What is the impact of flooding on the soil fertility status of soils? Should these fields be resampled? These are just a few of the questions resulting from the early season flooding that occurred in parts of the southern half of Wisconsin earlier this season.

 

As a general rule, we recommend soil sampling about every four years, except on sandy and loamy sand soils which should be sampled every two years.   Typically, crop nutrient removals over a four year period in most cropping systems will not change soil test levels enough to affect nutrient application rates. An exception to this would be high yielding corn silage or alfalfa cropping systems which may require testing more frequently than every four years due to large P and K removal rates.  

 

What happens when a soil is completely flooded for periods of a week or more? There are a few simple guidelines that we would recommend that you follow to determine if the fields should be retested following flooding. 

 

1. During the rain event, was there a significant amount of erosion (top of slope) or sediment deposition (bottom of slope)? If either of these occurred, resampling would be recommended and we would suggest splitting the field into zones based on where they fall on the slope and how the erosive event affected that part of the field.

 

2. Being submerged for a prolonged period of time will cause a shift in the oxidation-reduction potential of a soil. Under normal conditions, the microbial and chemical reactions occur in the presence of oxygen in the soil. Generally, within one or two days after flooding, essentially all of the free oxygen will be consumed, the activity of anaerobic micro-organisms increases, and soil reduction starts. Upon flooding an originally aerobic soil, reduction of the remaining oxygen takes place first, followed by nitrate, then manganese in neutral soils; or manganese, then nitrate in acidic soils. Later, ferrous iron may appear and still later the formation of sulfide and even hydrogen and methane may occur. Dissolved ferrous iron replaces part of the exchangeable cations resulting in increased dissolved Ca²⁺, Mg²⁺, K⁺ and Na⁺ following soil reduction. Upon return to aerobic conditions, the adsorbed Fe²⁺ is oxidized again to ferric oxide and the displaced cations will once again become exchangeable. (Bolt and Bruggenwert)

 

Following inundation, the pH normally increases in acid soils and decreases in alkaline soils. If submerged for 4-12 weeks the pH reaches fairly stable values between 6.5 and 7.0 in most soils. The pH increase in acid soils is mainly the result of the increase in carbonate alkalinity accompanying most reduction processes in soils. The pH of most calcareous soils drops 0.5-1.0 pH units upon submersion as a result of the production of CO₂ by anaerobic decomposition of organic matter.

 

To avoid introducing another variable into the interpretation of the soil test results, it is best to wait until the soil returns to its normal red-ox status. This may require waiting until next spring, but certainly at least until this fall. It would be best to resample this field at the same season as was done previously for comparison purposes. In other words, if the field was last sampled in the spring, retest next spring and if the last sampling occurred in the fall resample this fall. 

 

 

References

 

Bolt, G. H. and M.G.M. Bruggenwert. 1976. Soil Chemistry, A. Basic Elements