100 SOURCE AND ASSIMILATION OF NITROGEN. 



shrubs, and other wild plants ; but this is not suffi- 

 cient for the purposes of agriculture. Agriculture 

 differs essentially from the cultivation of forests, 

 inasmuch as its principal object consists in the pro- 

 duction of nitrogen under any form capable of 

 assimilation; whilst the object of forest culture is 

 confined principally to the production of carbon. 

 All the various means of culture are subservient to 

 these two main purposes. A part only of the carbonate 

 of ammonia which is conveyed by rain to the soil is 

 received by plants, because a certain quantity of it 

 is volatilized with the vapor of water; only that 

 portion of it can be assimilated which sinks deeply 

 into the soil, or which is conveyed directly to the 

 leaves by dew, or is absorbed from the air along 

 with the carbonic acid. 



Liquid animal excrements, such as the urine with 

 which the solid excrements are impregnated, contain 

 the greatest part of their ammonia in the state of 

 salts, in a form, therefore, in which it has completely 

 lost its volatility; when presented in this condition, 

 not the smallest portion of the ammonia is lost to 

 the plants ; it is all dissolved by water, and imbibed 

 by their roots. The evident influence of gypsum 

 upon the growth of grasses — the striking fertility 

 and luxuriance of a meadow upon which it is strewed 

 — depends only upon its fixing in the soil the am- 

 monia of the atmosphere, which would otherwise be 

 volatilized, with the water which evaporates.* The 

 carbonate of ammonia contained in rain-water is 

 decomposed by gypsum, in precisely the same man- 

 ner as in the manufacture of sal-ammoniac. Soluble 

 sulphate of ammonia and carbonate of lime are 

 formed ; and this salt of ammonia possessing no 

 volatility is consequently retained in the soil. All 

 the gypsum gradually disappears, but its action upon 



* It has long been the practice in some parts of the country to strew 

 the floors of stables with gypsum. This prevents the disagreeable odor 

 arising from the putrefaction of stable manure, by decomposing the 

 ammoniacal salts which are formed. — Ed. 



