150 THE ART OF CULTURE. 



the best plan which could be adopted. A field is 

 cultivated once every three years, and is in the inter- 

 vals allowed to serve as a sparing pasture for cattle. 

 The soil experiences no change in the two years 

 during which it there lies fallow, further than that 

 it is exposed to the influence of the weather, by 

 which a fresh portion of the alkalies contained in it 

 are again set free or rendered soluble. The animals 

 fed on these fields yield nothing to these soils 

 which they did not formerly possess. The 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 field, 

 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 sub- 

 stance, or in our own climate, in soils formed of 

 mouldered wood ; that its stalk under these cir- 

 cumstances attains no strength, and droops prema- 

 turely? 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 



