172 BIOLOGY OF PLANTS. 



come wholly decomposed ; and, when that is the case, the soil 

 becomes absolutely barren, and nothing can restore fertility 

 but the addition of alkalies and gravel ; hence the necessity 

 and utility of a rotation of crops ; for when all the potash has 

 been removed from the soil by one family of plants, other 

 plants may be substituted, which do not require this alkali for 

 their growth. 



In other cases, jTree alkali is needed. Hence the effect of 

 ploughing in green crops, and the utility of fallows. The 

 growing plant eliminates the potash from the feldspar,* and 

 it is then turned into the soil and is ready to be applied to 

 future crops. 



All kinds of grain contain, in the outer part of their leaves 

 and stalks, a large quantity of^ s'lViCRte of potash, which must 

 be derived from the soil. If now we increase the amount of 

 grass or grain by means of gypsum, a larger quantity of potash 

 will be eliminated, and the free alkali will be carried off, so 

 that in a short time, the crop will be diminished, and either 

 a fallow or some other means must be resorted to, to restore 

 that alkali. 



The planters of Virginia, according to Liebig, exhausted 

 their soils by cultivating for a century in succession, tobacco 

 and wheat on the same land without manure. By this pro- 

 cess, twelve hundred pounds of alkalies were in the course 

 of one hundred years, abstracted from every acre of soil. 

 Thus these lands were nearly deprived of alkali, and are 

 now barren wastes. Hence the necessity of returning the 



* There is not much danger that the alkali will be exhausted by 

 this process. The quantity contained in the rocks, is almost inex- 

 haustible, compared with that taken up by plants ; for it is found, 

 that the wheat straw, grown on an acre, takes up only twenty-two 

 pounds of potash, and the quantity in the soil would be sufficient for 

 the straw, during a period of three thousand years. The quantity of 

 potash on ap acre of granitic rock six inches in depth varies from 

 ninety to one hundred and twenty tons, but a fsir less quantity is 

 found in the same depth of soil. 



