EFFECTS PRODUCED BY PLANTS ON THE SOIL. 101 



the beginning of winter, and allowing it to remain in that 

 state, exposed to the action of the weather. By this means 

 the rocky particles of which the soil is composed, by the in- 

 fluence of the gases of the air and moisture, are gradually 

 made to crumble, and the alkalies contained in them set 

 free in a proper state to be taken up by plants. It is 

 evident that the various mechanical operations of the farmer, 

 such as frequent plonghings, &c. must facilitate this decom- 

 position of the mineral matters of the soil, and serve to im- 

 prove its productiveness. It was formerly supposed that the 

 fallowing of land was indispensable to renew its fertility when 

 exhausted, and that plants grown npon it separated from 

 their roots certain excrements which it was necessary should 

 be decomposed by free exposure to the air, and that by so doing 

 the land was "sweetened" and rendered again fit for bearing 

 crops. Many philosophers, however, consider that it has not 

 been satisfactorily proved that plants give out organic matters 

 from their roots, and are of opinion that the benefits of fallow- 

 ing are to be ascribed to its facilitating the liberation of the 

 inorganic principles of plants (42) which were locked up in the 

 soil. By this system, however, the farmer was obliged to 

 dispense for one year with any return from his field, either as 

 food for man or animals, and at present it is but seldom 

 employed in advanced agricultural districts, except on the most 

 stubborn clays where the thorough drain, and other means of 

 improving the texture of the soil, have not yet been adopted. 

 The introduction of what are teimed green crops into the 

 period formerly occupied by the unprofitable fallow, has not 

 only produced a complete revolution in our system of hus- 

 bandry, but greatly increased the produce of this country. 



144. Rotation of crops. A field, if made to produce the 

 same crops for a number of years in succession may, as has 

 been shown, in the grain and cattle sold off the farm, be 

 impoverished by the loss of all the inorganic matters which it 

 contained in a fit state to serve plants for food, and thus suffer 

 a general exhaustion ; or it may by the growth of a plant 

 requiring chiefly the alkalies or lime, supposing the active 

 </il* to contain the usual amount of these iugi*edieuts, be 



The term active soil has appropriately been employed by an English 

 chemist, Dr. Daubeny, to denote that portion of the soil of a field, in 

 ^vhich the mineral matters required by plants exist in a condition 

 capable of being taken up by the growing crop. 



