84 



PHYSIOLOGICAL PROCESSES IN PLANTS 



nitrogen removed from the soil by 1000 pounds each of the 

 different crops mentioned. Each square indicates a pound. 



FIG. 20. Showing the pounds of plant, food removed by 1000 pounds of Vir- 

 ginia leaf tobacco, and by 1000 pounds of clover. 



Conclusions. 1. How does clover compare with tobacco as 

 a user of nitrogen? 



2. If 1000 pounds of average barnyard manure contains 

 about 5 pounds of nitrogen, how much manure will be re- 

 quired to replace the nitrogen removed by 1000 pounds of 

 tobacco ? 



Observations. 1. Which of the crops shown in Figure 21 

 reduce soil fertility the most rapidly ? 



Ind 



VAieat 



Oats 



FIG. 21. Showing the amounts of the three most important plant foods that 

 are removed from the soil by 1000 pounds each of the grain of Indian corn, 

 wheat, and oats. 



2. How many pounds of nitrogen are removed by 1000 

 pounds of each of the three crops shown? 



