102 AGRONOMY 



several years in succession upon the same land, for instance, the 

 yield begins to decrease long before the available mineral food 

 has been used up. Thrifty crops of the same kind growing 

 elsewhere, if watered with extracts from such soils, give every 

 evidence of having been poisoned. When supplied with cer- 

 tain chemical elements, however, they regain their health. 

 Many wild plants exhibit similar peculiarities, and their 



Photograph by the University of Illinois 



FIG. 83. Wheat crop averaging 9.6 bushels to the acre 



The soil has been limed and a crop of legumes plowed under. Compare with 

 the following figure 



inability to grow in the same soil for any length of time is 

 attributed to an increase of the toxic elements. After growing 

 for a year or so in a given spot the old parts die, while the new 

 ones move out from the center in a constantly widening circle 

 known as a " fairy ring." Such rings are more common in 

 fungi, lichens, and ferns, but they also occur in flowering plants. 

 Most of these poisonous excretions appear to be harmful only 

 to the species that produced them, which explains one of the 

 benefits of a rotation of crops. By growing different crops on 



