Adding simple superphosphate or phosphogypsum to piles of manure or other organic residues has been recommended to reduce losses of ammonia by volatilization during composting. However, experiments have shown discrepant results when the efficiency of these additives and the adequate rate of application are evaluated. This discrepancy could possibly be explained by differences in the residual acidity of the additives utilized in those experiments. To check this hypothesis, a laboratory experiment was carried out using samples of phosphogypsum, simple superphosphate and triple superphosphate, each of which having two different levels of residual acidity (respectively 0.13 and 0.20%; 7.02 and 2.36%; 2.38 and 1.86%). Ten grams of an organic substrate obtained by mixing equal amounts (on a weight basis) of fresh chicken manure and cattle manure were placed in 1.6 L glass flasks. The additives were mixed to the substrate in rates of 0, 50, 100, 150 and 200 kg t-1. The amount of ammonia lost by volatilization was determined after 7, 14, 21, 28, 35 and 42 days by trapping the gas in a sulfuric acid solution placed in a small glass vial, maintained inside the flask. The acid remaining after the exposure to ammonia was determined by titration with 0.01 mol L-1 NaOH solution. The studied phosphogypsum samples showed to be ineffective in reducing ammonia loss from the manure. Simple superphosphate decreased the volatilization up to 4.8 times, with the sample with higher acidity being more effective. Triple superphosphate was the additive that best controlled NH3 volatilization when applied in low rates, but in the highest rate its efficiency was similar to that of simple superphosphate. It is suggested that the additives reduced volatilization because more stable ammonium compounds were formed by reactions of the NH3 gas with acids and, or, monocalcium phosphate found in those products.
simple superphosphate; triple superphosphate; manure composting; chicken manure; cattle manure