THE POTENTIAL PLANT FOOD 



8l 



In the case of barley, potassium is most in demand, 

 with nitrogen not far behind, and phosphorus within the 

 minimum range. 



Let us express these demands in terms of proportion, 

 using nitrogen and 100 as the units of comparisons for 

 each crop : we get : 



These variations appear still more noticeable when we 

 use the nitrogen of corn as the sole unit of comparison: 

 we get : 



These two tables show notable variations in food ele- 

 ments, utilized by different plants. Our comparison, in 

 this instance, is of cereals, only ; when the range of ob- 

 servation includes, not only cereals but also legumes, 

 fiber crops, the grasses, and root and vegetable crops, as 

 well, the differences are peculiarly striking and marked. 



Feeding demands of crops. The question naturally 

 arises : Is it likely that continuous cropping will exhaust 



