By Anthony Bly, SDSU Extension
Recent heavy rains have created small scale stagnant flooded soil. Nitrate-N leaching and denitrification are primary concerns for crop producers. However, other soil conditions are also affected. The rapid depletion of soil oxygen causes a shift in nitrogen cycle soil micro-organisms that lead to the denitrification processes. Conclusions from an extensive soil flooding study in Missouri found very small changes in microbial and fungi populations (Unger et al. 2009). In this study, soils were flooded for up to 5 weeks. They also found that total soil N (mostly organic N) and carbon were not significantly reduced.
Other soil chemical properties that could be affected are pH and phosphorus. Despite the original soil pH prior to flooding, the pH of most all flooded soils will gradually become neutral (The Chemistry of Submerged Soils, Advances in Agronomy, 1972). These processes are controlled by redox potential processes driven by the lack of soil oxygen. Gradually, in these flooded soils, phosphorus will become more available either from the reduction or dissolution of iron and calcium phosphates, respectively. This will not lead to increased crop productivity after flooding because oxidation processes will return the phosphates to their original associations. Soil redox potential (lack of oxygen) can also influence soil physical properties such as aggregate stability.
Soil aggregation is the clumping together of soil particles in larger sizes referred to as aggregates. Soil aggregate formation leads to improved soil structure (clumps of aggregate sticking together). Increased soil aggregation and soil structure improved water and air movement into and out of the soil. Soil water and air movement helps plants obtain nutrients and allows for improved root respiration needed for optimum crop productivity. As soil aggregates breakdown, the soil particles plug pores and voids that would help the soil to dry and return to normal after flooding. Researchers at Purdue found a 21% reduction in aggregate stability after 14 days of flooding in cultivated soils that might be irreversible (De-Campos et al. 2009). This is why surface crusting is an issue after flooded soils dry out. These researchers also found that aggregate breakdown was less in un-cultivated soils (7%).
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