Impact Of Flooding On Soil Fertility In The Red River Valley Of Manitoba

Spring flooding may cause large losses of fall applied nitrogen (N) fertilizer and residual nitrogen left over after the previous crop or fallow year. The following topics deal with losses from fall applied nitrogen fertilizer, other nutrient losses and how to determine the extent of nitrogen loss.

Losses Of Fall Applied Nitrogen Fertilizer

With flooded soil conditions, farmers may be concerned about the fate of nitrogen fertilizer applied last fall. Losses of nitrogen may occur in either of two ways, by leaching below the rooting zone on sandy soils and by denitrification on clay soils. In both instances, only nitrogen in the nitrate form is subject to loss. Nitrogen present in the ammonium form will not be affected.

Denitrification is the conversion of the nitrate form of nitrogen to nitrogen gas, which is lost to the atmosphere. this is the greatest threat of N loss in the Red River Valley, since the soils are predominantly fine-textured clays that become easily waterlogged.

The amount of nitrate nitrogen that is lost depends upon the soil temperature and the length of time during which the soil remains saturated. At 5 degrees Celcius the loss occurs slowly; loss is greatest at 25 degrees Celcius. A general rule of thumb for N losses due to denitrification is about 2 to 4 lb of lost N per acre per day when soil temperatures are 5 degrees Celcius. If a soil was under water for a week at this temperature, then 15 to 30 lb of nitrate - N per acre could be lost.

Fall fertilization practices increasing the potential for denitrification losses are:

Nitrate containing forms of nitrogen were applied last fall, for example ammonium nitrate (34-0-0) or UAN solution (28-0-0).
Ammonium forms (e.g. anhydrous ammonia (82-0-0) or urea (46-0-0) applied early in the fall to warm soil. In these situations ammonium may have been converted to the nitrate form of nitrogen.

As soils warm in the spring, any remaining ammonium-N from fall applied ammonia or urea will eventually convert to nitrate-N. If soils become water logged and warm after this conversion, then denitrification losses will increase.

It is difficult to predict denitrification losses, since several factors are involved. The following chart summarizes the estimated impact of individual factors on denitrification potential of flooded soils. If the fertilization practices and soil conditions for a flooded field were all rated as having a low impact (e.g. late season band application of anhydrous ammonia onto soil that was briefly flooded while the soil was cold), then the overall potential for N loss would be low.

Estimated Effect on Fertilization Practices and Soil Conditions on Denitrification Losses for Fall Applied Nitrogen Fertilizer

Factors	Low	Medium	High
Application Time in Fall early - warm soils late - cool soil (<5C)	√		√
Product urea or anhydrous ammonia (46-0-0 or 82-0-0)	√	√	√
Application Method broadcast banded	√		√
Soil Temperature During flood cold <5 C warm >5 C		√	√
Duration of Flooding brief - a few days several weeks	√		√

Other Nutrient Losses From Soil

Residual nitrate-nitrogen that was unused by the previous crop may also be affected by flooding. Farmers regularly utilize residual nitrogen to supply much of the present cropping needs, especially when full or partial fallow is practiced. According to reports from AgVise Laboratories, the average 2008 fall soil test levels for nitrate-nitrogen in Manitoba's Red River Valley were 40-45 lb of N per acre.

Residual nitrate-nitrogen is subject to the same leaching and denitrification losses as fertilizer nitrate-nitrogen. Therefore, this form of nitrogen loss may also be significant under flooded conditions. Residual nitrogen in the organic form (e.g. in legume crop residues or humus) is not affected.

Phosphorus is not seriously affected by periodic, short-term wetness. Long term repeated flooding might result in somewhat inflated soil P test levels. However, in early spring, wet soils are often cold and restrict crop root growth and soil P availability that increases crop response to P fertilizer. Keep in mind that if excess water delays seeding, that delay also allows soils to warm, reducing the response to fertilizer P.

Potassium is not directly affected by soil wetness. Indirect effects are similar to P with early spring soils often being cold which restricts and K availability and root exploration for K.

Sulphate-S is moderately susceptible to leaching in sandy soils. It tends to leach less than nitrate-N owing to its combining with soil calcium and forming less soluble gypsum in many soils. Sulphur volatilization may occur, similar to nitrate denitrification, if soil is flooded and the oxygen supply is exhausted. Sulphate S is used by bacteria to oxidize carbohydrates and producing hydrogen sulphide gas (H₂S). However, this occurs only in extreme cases of prolonged flooding, such as in marshes.

Micronutrients may also be influenced by soil wetness. Soluble manganese (Mn) concentrations may "explode" in flooded soils, interfering with iron(Fe) nutrition and causing iron chlorosis, especially in flax. Flooding of alkaline (high pH or high lime) soils causes a buildup of bicarbonate, which interferes with iron uptake and causes iron deficiency and chlorosis. This is especially seen in flax, soybeans and shelterbelts.

How To Determine The Extent Of Nitrogen Losses

It is difficult to determine how much fall applied nitrogen fertilizer or residual nitrogen might be lost on soils which are flooded prior to seeding. Ideally one would soil test prior to seeding. However, in many cases seeding the crop should be the most urgent priority. Furthermore, much of the nitrogen from fall applied ammonia (either as anhydrous ammonia or urea) may still be in the ammonium form at planting and will not show up in a nitrate soil test, especially if the fertilizer was applied close to freeze up.

Another option is to seed the crop and then test the soil within a couple weeks of seeding, while the crop is still small. Lab turnaround is generally fast at this time of year, so if additional N is required it can still be broadcast onto the crop. By waiting until after seeding, much of the ammonium from the fertilizer should have converted to nitrate, and the tillage and seeding operations should have mixed the fertilizer bands sufficiently with the soil to reduce sampling "hot spots".

Sample the soil from 0-6 and 6-24 inches to determine if extra nitrogen is needed. On fields where fertilizer was banded, ensure that many subsamples (>15) are taken to get a representative sample that is not extremely biased toward or against the location of a band.

If spring soil testing is not possible:

For fields not fertilized in the previous fall, fertilize according to general recommendations for Manitoba
For fields fertilized in the fall, carefully observe crop growth. If nitrogen deficiency (yellowing of lower leaves) appears, be prepared to top-dress the crop with additional nitrogen

Information compiled by
Manitoba Agriculture
Dr. Don Flaten, School of Agriculture, Faculty of Agricultural and Food Sciences, University of Manitoba
Dr. Geza Racz, Dept of Soil Science, Faculty of Agricultural and Food Sciences, University of Manitoba