Effect of pH on the behaviour of volatile compounds in organic manures during dry-matter determination

Abstract

Dry matter determination by drying at 105°C is one of the most widespread methods used in the characterization of manure. Loss of volatile compounds during dry-matter determination may lead to considerable errors in mass balances. Since the behaviour of most volatile compounds depends upon pH, these losses may vary from sample to sample. The loss of ammonia and volatile fatty acids from pig, cattle and poultry manure was studied as a function of pH. It was shown that above pH 8 all ammonia was volatilized and below pH 5 all volatile fatty acids evaporated. Total fixation of ammonia was achieved below pH 4. Above pH 10 all volatile fatty acids were fixed in the residue after drying. The three types of manure showed only slightly different patterns of losses of volatile compounds. The amount of acid or alkali needed to obtain the desired pH varied strongly between the various kinds of manure.