soil it will take more water to leach beyond the root zone.
  • How much rain has fallen, and how much at one time? Rains that soak the soil rather than run off will result in more leaching. Periods of soil drying between rains will cause water (and nitrate) to move upwards in the soil.
  • How long has the soil remained saturated? Denitrification occurs in saturated soils, but conversion of ammonium to nitrate does not.
  • How deep is the crop root zone? For a shallow-rooted crop like lettuce, nitrogen is effectively ?lost? to the crop if it leaches below 12 inches; for a deep-rooted crop like pumpkin, nitrogen could be available even if it leached two or three feet down.  Effective rooting depths for vegetable crops are listed in the Midwest Vegetable Production Guide for Commercial Growers (Table 5, p. 25).
  • How much nitrogen has already been taken up by the crop?

If nitrogen has been lost, is it worth replacing it? One tool to help evaluate whether additional nitrogen will benefit the crop is the soil nitrate test. This test has had most research done in the context of a pre-sidedress nitrate test, or PSNT. That test measures the amount of nitrate in the soil shortly before the time of sidedressing. If the nitrate level is above a certain value, it is unlikely that additional nitrogen will lead to higher yields. The value depends on the crop, and is determined empirically for different crops and environments. Much of the development of the PSNT for use in vegetables has been done in the Northeastern and Mid-Atlantic states. The critical values suggested by John Howell of the University of Massachusetts are a good starting point for using this test in Indiana. According to Howell, for sweet corn no additional nitrogen is needed if the nitrate level is 25 parts per million (ppm) or greater. For pumpkins, squash, tomatoes and peppers, no additional nitrogen is needed if the nitrate level is greater than 30 ppm. Although the PSNT has not been evaluated for use on vegetables in the Midwest, it has been studied in field corn in the Midwest, and the response is similar to that in the Northeast. This gives us some confidence that recommendations for vegetables developed in the Northeast will be reasonable in Indiana.

Soil samples for a nitrate test are normally collected about a week before normal sidedressing, but in this situation they can be collected when soil conditions permit. Take 15 to 20 cores 12 inches deep and mix them well. Dry one cup of the composite sample by spreading it thinly on a clean, non-absorbent material. Quick drying will prevent changes in nitrate levels of the sample. Send the sample to be analyzed for nitrate by a reputable soil testing laboratory that will provide results quickly. Use the guidelines above to determine whether or not to apply more nitrogen. Or, for a more quantitative approach, see the June 18 Issue of Purdue’s Pest & Crop Newsletter where Dr. Bob Nielsen has provided guidelines for calculating rates of nitrogen to apply to field corn based on a soil nitrate test and the amount of preplant nitrogen applied and considering the stage of crop growth. The principles presented there should be relevant for vegetables where fertilizer is broadcast as well. 

Other factors worth considering when deciding whether and how much to apply include the cost and practicality of application, whether the initial nitrogen application was high enough that some loss could be afforded; whether the crop is still healthy enough to produce the quality and yield expected; and whether it is still early enough in crop development for the application to make a difference.

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