454 Transactions of the American Institute. 



Others claim that it acts only as a constituent of plants. This 

 cannot be true, because, if it were so, much more than a bushel per 

 acre might be profitably used upon common land as manure, as it is 

 perfectly obvious that common manure, bone dusts, and guanoes of 

 all kinds, can be used to any extent almost, with results correspond- 

 ing to amounts employed, while in the case of plaster, generally 

 speaking, no appreciable results follow beyond a certain limited 

 quantity. 



Others call plaster the saliva or gastric juice of vegetation. This 

 cannot be the explanation of its action, for if fifty bushels be 

 applied, where one bushel is enough per acre, no injurious effects 

 follow, as would be the case from excess or over action, if plaster 

 performed any function at all to be assimilated to that of saliva or 

 vegetable gastric juice. 



Some say it is simply a stimulant of vegetation. This explanation 

 will not do ; because there is no doubt plaster permanently improves 

 the soil if the vegetation produced by it is not removed from the 

 land. Others attribute its powers to its possessing the power of 

 supplying water and carbonic acid to plants. This explanation will 

 not hold, because of the fact that only a very small quantity of 

 plaster will act when sown upon land in its ordinary condition, and 

 it is only another form of expressing the first explanation of the 

 action of plaster, viz., — that it acts as a manure, which we have 

 shown to be untrue. Others say it owes its power to the fact of its 

 supplying sulphuric acid to plants, which is but a modification of 

 other explanations which I have shown to be untenable. The ques- 

 tion still remains, how does plaster act? I think I can answer it in 

 great part, if not to the full satisfaction of the inquirer. It acts as a 

 condenser of the ammonia of the atmosphere and of the soil. 

 Gypsum is composed of sulphuric acid and lime. A bushel of this 

 substance, if it acted as a manure, would clearly exert but a very 

 small effect upon an acre of land by its chemical action upon plants, 

 or by entering into combination in the growth of crops. By observa- 

 tion, however, it is concluded that gypsum acts as a condenser 

 of ammonia, which is found everywhere in the atmosphere and in all 

 soils to a limited extent — that it holds this most subtle and powerful 

 manure in its grasp, fixes it and gives it out to the growth of plants 

 as they require it, instead of allowing the ammonia to pass away and 

 remain unfixed and unadapted to plant growth. There are a few 

 phenomena which seem to me to fully establish the fact that this is 

 the true explanation of the action of 'plaster, that only a certain 



