Fallow Syndrome (Don’t Skimp on Starter P!)

Several million acres went unseeded in the northern plains last year due to excessive rainfall and flooding. One nutrient issue that comes along with flooding is called “fallow syndrome.” “Fallow syndrome” is primarily a phosphorus deficiency caused by a lack of mycorrhiza fungi in the soil. Mycorrhiza (Vesicular arbuscular mycorrhiza or VAM) is a beneficial fungus found in most soils and needs living plant roots to flourish. This fungus has a symbiotic relationship with plants and is especially important for grass crops like corn. VAM fungi act like an extension of the plant roots, to help them absorb more nutrients like phosphorus and zinc.

If all plant growth on unseeded fields is controlled during the summer, these fields are set up for “fallow syndrome.” The population of VAM fungi decreases greatly without any active plant growth. If the unseeded fields had a cover crop mix (including some grasses) seeded on them early in the summer, the VAM fungi population is probably OK and “fallow syndrome” should not be an issue.  If only turnips and radish were used as a cover crop, fallow syndrome will be an issue because these crops do not support the VAM fungi.

For those acres that were kept free of weeds, “fallow syndrome” must be considered in your phosphorus fertilizer planning.  Corn growers who already place starter P fertilizer with or near the seed, should not have any issues with “fallow syndrome.” They just need to increase the rate to about 30-40 lb/a P2O5 in their starter. Some university research shows the starter rates should be even higher at 40-60 lb/a if the band is safely placed an inch or two from the seed row.  A rate of about 10 gallons of 10-34-0 would be the limit for seed placed fertilizer with corn seeded on 30″ spacing on a fine textured soil with good moisture in the seed zone (see table 2). If you usually use only 2-3 gallons of a low phosphorus concentration liquid fertilizer, that will not be enough P to avoid fallow syndrome and yield loss. If a corn grower does not have the equipment needed to apply a high rate of P fertilizer in a band near or with the seed at planting, “fallow syndrome” will likely cause yield loss. Research has shown that broadcasting P fertilizer (even at high rates) will not fix the “fallow syndrome” induced P deficiency (see table 1). A high rate of
starter P is required.