PLANTS. 



71 



the quality of the soil is tested; and, if it be poor, 

 one heavy crop of seeds will so exhaust it that a 

 crop of the same kind the succeeding year will not 

 be worth the harvesting. 



Weeds, of course, rob the soil for their seeds; 

 but, since they either fall upon the ground or are 



MEASURING WHEAT AT THE THRASHER. 



plowed under, the rich substances they take from 

 the soil are returned to it again. Useful, seed- 

 bearing crops are usually taken away from the soil 

 on which they were grown, little or none of their 

 substance ever being returned. In this way soil, 

 whether it be rich or poor, will, sooner or later, 



