84* FOOD OF THE VEGETATING PLANT. CHAP. II. 



of fallowing, and of trenching or deep ploughing, 

 which must have nearly the same effect. 



If any one asks how the fertility of a soil is 

 restored by the means now stated, it will be suffi- 

 cient for the object of the present section to reply 

 that, in the case of draining, the amelioration is 

 effected by means of its carrying off all such super- 

 fluous moisture as may be lodged in the soil, which is 

 well known to be prejudicial to plants not naturally 

 aquatics, as well as by rendering the soil more firm 

 and compact. In the case of burning, the ameliora- 

 tion is effected by means of the decomposition of 

 the vegetable substances contained in the turf, and 

 subjected to the action of the fire, which disperses 

 part also of the superfluous moisture, but leaves a 

 residue of ashes favourable to future vegetation. In 

 the case of the rotation of crops, the fertility is not 

 so much restored as more completely developed and 

 brought into action ; because the soil, though ex- 

 hausted for one species of grain, is yet found to be 

 sufficiently fertile for another, the food necessary to 

 each being different, or required in less abundance. 

 In the case of the repose of the soil, the restored 

 fertility may be owing to the decay of vegetable 

 substances that are not now carried off in the annual 

 crop, but left to augment the proportion of vegetable 

 mould ; or to the accumulation of fertilizing parti- 

 cles conveyed to the soil by rains; or to the con- 

 tinued abstraction of oxygen from the atmosphere. 

 In the case of fallows, it is owing undoubtedly to the 



