Options to Reduce the Risk of Fallow Syndrome

  • Corn planted into soil that was flooded or fallow the previous season may be affected by Fallow Syndrome.
  • Symptoms typically are phosphorus deficiency and slow early growth.
  • Caused by low populations of beneficial soil fungi that assist corn in nutrient and water absorption.
  • Management options for corn ground may include planting a cover crop or banding starter fertilizer in the spring.


Conditions That Promote Fallow Syndrome

Cool, wet spring weather can result in planting delays or prevented planting. Extensive flooding, hail damage, or other weather related problems can damage or destroy standing crops. Producers are faced with decisions about what to do on these acres for the remainder of the season. In some cases the only option is to abandon planting intentions and the acres for that season. Leaving acres fallow, without crop or weed cover, can lead to problems next season. 

Fallow  yndrome  FS), Flooded Soil Syndrome, and Post Flood Syndrome are similar conditions usually encountered in corn or small grain crops on acres that were flooded or fallowed the previous season. Corn-Following-Sugarbeets Syndrome (CFS) . may occur when corn is planted after sugarbeets, particularly in flooded or saturated fields during extended cool temperatures. CFS has similar symptoms to FS, but can also be a result of the high water and nitrogen use and soil compaction that can be associated with sugarbeet production. FS appears to be caused by the absence of beneficial, root colonizing soil fungi, vesicular-arbuscular mycorihize (VAM), that forms a symbiotic relationship with most plants that assists with nutrient uptake.1

Figure 1. Stunted, purple corn due to phosphorus deficiency is a common When 60 to 80 lbs of P/acre were applied as a starter fertilizer, symptom of corn with fallow syndrome.

VAM hyphae grow inside and outside roots to serve as an extension of the roots for water and nutrient uptake. External hyphae of VAM, can be up to 100 times longer than plant roots and extend the nutrient absorption zone.2 VAM do not survive in saturated, flooded, or fallow soils and need plant roots to survive or populations will decline significantly.

Most agricultural plants and weeds can be hosts for VAM, with the exception of brassica species (e.g. canola, cabbage, broccoli, etc.) and sugar beets. When a non-host crop is planted, VAM populations decrease because a host crop is essential for completion of the VAM lifecycle. The amount of time it takes for VAM populations to rebound is relative to the extent of decline in population. Generally, the effects of FS are no longer evident two years following the occurrence that caused the decline in the VAM populations.


Identifying Fallow Syndrome in Corn

Corn and small grains typically show symptoms of phosphorous (P) or zinc (Zn) deficiency and slow early growth on FS soils. Soybean plants have a lower risk potential for FS and may not require special management procedures.1 The primary symptoms of FS are a corn crop deficient in P or Zn accompanied by slow, uneven early growth. The leaves of young phosphorus-deficient plants are bluish-green and slightly narrowed, turning reddish-purple starting at the tips and along the edges. Leaf tips may die. Zn deficiency symptoms are rare beyond the seedling stage. Yellow to white bleached bands appear on the lower part of leaves while the midvein, margins, and tips remain green. 


Fallow Syndrome Effect on Yield Potential

There is limited research available regarding the potential effects of fallow syndrome on corn yield. A 1994 study conducted in Iowa and Missouri compared corn planted into ground  that was flooded the previous year with adjacent non-flooded ground.2  Corn planted in previously flooded ground that received only 25 lbs of P/acre in starter fertilizer showed P deficiency symptoms and yielded 32 bu/acre less than the non-flooded ground. When 60 to 80 lbs of P/acre were applied as a starter fertilizer, the yield penalty from previously flooded ground ranged from 7 to 16 bu/acre and plants did not exhibit any P deficiency symptoms.


Options to Reduce Risk of Fallow Syndrome

Cover Crops. Planting a cover crop on soils where crops can’t be planted, were destroyed by weather events or flooding, or on prevented plant acres can help reduce the risk of FS. Cover crops can provide other benefits such as provide a forage crop, scavenge nitrogen, provide erosion protection, prevent weed growth, and improve soil tilth.1 A legume cover crop can provide a residual nitrogen benefit for succeeding crops. Most cover crops are hosts for VAM and can be planted in areas prone to FS to help reduce the risk of FS the following season. There are several cover crops that can provide good late season forage that also maintain VAM populations (Table 1). Some cover crops do not support VAM, such as canola, mustard, radish and turnip and will not help reduce the risk of FS. Consult local experts for recommendations for crops, planting dates, and rates suitable for different production areas.

A factor to consider when selecting a cover crop is the plantback or rotational restrictions for herbicides that were applied to the destroyed crop. Information on cover crop tolerance may be limited and producers should consult individual herbicide labels to understand the use restrictions and precautions before planting a cover crop.

Starter Fertilizer. In corn, a banded application of P or Zn can help alleviate the effects of FS or CFS. Banded starter is more effective than broadcast applications because banding puts the P or Zn in close proximity to the small, developing root system for rapid plant uptake. Applying 60 to 80 lbs of P/acre as a starter fertilizer can help overcome the effects of fallow syndrome.2 When applying these high rates, the starter should be applied 2 inches below and 2 inches to the side of the seed row, not in furrow.3

Alternative crops. Some crops are more tolerant to low VAM populations. Soybean and sorghum show less of a negative response to low VAM populations than corn and may be alternative options.

Inoculants. Attempting to rebuild populations by adding VAM fungi to the soil is not feasible based on availability and cost of VAM inoculum.

FS can occur on soils that are saturated or flooded where crops were destroyed, or when corn is planted following a crop such as sugarbeet that does not support VAM. Corn is sensitive to low populations of VAM. If possible, a cover crop that hosts VAM should be planted in fields rather than leaving a field fallow.


1Stahl, L. 2014. Reduce risk of fallow or flooded soil syndrome with cover crops. University of Minnesota. http://blog-crop-news.extension.umn.edu.

2Ellis, J.R. 1998. Post flood syndrome and vesicular-arbuscular mycorrhizal fungi. Journal of Production Agriculture. Vol 11, no. 2: 200-204.

3Sawyer, J., Mallarino, A.P., and Al-Kaisi, M. 2011. Flooded soil syndrome. Flood Recovery for Cropland. University of Nebraska-Lincoln and owa State University Extension.

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