J 62 ROTATION OF CROPS. 



we compare it with the quantity removed from the soil in three 

 months by a crop of wheat of equal weight ! 



It follows, then, from the preceding observations, that the ad- 

 vantage of the alternate system of husbandry consists in the fact 

 that the cultivated plants abstract from the soil unequal quantities 

 of certain nutritious matters. 



A fertile soil must contain in sufficient quantity, and in a form 

 adapted for assimilation, all the inorganic materials indispensable 

 for the growth of plants. 



A field artificially prepared for culture, contains a certain 

 amount of these ingredients, and also of ammoniacal salts and de- 

 caying vegetable matter. The system of rotation adopted on such 

 a field is, that a potash-plant (turnips or potatoes) is succeeded by 

 a silica plant, and the latter is followed by a lime-plant. All these 

 plants require phosphates and alkalies — the potash-plant requiring 

 the largest quantity of the latter and the smallest quantity of the 

 former. The silica plants require, in addition to the soluble silica 

 left by the potash plants, a considerable amount of phosphates; 

 and the succeeding lime-plants (peas or clover) are capable of 

 exhausting the soil of this important ingredient to such an extent, 

 that there is only sufficient left to enable a crop of oats or of rye 

 to form their seeds. 



The number of crops which may be obtained from the soil de- 

 pends upon the quantity of the phosphates, of the alkalies, or of 

 lime, and the salts of magnesia existing in it. 



The existing provision may suffice for two successive crops 

 of a potash or of a lime-plant, or for three or four more crops of 

 a silica plant, or it may suffice for five or seven crops of all taken 

 together ; but after this time, all the mineral substances removed 

 from the field in the form of fruit, herbs, or straw, must again be 

 returned to it ; the equilibrium must be restored, if we desire to 

 retain the field in its original state of fertility. 



This is effected by means of manure. It may be assumed that 

 the soil receives again, in the roots and stubble of the cereals, or 

 in the fallen leaves of Irees, as much carbon as its humus yielded 

 in the form of carbonic acid at the commencement of a new vege- 

 tation ; in like manner, the herb of the potatoe and the roots of the 

 clover remain in th<» soil. The remains of these plants enter into 



