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Flooding Effect on P Nutrition
Floods that destroy crops or prevent planting often produce a condition called Flood Syndrome the next cropping year. Crops after flooding are often P deficient, although soil-testing procedures often find more available P in soils after flooding than in nonflooded soils. The Flood Syndrome and "long fallow disorder" are both characterized by P deficient plants. In the case of long fallow disorder, Zn deficiency is also a problem. Absence of a mycorrhizal host plant during the fallow period decrease mycorrhizal colonization potential for the succeeding crop and results in P deficiency symptoms in plants that are mycorrhizal dependent, such as corn, soybean, sunflower, and cotton. Corn plants are stunted and show severe P deficiency when mycorrhizal hyphae or spores producing colonization are greatly decreased in soil. Similar problems occur with soybean and wheat plants after fallow in the USA.
Flood Syndrome is very similar to the long fallow disorder discovered in Australian (and in North and South Dakota) in that the effect on plants is associated with a marked decrease in VAM fungal hyphae or spores producing colonization. In a greenhouse study on flooded soils, root colonization by VAM fungi and their activity, as measured by arbuscule production, was not affected. The results of this experiment would support the hypothesis that lack of host plants during an extended fallow was the reason for decreasing VAM fungal hyphae or spores producing colonization and not extended water saturation of soil. During the year following flooding and fallow, the growing plants were a host for VAM fungi and the population was increased. However, extreme loss of VAM fungi may require more time to rebuild the population. In some cases, a plant which is less dependent upon VAM fungi can be planted. Soybean is more tolerant to decreased VAM fungal colonization and sorghum has even greater tolerance. In general, the shorter and coarser the root system, the greater the host plant dependence on mycorrhiza.
Is P fertilizer treatment adequate to overcome the loss of the arbuscular-mycorrhizal fungus from soil?
Although there is no treatment without fertilizer applied, plants with the lowest rate of 25 lb P/acre had severe P deficiency, which resulted in 32 bu-acre yield decrease in the flooded field, compared with the nonflooded field. Broadcasting P fertilizer did not have a significant impact on P uptake by plants. When 60 to 80 lb P/acre as starter fertilizer was applied to plots, plants did not show any P deficiency but flooded fields had a 7 to 16 bu/acre decrease in grain yield. An alternative for P fertilization, when soil P is adequate, could be to grow a fall cover host crop to rebuild the VAM fungal population.
Source: Taken from Research article "Post Flood Syndrome and Vesicular-Arbuscular Mycorrhizal Fungi" J. Prod. Agric., Vol 11, no. 2, 1998, p 155-156. J.R. Ellis
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