210 ON THE CHEMICAL CONSTITUENTS OF SOILS. 



viewed as such. These salts do not, however, occur 

 simultaneously in all plants. Thus, in saline plants, 

 soda is the only alkali found; in corn plants, lime 

 and potash form constituents. Several contain both 

 soda and potash, some both potash and lime ; whilst 

 others contain potash and magnesia. The acids 

 vary in a similar manner. Thus one plant may 

 contain phosphate of lime, a second, phosphate of 

 magnesia, a third, an alkali combined with silicic 

 acid, and a fourth, an alkali in combination with a 

 vegetable acid. The respective quantities of the 

 salts required by plants are very unequal. The 

 aptitude of a soil to produce one, but not another 

 kind of plant, is due to the presence of a base w^hich 

 the former requires, and the absence of that, indis- 

 pensable for the development of the latter. Upon 

 the correct knowledge of the bases and salts requi- 

 site for the sustenance of each plant, and of the 

 composition of the soil upon which it grows, depends 

 the whole system of a rational theory of agriculture; 

 and that knowledge alone can explain the process 

 of fallow, or furnish us with the most advantageous 

 methods of affording plants their proper nourish- 

 ment. 



Give, — so says the rational theory, — to one plant 

 such substances as are necessary for its development, 

 but spare those, which are not requisite, for the 

 production of other plants that require them. 



It is the same with regard to these bases as it is 

 with the water which is necessary for the roots of 

 various plants. Thus, whilst one plant flourishes 

 luxuriantly in an arid soil, a second requires much 

 moisture, a third finds necessary this moisture at 

 the commencement of its development, and a fourth 

 (such as potatoes) after the appearance of the blos- 

 som. It would be very erroneous to present the 

 same quantity of water to all plants indiscriminately. 

 Yet this obvious principle is lost sight of in the 

 manuring of plants. An empirical system of agri- 

 culture has administered the same kind of manures 



