PHYSIOLOGICAL PROCESSES IN PLANTS 



85 



3. How does the amount of nitrogen removed compare with 

 the amounts of the other substances ? 



Conclusion. Compute the amount of manure necessary to 

 offset the nitrogen removed by each of the grains mentioned. 



Mi lie 



(Strawberries 



FIG. 22. Showing the amounts of nitrogen, phosphoric acid, and potash re- 

 moved from the soil when 1000 pounds each of beef, milk, and butter are 

 sold. 



Observations. 1. How does beef compare with wheat as a 

 remover of nitrogen? 



2. How does dairying compare with crop raising as a re- 

 mover of nitrogen ? 



Conclusions. 1. As a- rule, which do you consider the more 

 expensive operations, crop raising or dairying? 



Note. We must remember that total costs depend on the 

 amount of plant food removed, the labor required, and the mar- 

 ket price. 



2. Suggest two means of returning nitrogen to soil that has 

 been exhausted by repeated crops of the grains. 1 



h. Crop Rotation 



Observations. 1. What would be the advantage of planting 

 rows of cowpeas between rows of corn after the corn no longer 

 needs cultivating ? 



2. Many farmers, plow grain stubble under in the autumn, 

 and immediately plant some tubercle-bearing plant. Explain. 



1 Supplies of nitrogen bacteria may be obtained for experimental purposes 

 by applying to the United States Department of Agriculture, Washington, 

 D.C. Directions for carrying on the experiments accompany the supplies. 



