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Soil fertility considerations and fallow syndrome

Many growers have questions about management in 2014, particularly about the possible effect of fallow syndrome, due to increased prevented plant corn acres this year.

Fallow syndrome is the result of reduced colonization of plant roots by vascular arbuscular mycorrhizae (VAM), which are important in the uptake of elements such as phosphorus and zinc. Fallow syndrome does not affect all crops nor will it likely be an issue for all prevented planting acres.

Corn will likely show the greatest potential impact on grain yield from fallow syndrome. The effects are clearly visible as small purple plants early in the growing season.

The greatest risk for fallow syndrome will be in fields that remained clear (fallowed) throughout the prior growing season. Fallow syndrome can happen in Minnesota. It has been found when corn is grown after sugarbeet, which is not a host crop for VAM.

Brassicas are also not hosts for VAM. Forage radish and turnips are Brassica family crops that have been seeded on prevented planting acres. However, research has not identified that forage radish would necessarily induce fallow syndrome, but the potential risk may be higher. In cases where oat or other small grain crops were seeded, there should be a low risk of fallow syndrome since small grains host VAM. Remember, even though the potential risk may be higher, there is no guarantee that treatment is needed or will be economical.

For corn, the best way to mitigate potential negative impacts of fallow syndrome is applying small rates of banded phosphorus and chelated zinc directly on the seed as a starter fertilizer. There is not a known, exact rate for all circumstances, but a normal application of five gallons per acre of 10-34-0 may be enough.

Banding is important under fallow syndrome. Broadcasting extra phosphorus and zinc has not been shown to effectively treat the problem. The potential risk of fallow syndrome for other crops such as soybean is low, and special management is not required. 

Specialty fertilizer products are available that are marketed to increase root growth and potentially help with VAM colonization. However, there is no known product that would work as effectively as a small rate of "pop-up" starter fertilizer.

A concentrated band of nutrients near the roots is important--and the best management strategy--when managing fields prone to fallow syndrome. Ensure the starter is not applied at too high of a rate. Over-application still may result in a high risk for root damage to corn even if conditions are right for a higher chance of an economic return. This is also especially true for soybean where fertilizer application directly on the seed is not recommended and the risk for fallow syndrome is very low.

To learn more about nutrient management from Extension,

Daniel Kaiser is a nutrient management specialist with University of Minnesota Extension.