by Dan Kaiser, Extension Nutrient Management Specialist
With record precipitation across much of the Midwest in 2019, many fields went unplanted. Fields that were in a prevent plant situation last year may be at risk of fallow syndrome in 2020. When managing fields prone to fallow syndrome, a low rate of “pop up” starter fertilizer is the best management strategy.
What is fallow syndrome?
Fallow syndrome is caused by decreased colonization of plant roots by beneficial fungi called vesicular-arbuscular mycorhizae (VAM). VAM act like an extension of the plant’s roots, helping crops such as corn take up phosphorus and zinc. The population of VAM in the soil goes down rapidly in a year with little plant growth, which can negatively impact crop uptake of phosphorus and zinc the following growing season. Fallow syndrome often causes purple coloring of corn plants early in the growing season due to reduced update of phosphorus, which can lead to yield loss.
Which fields and crops are at risk?
Corn following fallow fields has the greatest potential for yield loss from fallow syndrome. Other crops, such as soybean, are at low risk. Corn following some crops and cover crops also requires special management considerations, including corn following sugar beets. Cover crops on prevent plant acres can help reduce the risk of fallow syndrome, but it depends on the species of cover crop.
If you seeded oat or small grain crops as a cover last year, the risk of fallow syndrome this spring is very low. Brassica cover crops are not VAM hosts, so they may require management considerations. Research on forage radish, from the brassica family, has not shown negative impacts on the following corn crop, but the potential risk may be higher.
Any plant roots in a field, including weeds, can reduce the risk of fallow syndrome for the following crop. If weeds were present for part of last year’s growing season, there is a reduced chance for fallow syndrome to occur. Fields with extremely high soil P tests, particularly manured fields, are likely at reduced risk. However, research has not identified a specific soil P test above or below which fallow syndrome is more or less likely to occur.
Remember, even if the potential risk for fallow syndrome is high, there is no guarantee that treatment is needed or will be economical.
What to do about fallow syndrome?
You can’t do much about fallow syndrome once you identify the problem, so it’s best to mitigate risk in advance. If you’re planning on planting corn in a field that was fallow last year, the best course of action is to band phosphorus and chelated zinc directly on the seed as a starter.