Dr. Ajay Nair
Department of Horticulture
Iowa State University
With respect to vegetable crops, recent flooding in western Iowa has created a tough situation for growers. Given the time of the year of this catastrophic event, there were no vegetable crops standing in the field, however, water/runoff from surface waters such as rivers, lakes, or steams could have overflowed and run into fields. This water most likely could contain chemical and biological contaminants that may be harmful to the health of humans and animals. Two primary types of contamination that are of concern for food crops include chemical and biological. Chemical contamination includes pesticides, agricultural chemicals, heavy metals, and petroleum products. Not all the field will be equally contaminated as the degree of contamination depends on severity of flood, proximity to operations using chemicals, or runoff from roadway. Biological contamination may include bacteria, parasites, and viruses that originate from upstream farms, rural septic systems, overflow from industrial sewage systems, and raw manure or feces.
Now the concern is whether these fields could be planted into vegetable crops this year. Although, conditions will and in some cases have improved, flooded fields will need some time to recover fully. Depending upon the severity of flooding growers could experience saturated soils, nutrient leaching, deficiencies, and above all limited oxygen for plant growth. In addition to flooding, frequent rain events have saturated the soil leaving little room for air. Ideally, for good root growth 50 percent of the soil pore space should be filled with air. As the soil drains, air is drawn into the soil, but when it rains, the water forces the air out of the pores. Unfortunately, the flooded fields often develop a hard surface layer that prevents air from entering. Any tillage that can be done to break that seal is beneficial. If growers experience longer duration of saturated conditions due to continuous rain events, spraying crops with nitrogen, phosphorus and potassium can help plants through stress periods. Use a low salt liquid fertilizer to supply 4 to 5 lb. nitrogen, 1 lb. phosphate (P2O5) and 1 lb. potash (K2O) per acre. Since nitrogen is the key nutrient to supply, spraying with urea ammonium nitrate (28 % N solution) alone can be helpful. For spraying, use 5 to 20 gallons of water per acre. The higher gallons per acre generally provide better coverage.
The bigger issue is biological contamination. Although improving weather and soil conditions bring situation to normal and could possibly reduce pathogen load, the potential impact of flooding on food crops is likely to depend on the degree and duration of crop exposure to flood waters. For example: the volumes of flood waters (how deep) and/or how long the flood waters were present in the field before receding; how quickly the field began to dry out after flood waters receded, taking into consideration soil type, topography and drainage. The Food and Drug Administration (FDA) recommends not replanting in flooded fields if floodwaters have not receded and the soil has not sufficiently dried. It recommends the following assessment for formerly flooded production ground:
- Assessing field history and crop selection.
- Determining the time interval between the flooding event, crop planting, and crop harvest.
- Determining the source of floodwaters (e.g., drainage canal, river, pond, or irrigation canal) and whether there are significant upstream potential contributors of human pathogens.
- Allowing soils to dry sufficiently and reworked prior to subsequently planting crops on formerly flooded production ground.
The waiting period before growers can replant depends on conditions such as temperature, weather, and soil type. Currently, FDA has not completed studies to determine the length of waiting time that is generally considered safe for replanting. State, industry, and university extension specialists have recommended a 30-60 day waiting period and/or soil testing prior to replanting. While this period is generally considered sufficient for fecal contamination to decline, chemical contaminations, if present, may continue to remain in the flood-affected soil. Growers should certainly check the FDA Guidance Document that discusses safety of flood-affected food crops for human consumption (https://www.fda.gov/regulatory-information/search-fda-guidance-documents/guidance-industry-evaluating-safety-flood-affected-food-crops-human-consumption)
An approach which vegetable growers could take is to work the ground and plant a cover crop or green manure before they plant their cash crop. This may lead to not planting/growing a cash crop in the spring but offers a safe passage to grow summer or fall vegetable crops. Rather than letting the field sit fallow and go to weeds in the spring, growers can use the opportunity to plant a cover crop or green manure. Some cover crops to consider include buckwheat, cowpeas, sorghum sudangrass, sunnhemp, and crops in the brassica family such as oilseed radish and yellow mustard. Brassica cover crops might perform to their fullest potential under hot conditions but they should produce decent biomass in May and June. When mowed and tilled into the soil, both oilseed radish and yellow mustard produce a compound called Isothiocyanate which acts as a natural fumigant and can reduce pathogen load in the soil. More information on short duration cover crops that could be planted in Iowa is available at https://store.extension.iastate.edu/product/14481.