ROTATION. 341 



The water of the Artesian well at Grenelle, near Paris, according 

 to the analysis of M. Payen, contains, in 100,000 parts : 



Carbonate of lime 6.80 



Carbonate of niiignesia 1.42 



Bicarbonate of potash 2.96 



Sirtphate of potash 1.20 



Chloride of poUissium 1.09 



Silica 0.57 



Yellow matter, not defined 0.02 



Organic azotized matter 0.24 



14.30 



CHAPTER VII. 



OF THE ROTATION OF CROPS; 



§ 1. OF THE ORGANIC MATTER Oii' MANURE AND OF CROPS. 



It is known that the atmosphere and the organic matters diffused 

 through the earth concur simultaneously to maintain the life of 

 plants ; but how far each contributes is undetermined. We shall now 

 study the theory of the exhaustion of the soil by culture, and the 

 rotation of crops. 



When a succession of crops is grown upon fertile land without 

 renewal of manure, the produce gradually diminishes ; and after a 

 certain period, if it be grain, the quantity which at the outset was 

 eight or nine times the amount of the seed, will be reduced to three 

 times or even to twice the seed. Thus crops impair the fertility of 

 the soil, and eventually exhaust it. 



It has been long admitted that different species of plants manifest 

 great diversity in their powers of exhaustion. Certain kinds, indeed, 

 as trefoil and lucerne, far from exhausting it, communicate new 

 vigor. As a general rule, however, every plant may be said to 

 impoverish the soil in which it grows. This impoverishment is al- 

 ways manifest when the plant after maturity is completely removed, 

 but is less sensible when much rubbish is left. Thus, for example, 

 clover, after yielding two crops, which are generally cut as fodder, 

 might still yield a third ; this last, however, is generally ploughed 

 into the ground as manure, being buried along with a considerable 

 quantity of roots. This plan of meliorating the soil by the cultiva- 

 tion of trefoil is what is called manuring by smothering ; a method 

 practised from a remote period in the south of Europe, and which 

 offers decided advantages in those districts where there is abun- 

 dance of pasture land. Hence, in smothering trefoil, the soil is 

 amended at the expense of the nutritive matter it contains. 



Thaer, who endeavored to make theory and practice mutually 

 agree, laid it down as a rule, that the exhaustion occasioned b^ 



29* 



