Nov. 8, 2011

The author, Mahdi Al-Kaisi, is an extension soils specialist in the Iowa State University Department of Agronomy and a member of a joint UNL-ISU team addressing cropland problems from the 2011 Missouri River Flood. For more information and recommendations related to flooded cropland, see flood.unl.edu/crops.

Following this summer's flood, cropland in western Iowa and eastern Nebraska may sustain economic and environmental consequences if the soils are not planted to a crop or are left unattended. 

When soil is saturated for an extended time, it changes, and its biological, chemical, and physical soil health can be affected. The greatest concern for its biological health is when the soil is left unplanted to any crop or cover crop. Growing plants in these affected areas will help build the microbial community in the root zone, which is essential to nutrient cycling, especially phosphorous. The potential for long-term soil damage in areas of significant flooding needs to be considered when planning for next season’s crop management.

Biological, Chemical and Physical Soil Health

Flooded soil may experience what is called “post flood syndrome,” similar to the 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 next growing season.

The AM fungi are colonized around the root systems of crops in a mutually beneficial (symbiotic) relationship. The fungi benefits from the host plant roots and the crop benefits from the increased nutrient uptake zone developed by the fungal hyphae (threads that make up the mycelium of fungi). Unplanted flooded areas can 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.

In addition to soil biological changes caused by flooding and the absence of an active root system, chemical and physical changes can occur.  Most chemical changes will be induced by temporary changes in oxidation and reduction conditions. However, physical, chemical, and biological changes in soil such as aggregate stability, soil structure, pH, etc., can be significant, especially if there is no growing crop.

Measures to Manage Previously Flooded Soils

Research has shown that growing plants such as cover crops, row crops, and even weeds can increase the AM recolonization and ultimately the availability of phosphorous, which is the nutrient most affected by a reduction in mycorrhizae population. If your fields were saturated by flooding, consider the following management steps to help prepare your fields for future crop production.

Land Leveling and Sand Cleaning – Sand cleaning depends on the depth of accumulation.

• If sand is 2-4 inches deep, it can be incorporated in soil using normal field operations. Otherwise, minimum soil disturbance is advisable to promote even weed growth till next spring.
• If sand is up to 6 inches deep, then moldboard plow to a depth twice the sand depth to incorporate.
• Sand 8-24 inches, it is advisable to consider spreading to areas with less sand and incorporate with special deep tillage equipment. However, it is advisable not to move sand to fill lower or severally eroded areas in the field without proper top soil to cover the sand.
• Sand above 24 inches deep, evaluate cost of removing sand or stockpile to decide whether to remove the sand.
• In case of severe erosion and deep cuts, top soil 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, and may need to be collected in the spring.
• Allow time for P reactions after soils aerate.
• Potassium (K) deficiency can occur due to soil compaction.
• Soil tests could increase from sediment deposition.

Cover Crop

• Use a cover crop immediately after soil dries to promote growth of microorganisms that are essential for nutrient cycling.
• Planting conditions should provide good seed-to-soil contact for cover crop success.
• Consider overwintering cover crops to provide additional benefits of continuous growth in the spring prior to planting.
• When planting soybean, as a precaution seed should be inoculated with Bradyrhizobium japonicum to ensure nodulation and N fixation.
• AM fungi inoculation of soil is not feasible.
• Once soils become aerobic, soil microflora will recover naturally.

Observations from Previously Flooded Soils in Iowa

• Corn showed purple leaves that disappeared in a week.  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 on crop.
• Crops planted after a fallow/flood period grew poorly.
• There were some P deficiency symptoms in crops – for corn these were 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 two or more times the normal recommended rate may be needed.

Mahdi Al-Kaisi
Department of Agronomy, Iowa State University