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Reduce Risk of Fallow or Flooded Soil Syndrome with Cover Crops

by Lizabeth Stahl, Extension Educator in Crops, Fabian Fernandez and Daniel Kaiser, Extension Nutrient Management Specialists

The challenging spring of 2014 has resulted in wide-spread planting delays in parts of the state and a significant amount of acres that remain unplanted at this time. If the decision has been made to take the "prevented planting" option for insurance purposes, the question remains about what to do with these acres. In other parts of the state, extensive flooding and/or severe hail has significantly damaged standing crops. In either case, leaving the ground bare greatly increases the risk of not only soil erosion, but also the risk of "Fallow Syndrome" the following year.

Fallow syndrome can severely limit crop growth in soils where no crop or weed growth occurred the previous year. When there is no plant growth in an area for an extended period of time, populations of "good fungi", called active arbuscular mycorrhizae (AM), are dramatically reduced because AM fungi need actively growing roots to survive. AM fungi assist in the uptake of phosphorus and other nutrients with limited mobility in the soil such as zinc. Fallow syndrome has also been found to occur in Minnesota when corn is grown following sugar beet, since sugar beet it not a host crop for AM.

Corn and small grains tend to be more affected, with corn showing the greatest potential impact on grain yield from fallow syndrome. The potential risk for other crops, such as soybean, is low and special management will not be required. Prevented plant acres, acres where severe hail wiped out crops, and drowned-out spots in fields are at risk of fallow syndrome. Planting some kind of an annual crop in these areas can help maintain levels of AM fungi in the soil and thus reduce the risk of fallow syndrome the following year.

From a biological perspective, weeds could serve as a "cover crop" to help prevent fallow syndrome, but the resulting seed production and contributions to the weed seedbank would lead to increased weed management issues in the future.

Benefits of a cover crop

Planting a cover crop on prevented plant acres, hailed out areas, or drowned-out spots in the field can provide multiple benefits beyond addressing the issue of fallow syndrome. For example, if nitrogen was applied but corn was not able to be planted, a cover crop that helps scavenge nitrogen might be a good choice.

Nitrogen that was already applied will all transform to nitrate in the course of a growing season. Once in nitrate form, nitrogen can be lost by leaching or denitrification if we have a wet fall or wet spring again in 2015. A cover crop can keep nitrogen in the field by taking up currently available nitrogen and converting it to organic nitrogen (plant material). This organic nitrogen would then need to be mineralized back to ammonium and then be converted to nitrate before there is a concern for nitrogen loss. By cycling nitrogen this way, it is possible to protect the nitrogen investment currently in the field for the 2015 crop and reduce the chance of nitrogen loss to the environment.

In addition to protecting the nitrogen investment, cover crops may help maintain other nutrients and soil organic matter by reducing soil erosion risks. Cover crops can also vary in their effectiveness at fighting weeds, alleviating compaction, and providing forage value. Note that brassicas, such as forage radish, are not hosts for AM.

The U of MN Fact Sheet, "Prevented Plant Cover Crop Options" (CC.for.PP.factsheet.June.2013.pdf), lists suggested planting dates and seeding rates for cover crops, and also compares a number of cover crops in their effectiveness at providing various benefits. A list of cover crop seed sources and pilots for aerial seeding has also been compiled by the U of MN (current as of July, 2013:

Further details about using a cover crop for prevented plant acres (i.e. seeding date considerations, using the cover crop for hay or forage, etc.) are discussed on the U of MN Hail and Flooding Website at Be sure to check with your local FSA and crop insurance representatives to ensure you are in compliance with rules and regulations related to cover crop use on prevented plant acres.

Herbicide considerations with cover crops

There is also the question of what cover crop(s) can be planted when a herbicide was applied earlier in the growing season. Information is very limited if not non-existent regarding plant-back or rotation restrictions for various cover crops. Be sure to check the label of all herbicides applied where you wish to plant a cover crop. Crop injury ranging from minor to severe stand reductions could result. If a cover crop is being planted for soil erosion protection or to prevent fallow syndrome, the grower assumes all risk if the cover crop is not on the herbicide label. You MUST follow rotational restrictions if you plan to graze, feed, or harvest a cover crop, however, to ensure the safety of the food and feed chain.

Managing fallow syndrome in 2015

If it is not feasible to plant any crop yet this year and corn or small grains will be planted next year, a banded application of phosphorus at planting next year may help alleviate the effects of fallow syndrome. For corn, the best way to mitigate potential impacts of fallow syndrome is to apply banded phosphorus and chelated zinc directly on the seed as starter fertilizer. While research hasn't identified the exact rate to use for all circumstances, a normal application of 5 gallons per acre of 10-34-0 may be enough. Under fallow syndrome, banding is important and recommended even if soil test values for phosphorus are high. Broadcasting extra phosphorus and zinc previously has not been shown to effectively treat the problem.

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