Fallow- and Flooded-Soil Syndromes: Effects on Crop Production

Fallow Soil Syndrome has been a problem for farmers in arid areas. The first observations were made in the 1940s when crops failed to produce as expected when planted to fallowed fields. Recent flooding in the midwestern U.S. states caused extensive crop damage and left many areas barren of plant growth. There was concern that crop production problems would develop when subsequent crops were planted to previously flooded areas. Like Fallow Soil Syndrome, the symptoms of Flooded Soil Syndrome are P deficiencies developing in crops even when adequate soil P concentrations are measured. Commonly used soil chemical extracts are not reliable when attempting to predict occurrence of the problem. Soil vesicular-arbuscular mycorrhizae (VAM) fungi have been shown to increase host plant uptake of relatively immobile nutrients like P. In studies of fallowed soils, VAM colonization rates are lower than similar unfallowed soils. Decreased VAM colonization rates appear to be a factor in development of the disorder. Corn (Zea mays L.) is probably the crop most susceptible to Fallow and Flooded Soil Syndromes. Phosphorus deficiencies have successfully been corrected using a starter P fertilizer applied in a band at planting. Broadcast fertilizer treatments have been ineffective on corn but effective for soybean [Glycine max (L.) Merr.]. Research QuestionThe first observations of Fallow Soil Syndrome were noted in wheat in the 1940s. Crops planted to previously barren land failed to produce as expected. Recent flood damage to agricultural crops left many barren areas and caused concern about subsequent crop production problems. The most common crop growth problems are classic P deficiency symptoms. The purpose of this paper is to summarize available information and recommend possible solutions to correct the problem. Literature SummaryAreas where Fallow Soil Syndrome and Flooded Soil Syndrome develop frequently have adequate soil test levels of P for crop growth. Phosphorus deficiencies still develop, suggesting that commonly used soil chemical extractants are not reliable tools to detect the problem. Soil microorganisms have been shown to play an important role in P nutrition of plants. In fallowed soils, there is a detectable decrease in numbers of beneficial organisms, specifically vesicular-arbuscular mycorrhizae (VAM) fungi. There is evidence to suggest soil VAM populations also decrease after flooding. Applied QuestionsWhat mechanisms are responsible for causing the P deficiencies characteristic of Fallow Soil Syndrome and Flooded Soil Syndrome? Soil from flooded areas usually tests adequate or high for available P, based on commonly used chemical extracting procedures. Plants growing in these areas frequently develop classic P deficiency symptoms. Apparently, more soil P is extracted from these soils than is actually available to growing plants. This suggests that part of the problem may be the result of changes in P solubility. In fallowed soils, decreased VAM colonization rates seem to be a major factor. How can the problem be corrected? Corn is probably the crop most susceptible to Fallow Soil Syndrome and Flooded Soil Syndrome. Starter P fertilizer applied in a band at planting is the most effective way to prevent P deficiencies from developing. Rates of applied P almost twice those normally recommended are necessary. If proper application equipment is unavailable then a less susceptible crop, soybean for example, should be planted.