Gypsum Effect on Broiler Litter pH.

Previous work has shown that addition of gypsum to broiler litter can decrease ammonia volatilization in part due to a decrease in pH, but the mechanism for that pH decrease is not clear. We conducted two laboratory studies to elucidate the mechanism involved in pH reduction. In one study, broiler litter with or without 20% flue-gas desulfurization gypsum was incubated at room temperature for 24 h, with pH measurements made every hour for the first 4 h and at the end of the study. Additionally, the broiler litter buffering capacity and initial amount of bicarbonate present were determined using a titrimetric method. Broiler litter pH decreased 0.2 units immediately after adding gypsum and continued to decrease over 24 h, reaching a difference of 0.4 pH units with respect to un-amended broiler litter. The broiler litter buffering capacity was 229 mmol H⁺/(pH unit)/kg and the amount of bicarbonate present was sufficient to release 51 mmol H⁺/kg if it precipitated as CaCO ₃. Such a release of H⁺ would cause a reduction in pH of 0.22 units, similar to the pH reduction observed immediately after adding gypsum. To better understand and confirm this possible precipitation of CaCO ₃ in the presence of gypsum, an additional study was conducted in which broiler litter amended with urea (to generate bicarbonate ions) with or without 20% gypsum was incubated at room temperature for 48 h, after which CO ₂ released, bicarbonate/carbonate in solution, and precipitated CaCO ₃ were measured. The estimated carbon balance showed 92 to 99% recovery of  the C added with urea, with values for CO ₂, and bicarbonate/carbonate in solution being lower, and values for precipitated CaCO₃being greater in the treatment with gypsum than in the treatment without gypsum. These results indicate that hydrolysis of urea commonly present in broiler litter leads to enhanced CaCO ₃ precipitation in the presence of gypsum, which in turn appears to be responsible for the decrease in pH and reduced ammonia loss observed with gypsum addition.