COMPOSITION OF SOILS. 153 



weeds upon which they live spring from the soil, 

 and that which they return to it as excrement must 

 always be less than that which they extract. The 

 fields, therefore, can have gained nothing from the 

 mere feeding of cattle upon them ; on the contrary, 

 the soil must have lost some of its constituents. 



Experience has shown in agriculture that wheat 

 should not be cultivated after wheat on the same 

 soil, for it belongs with tobacco to the plants which 

 exhaust a soil. But if the humus of a soil gives it 

 the power of producing corn, how happens it that 

 wheat does not thrive in many parts of Brazil, where 

 the soils are particularly rich in this substance, or 

 in our own climate, in soils formed of mouldered 

 wood ; that its stalk under these circumstances 

 attains no strength, and droops prematurely ? The 

 cause is this, that the strength of the stalk is due 

 to silicate of potash, and that the corn requires 

 phosphate of magnesia, neither of which substances 

 a soil of humus can afford, since it does not contain 

 them ; the plant may, indeed, under such circum- 

 stances, become an herb, but will not bear fruit. 



Again, how does it happen that wheat does not 

 flourish on a sandy soil, and that a calcareous soil is 

 also unsuitable for its growth, unless it be mixed 

 with a considerable quantity of clay?* It is because 

 these soils do not contain alkalies in sufficient quan- 

 tity, the growth of wheat being arrested by this 

 circumstance, even should all other substances be 

 presented in abundance. 



It is not mere accident that only trees of the fir 

 tribe grow on the sandstone and limestone of the 

 Carpathian mountains and the Jura, whilst we find 



* In consequence of these remarks in the former edition of this 

 work, Professor Wohler of Gottingen has made several accurate analy- 

 ses of different kinds of limestone belonging to the secondary and 

 tertiary formations. He obtained the remarkable result, that all those 

 limestones, by the disintegration of which soils adapted for the culture 

 of wheat are formed, invariably contain a certain quantity of potash. 

 The same observation has also recently been made by M. Kuhlmann 

 of Lille. The latter observed that the efflorescence on the mortar of 

 walls consists of the carbonates of soda and potash. — L. 



