“In agriculture plaster of Paris has by no means the importance of lime; nevertheless it produces excellent results on clover, sainfoin, and lucerne. It is used in the spring for sprinkling the young leaves when they are still damp with the morning dew. Still, foggy weather is the most favorable for this work. Plaster also acts well on rape, flax, buckwheat, and tobacco, but has no effect on cereals.
“The intelligent farmer puts plaster of Paris to still another use. In every dunghill there is always going on a slow combustion, or fermentation, giving forth ammonia in vaporous form; and this ammonia escapes into the air as a total loss, whereas it ought to be retained as far as possible in the manure, since the compounds of ammonia constitute the source whence plants obtain nitrogen. Therefore to prevent this waste, plaster is sprinkled over the dunghill. Sometimes, too, it is sprinkled over each layer of manure as the pile rises. The plaster absorbs the ammoniac vapors, gives them a little of its sulphuric acid, and converts them into a compound, sulphate of ammonia, which is proof against vaporization. Hence we say that plaster of Paris fixes ammonia, that is to say prevents its being dissipated.