Will fallow syndrome be an issue in 2019 prevented planting acres?

Rebecca Vittetoe
Iowa State University Extension Field Agronomist

In 2019 Iowa had over 460,000 prevented plant acres. As we prepare for the 2020 growing season, one concern we sometimes hear about with prevented plant acres is fallow syndrome. Fallow syndrome happens when the populations of “good fungi” known as vesicular-arbuscular mycorrhizae (VAM) are reduced due to the lack of actively growing roots. These “good fungi” help with nutrient cycling and plant nutrient uptake, particularly for phosphorus (P) in corn. Fallow syndrome is only a concern if there was little to no plant growth in a field last year (i.e. no cover crop and no weeds).

To manage for fallow syndrome, these are our best recommendations based on the knowledge we have:

• If the field is to be planted to soybean, fallow syndrome is typically not a concern. Starter fertilizer is not suggested for soybean. If starter is used, it should not be placed in-furrow with soybean planting.

• If the field was under water for a considerable period of time (several months) last year, inoculating soybean with rhizobia may help with nodulation.

• For corn, research has been inconsistent regarding yield response with fertilizer application.

- If no P has been applied and fallow syndrome is suspected, suggestions have been to apply either a high P starter rate placed 2x2 (two inches beside and two inches below the seed) or a high P broadcast rate (approximately twice the normal recommended P rate) if soil tests are low and no starter equipment on the planter.

- If an annual P rate was applied in 2019 on acres that were prevented plant, a normal P starter rate placed 2x2 or in-furrow could help is fallow syndrome is suspected.

- If a 2-year rate of P was applied in 2019 on acres that were prevented plant, the -additional need for P fertilizer is unlikely.

- Sometimes a yield response has occurred to a higher starter N rate (25 lb N/acre, away from the seed) rather than to P.

Additional resources

• If soils were flooded for a considerable time last growing season, check out the fact sheet “Flooded Soil Syndrome” at tinyurl.com/wgo7gdz.

• The University of Minnesota recently published the article “Spring Management of Prevent Plant Acres” at tinyurl.com/r9pvztj, which shares some additional information on how to management prevented plant acres that either had cover crops planted on them or had no weed control management used. Please note that fertilizer recommendations vary from state to state. Thanks to Liz Stahl, Anna Cates, and Lisa Behnken for allowing us to share their article.