y 



CHAPTER X 



CROP ROTATION — ITS PURPOSE AND PRACTICE 



IT has been found that where the same crop is grown 

 on the land year after year, no matter how well sup- 

 pUed with fertiUzers, the yield cannot be kept up 

 profitably, and often the soil refuses to produce that crop 

 at all. 



No crop is ever entirely removed from the soil. The 

 roots at least remain there to decay, and it seems that 

 plants soon get tired of feeding on their own decay. Some 

 have supposed that there are deleterious excreta thrown 

 off by the growing roots of plants, and that these are poi- 

 sonous to that particular plant while not so to plants of a 

 different character. But no true excretory process has 

 ever yet been discovered in plant life, though it is believed 

 that the roots do exhale carbon dioxide in the soil. But 

 this would only tend to render the insoluble plant food in 

 the soil more available, and would not be at all poisonous 

 to plants. 



In the experiments at Rothamsted, England, conducted 

 by Lawes and Gilbert, they cultivated Irish potatoes on a 

 piece of land continuously for a long series of years until it 

 finally refused to produce any potatoes. But when it was 

 sown to barley it made a crop of seventy-five bushels per 

 acre. The cause of this probably was that plants select 

 plant food from the soil differently. Irish potatoes call 



148 



