Floods of 1993 and 2011 caused what is known as “flooded soil syndrome” which is also known as fallow syndrome. It happens when soil conditions contribute to the destruction of vasicular arbuscular mycorrhizae (VAM or AM). Arbuscular mycorrhizae are beneficial fungi that thrive when there are active roots for them to form a symbiotic relationship with. Mycorrhizae look like long straws in the soil. (Figure 1.) Without active roots, AM don’t survive well.
Figure 1. Stained image of arbuscular mycorrhizae. Image credit – Jim Ellis, USDA 1994.
Reduced AM populations may appear to resemble phosphate deficiency. It may be that AM increases the ability of roots to absorb soil P. The year following flooding corn may appear shorter and may have some purpling. Soybeans may be less affected to reduced AM populations. Sugar beets may have some sensitivity to flooded soil syndrome as well. Crop conditions may appear to have large areas where the crop is doing poorly, may be purple, or may be bare altogether. The area in Figure 2 was fumigated to simulate loss of AM. The corn is shorter than the surrounding crop and sometimes not present at all.
Avoiding Flooded Syndrome
John Sawyer and associates put together a summary on flooded soil syndrome after the 2011 flood (https://flood.unl.edu/311d6f55-3508-4cca-a88c-5f55d6ca077d.pdf). If a crop can be planted, the crop may benefit from high rates of banded phosphorus – 2x the normal rate. Spring applied phosphorus is effective. Planter applied phosphorus will work if sufficient distance from the seed can be achieved. There is little work on in-furrow products but necessary P rates have potential to reduce germination rates.
If you can’t plant a crop, consider planting a cover crop. Active roots aid AM colonization.
Tillage also seems to reduce AM populations with or without flooding conditions.
Mahdi Al-Kaisi at Iowa State University has put together some ideas for post flood soil management. It would be worthwhile checking our his recommendations (https://crops.extension.iastate.edu/cropnews/2019/03/management-considerations-post-flooding-soils)
Raun Lohry, Ph.D.
President, Fluid Fertilizer Foundation