Soil Nutrient Management After the Flood

Erosion and deposition, common after major flooding, may have significant impacts on how producers manage nutrients on affected fields in subsequent years. As fields are reconstructed to even out effects of erosion and/or deposition, new soil profiles are created. These “new” soils may have completely different physical and chemical characteristics than the pre-flood soil. This fact sheet addresses some guidelines on managing these soils.

Soil test

A good soil testing program is essential on these soils. Take the approach that a new field has been purchased and nothing is known about the soil, because, in many cases, this is a good representation of the situation. Do not rely on previous soil test results from the field to build nutrient management plans. Affected fields should be retested and nutrient management plans should be based on these results. In addition to general fertility (pH, P, K), organic matter and zinc levels should also be determined. Sand deposits are often very low in organic matter and nutrients. Significant changes in organic matter levels could impact herbicide choice and recommended application rates. Lime, phosphorus, potassium, and micronutrient recommendations can be made based on new soil test results and crops to be grown.

Fields with substantial soil deposition and erosion may require more intensive sampling. Where portions of a field have been damaged by moving water and sand or silt deposition, resulting nutrient levels may be considerably different from other areas of the field where minimal damage was done. Separate sampling of those areas could result in very different nutrient management plans. Use good techniques of sampling, but just stay within the given areas where soils are obviously different.

Nitrogen

Flooding or just extended very wet periods, can impact nitrogen management. Flooding, with the erosion and deposition, tends to reduce the organic matter pool in soils. Reduced organic matter means less mineralization release of N from this pool during the growing season. Extended wet periods increase potential for leaching and/or denitrification in soils. Leaching is the downward movement of nitrate nitrogen (NO₃-N) with soil water. Leaching is more of a problem on coarse textured soils. Denitrification occurs in saturated soils and is conversion of NO₃-N to N₂ gas. Due to a combination of all these factors, low residual NO₃-N levels are common in soils following flooding or extended wet periods.

A nitrogen soil test is advisable on flood affected and all fields to determine residual NO₃-N levels. Details on how to take N soil tests (and routine tests) can be found in Extension Bulletin MF-734 Revised, “Kansas State University Soil Testing Laboratory”. Results of an N soil test coupled with expected crop yield allow accurate N fertilizer recommendations. If very low residual N levels are confirmed by soil testing, the full recommended rate of N fertilizer for a given crop and yield goal should be applied.

Special Situations

Fallow Syndrome

Often corn grown in fields that were “fallowed” the preceding year will show nutrient deficiency symptoms, reduced growth, and a poorly developed root system. This condition is known as fallow syndrome and can best be described as an environmentally induced phosphorus deficiency. The most widespread occurrence of this phenomenon was following the PIK year, but has been observed in fields flooded the previous year. This phenomenon has not been observed in areas of Kansas where fallowing is a normal practice.

While not all the details are known, University research has shown the lack of live root material in the “fallow” year reduces the population of some beneficial soil microorganisms (vesicular arbuscular mycorrhizae or VAM). These soil fungi can increase uptake of nutrients by acting as extensions of the plant root system. If little or no live root material is present, populations of the fungi decline. Moderate populations of weeds, crop or cover crop can sustain populations of these fungi and prevent the occurrence of fallow syndrome.

A reduced amount of nutrient recycling may also play a role. Nutrients leached from plant residues after crop maturity are easily taken up by the next crop and their absence may reduce plant growth.

The effects of fallow syndrome generally occur only with a lack of plant growth the previous year. Moderate amounts of crop or weed growth are sufficient to prevent fallow syndrome. If no cover exists on affected fields, it may be desirable to plant a cover crop, such as wheat or a winter annual legume.

If a cover crop or other cover is not used, starter fertilizer with high phosphorus content should be applied in a band at planting. Broadcast applications have not proven effective. Fallow syndrome does not occur as often on soils with higher phosphorus test levels, but critical levels have not been determined. Corn is most sensitive to fallow syndrome but other crops including soybean can be affected.

Fallow syndrome is not likely to be a widespread problem in Kansas, but may show up in fields severely affected by flooding.

Micronutrients

On fields where heavy sand deposits are incorporated into existing soil, micronutrients (particularly zinc and sulfur) may be a bigger concern. Soil testing will help identify need.

Ray E. Lamond

Extension Specialist, Soil Fertility and Management

          November 1993