THE USE OF FERTILIZERS 219 



increasing need for greater quantities of fertilizers. This is 

 doubtless the case in many instances, and arises from neglect 

 of other factors affecting soil productiveness. As we have 

 seen, certain fertilizers cause the soil to lose lime, which 

 results in soil acidity. Organic matter is allowed to decrease, 

 and this causes the soil to become compact and poorly aerated, 

 and thus, one bad condition leads to another and crops be- 

 come poorer in spite of increased applications of fertilizer. 



QUESTIONS 



1. Why are some crops able to draw abundant nourishment from 

 soils on which other crops yield poorly ? 



2. How do wheat and corn differ in their need of plant-food 

 materials ? 



3. Why is nitrate of soda particularly beneficial to grass ? 



4. What two fertilizer materials are generally useful on legumes ? 



5. What fertilizer material is required in large amounts by most 

 root crops ? 



6. What plant nutrient is especially needed by vegetables that 

 are expected to make a rapid and succulent growth ? 



7. In what ways are green-manures of use in orchards ? 



8. Plan a fertilizer test similar to that shown in Pig. 31, but 

 to be used with a crop of timothy instead of small grain. 



9. To what crops in a rotation of corn, oats, wheat, and grass 

 would you apply fertilizers ? 



10. Explain what is meant by the limiting factor in plant growth 

 with respect to the use of fertilizers. 



11. What is meant by the law of diminishing returns ? 



12. Name five soil factors within the control of man that influ- 

 ence the availability of fertilizers. 



13. Give two reasons why a sandy soil responds more promptly 

 to fertilizers than does a clay soil. 



14. Explain why soils sometimes demand an increasing use of 

 fertilizers to maintain their productiveness. 



LABORATORY EXERCISES 



Exercise I. — Fertilization of standard rotations. 

 The fertilization of the rotation is the ultimate and final consider- 

 ation of any systematic use of fertilizers. While the fertilization as 



