Manure-loaded soils contain fractions of soluble and insoluble, organic and inorganic P. With time, a portion of the insoluble organic P can be mineralized to a soluble P fraction. Evidence indicates that soil amendments can significantly reduce the amounts of soluble P leached from these soils. These amendments include those investigated in these studies: calcium carbonate, gypsum, AI sulfate, and Fe sulfate. Calcium carbonate was effective when used to raise and maintain the soil pH in the range 7.0 to 7.5. Maintaining this pH is important because solid-phase P will be released should soil acidification occur over time. Gypsum was effective at all pH ranges under anaerobic conditions, but to varying degrees under aerobic conditions. Gypsum was highly effective in Spodosols with high loads of manure. Calcium carbonate and gypsum applied alone or in combination were more effective in reducing soluble P when Al a n d / o r Fe are applied in combination. However, there are recognized limitations. Aluminum can be toxic in acid soils (pH < 5.2), and costs of either Al or Fe may additionally limit the practical use of the AI and Fe sources.

Gypsum as a soil amendment had an impact on bacterial recoveries, but the reason for bacterial suppression was unclear. Bacterial suppression would also occur under anaerobic conditions. Because reagent-grade gypsum was used in this study, the role of toxic trace metals or other impurities can be excluded. Similarly, the data do not indicate that nutrient limitation at increasing gypsum application would have limited bacterial growth with time. The retention of phosphate in soil due to gypsum amendment warrants further investigation in view of the impor- tance of soluble phosphate in the eutrophication process.

The results suggest that land disposal/application of gypsum, as a waste material from the construction industry (gypsum board), coal-burning power plants (flue-gas desulfurization gypsum), and phosphate production (phosphogypsum) will have an impact on soil microorganisms by cycling and mobilization of nutrients.  

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