REVIEW OF PRECEDING THEORIES. 203 



The action of an artificial supply of ammonia as a source of 

 nitrogen, is limited, like that of humus as a source of carbonic 

 acid, to a gain in point of time ; in other words to the ac- 

 celeration of the development, in a given time, of our cultivated 

 plants. 



By means of ammonia, in the form of animal excrements, we 

 increase the quantity of the constituents of blood in our cultivat- 

 ed plants — an action which the carbonate or sulphate of ammonia 

 alone never exerts. 



In order to obviate any misunderstanding, we must again draw 

 attention to the fact that this explanation is not in any way con- 

 tradicted by the efFects produced on the application of artificial 

 ammonia, or of its salts. Ammonia is, and will continue to be 

 the source of all the nitrogen of plants : its supply is never in- 

 jurious ; on the contrary, it is always useful, and, for certain 

 purposes, indispensable. But, at the same time, it is of great 

 importance for agriculture, to know with certainty that the sup- 

 ply of ammonia is unnecessary for most of our cultivated plants, 

 and that it may be even superfluous, if only the soil contain a 

 sufficient supply of the mineral food of plants, when the ammonia 

 required for their development will be furnished by the atmo- 

 sphere. It is also of importance to know, that the rule usually 

 adopted in France and in Germany of estimating the value 

 of a manure according to the amount of its nitrogen, is quite 

 fallacious, and that its value does not stand in proportion to its 

 nitrogen. 



By an exact estimation of ihe quantity of ashes in cultivated 

 plants, growing on various kinds of soils, and by their analysis, 

 we will learn those constituents of the plants which are variable, 

 and those which remain constant.* Thus also we will attain a 



* The following analyses of ashes may be added to those formerly given : 



