Frequent and excessive rainfall throughout Indiana during the past several months has resulted in extensive flooding of river bottoms and ponding in low-lying areas of other fields. Some river bottom ground has yet to be planted for the first time this year, some has been replanted two or more times but is still devoid of crop growth because of continued flooding. Many fields throughout the state contain "wet holes" that have ponded numerous times this growing season, killing young corn or soybean, and have not been replanted.
Where such fields or parts of fields have been left fallow due to flooding or ponding, corn growth the following year sometimes displays symptoms of phosphorus deficiency even if adequate levels of soil phosphorus exist. Recently published research from USDA-ARS at the Univ. of Nebraska sheds some light on this phenomenon known as Post Flood Syndrome. The complete citation is : J.R. Ellis, Post Flood Syndrome and Vesicular-Arbuscular Mycorrhizal Fungi, J. Prod. Agric. 11:200-204 (1998).
The fungi referred to in the article's title (acronym: VAM) are plant fungi that routinely colonize roots of crops in a mutually beneficial (symbiotic) relationship. The fungi benefit from the host plants' resources, the crop benefits from the increased nutrient absorption zone offered by the fungal hyphae (the many threads that make up the mycelium of fungi).
The bottom line of the research is that prolonged fallow periods, such as caused by flooding or ponding, significantly reduce the populations of the VAM fungi in the soil. Following the seeding of a subsequent crop, the VAM fungi slowly recolonize and restore their populations. Until the populations of the VAM fungi are sufficient, plant nutrient uptake (especially phosphorus) is restricted and nutrient deficiency symptoms can occur.
The author suggests several alternatives to forestall the development of the Post Flood Syndrome in corn. One is to apply 60-80 lbs per acre of starter phosphorus to the following corn crop, equal to 16 to 21 gallons of 10-34-0 starter fertilizer (easily accomplished by many Indiana corn growers). Another alternative is to seed a host cover crop in the fall to encourage the regeneration of the VAM fungal populations (seeding the areas to corn or soybean now would accomplish the same thing). While VAM fungal inoculants exist, the author suggests that using them would be too expensive for field crops.