360 BELATIONS OF ELEMENTS 



philosophers* have maintained, and that this assimilated, decom- 

 posed, and recomposed anew by the plant, is the source of its azotized 

 constituents. 



§ 2. OF THE RESIDUES OF DIFFERENT CROPS. 



The vegetable matter which is produced in the course of a season 

 is never found entirely in the crop. A certain quantity of it, for 

 instance, always remains in the ground. It is, therefore, a point of 

 interest to ascertain what quantity of elementary matter is left in the 

 soil after each kind of crop in the rotation ; precise knowledge of 

 this description may even be important in calculating rotations, for 

 it is obvious that the remains of the crop now on the ground must 

 influence that which is to follow, and in the course of a rotation the 

 sum of the residuary matters must be regarded as a supplement oi 

 addition to the manure put into the ground at its commencement. 



In the systems of rotation very generally followed at the present 

 time, the infloence of these residuary matters is manifest, and it is 

 partly by their means that we can explain how a quantity of manure, 

 frequently very moderate, should suffice for the whole of the crops 

 in a productive rotation. The remarkable effect of clover has not 

 failed to arrest attention even from the most unobserving. The 

 wheat crop which comes after our drill crop in Alsace, beet or 

 potatoes, averages from 18 to 20 bushels per acre ; but the wheat 

 crop that succeeds our clover averages from 23 to 24 bushels per 

 acre. 



The improvement of the soil, so obvious in connection with clover, 

 in all probability also occurs in connection with the residues of other 

 crops ; but as in most instances the residue merely compensates the 

 loss, or lessens its extent, the effect produced is less remarkable, 

 and is less, indeed, in amount. All the world acknowledge, then, 

 that the residues of the crops that enter into a rotation compensate 

 in greater or less degree for what is carried away in the shape of 

 harvest, and that in some cases they even add to the fertility of the 

 soil ; for in growing crops that leave a large quantity of residue, it 

 is precisely as if a smaller quantity were taken from a given extent 

 of surface. But what is the amount of residue or refuse which is 

 returned to the soil by such and such a crop ? What, in a word, is 

 the value of this residuary matter considered as manure ? This is 

 a point upon which only the most vague and indefinite ideas are 

 generally entertained ; and it was with the purpose of substituting 

 positive facts for mere guesses, that I determined on weighing and 

 analyzing the vegetable residue of the several crops that enter as 

 elements into our more usual rotations. 



My experiments were made upon breadths of land which varied 

 from 120 to 500 square yards in extent. The clover roots and 

 stubble were taken up with the spade, and before being dried, were 

 freed from adhering earth by washing. The beet-leaves and pota- 

 to-tops were dried at once in the oven ; and it was from each of the 



* Saoisore Recberches Chimiques, and Liobig, Agricultural Chemistry. 



