Management Considerations for Flooded Soils

Heavy rainfall can have economic and environmental consequences if flooded fields are left barren. Long-term damage to soil needs to be considered when planning for next season's crop.

Biological, chemical and physical soil health

Biologic health: Several changes take place when soil is flooded for an extended period of time. One potential change is the biological health of the soil, with the greatest concern being soil left unplanted to any crop or cover crop. The existence of plants in such areas will help sustain the microbial community in the root zone, which is essential to nutrient cycling, especially phosphorous (P), but also zinc, and other less mobile nutrients.

Flooded soils may experience what is called post flood syndrome, similar to fallow syndrome, where the land is left unplanted to any crop for the entire season. Flooded soils will encounter problems caused by the reduction of soil arbuscular mycorrhizae (AM) fungi colonization rates the following growing season.

AM fungi colonize the root systems of crops in a mutually beneficial (symbiotic) relationship. The fungi benefit from the host plant roots, and the crop benefits from the increased nutrient uptake zone developed by fungal hyphae (threads that make up the mycelium of fungi). Unplanted, flooded fields may be affected next season due to the absence of a root system that is essential to maintaining this microbial community that contributes to nutrient cycling.

Weeds can serve as a “crop” to help reduce the chances of fallow syndrome, but one must take into consideration the resulting seed production and impacts on future weed control.  If you choose this method of leaving some “crop” in these areas, be sure to clip weeds frequently to reduce seed production. 

Chemical: Most of the chemical changes will be induced by temporary changes in oxidation and reduction conditions. However, physical-chemical-biological changes in soil such as aggregate stability, soil structure, pH, etc., can be significant, especially if there is no growing crop. Cover crops can be grown to scavenge any remaining nitrogen.

Physical:  Bare soil can be impacted by both wind and water erosion, both of which can cause a decline in crop productivity.  Seeding a cover crop can help protect soils from erosion by mitigating raindrop impact and soil particle detachment and further movement of soil off-site. In addition, cover crops can slow water moving across the landscape. Cover crops can also help protect soils from wind erosion.

Steps to manage previously flooded soils

Depending on the erosion or deposition of material in flooded fields, steps may need to be taken to restore field conditions for farming practices.

Land Leveling and Sand Removal 

• Sand a few inches (i.e., 2-4 inches deep) can be incorporated in soil using normal field operations. Otherwise, minimum soil disturbance is advisable.
• If sand is up to six inches deep, then moldboard plow to a depth twice the sand depth to incorporate.
• If sand is 8-24 inches deep, it is advisable to consider spreading it to areas with less sand and incorporate with special deep tillage equipment. It is not advisable to move sand to fill lower or severally eroded areas in the field without proper topsoil to cover the sand.
• For sand more than 24 inches deep, evaluate the cost of removing or stockpiling sand.
• In the case of severe erosion and deep cuts, topsoil from surrounding fields should be used to fill such areas.

Soil Testing

• Soil testing should be conducted after any land leveling is done.
• Soil samples should not be collected immediately after soils dry.
• Allow time for phosphorus (P) reactions in soils to take place after soils aerate.
• Potassium (K) deficiency can occur due to soil compaction.
• Soil test levels could increase from sediment deposition.

Cover Crops (if a crop cannot be planted in 2024)

Research documents growing plants, such as cover crops, row crops and other crops, can increase the AM recolonization and ultimately the availability of phosphorous, which is the nutrient most affected by reductions in mycorrhizae population.

• Use a cover crop immediately after soil dries to promote growth of microorganisms that are essential for nutrient cycling.
• Planting conditions should provide good soil to seed contact for cover crop success.
• Consider a cover crop that overwinters to provide additional benefits of continuous growth in the spring prior to planting.
• Establishment of a cover crop can help suppress weeds, preventing a buildup of the weed seedbank.
• Cover crops can help reduce erosion.

Cover crop options in prevented planting fields and forage options with prevented planting fields provides a list of cover crop options for your fields.  Consult your crop insurance provider before making decisions.

Before deciding on a cover crop species for prevent plant fields, read the label from herbicides that were used in the past growing season as well as any that may have been used this growing season. Check for grazing restriction (if applicable), in addition to rotation restrictions before planting to understand any possible herbicide interactions of past herbicides and the cover crop.  Practical Farmers of Iowa has a search tool to find seed supplies as well as service providers for cover crops.

Note: Contact your crop insurance agent if you have crop loss and prior to planting a cover crop or other crop so you understand coverage.   

Other Considerations

• When planting soybean, as a precaution, seed should be inoculated with Bradyrhizobium japonicum to ensure nodulation and nitrogen fixation.
• AM fungi inoculation of soil is not feasible.
• Once soils become aerobic, soil microflora will recover naturally.

Observations from Previous Research

• Corn growing on flooded soils showed purple leaves that disappeared within a week.
• Flooded fields with weeds or without tillage showed less purpling than those tilled to control weeds.
• Fields with high manure application history (i.e., feedlots) showed no adverse effect for flooded soils on crops.
• Crops planted after a fallow/flood period grew poorly.
• P deficiency symptoms in crops — for corn it is slow early growth and purple coloration.
• Flooded soils may have normal P test level and low AM population.
• To alleviate P deficiency, high banded P rates are needed — twice or more than the normal recommended rate.

This article was adapted from a March 2019 article in Integrated Crop Management News written by Mahdi Al-Kaisi.