34 THE PLANT FOOD SUPPLY 



product of very high value per acre, and any material increase 

 in yield or quality that can be brought about by additional appli- 

 cations of plant food is usually attended with corresponding profit. 

 Of course, there is a limit beyond which it is not profitable or 

 expedient to go, but that limit is seldom reached if intelligence is 

 used in the selection of the forms of plant food that are to be applied 

 and the method and time of their apphcation. 



QUESTIONS 



1. WTiat three elements of plant food is it usually necessary to apply to 



soils? 



2. T\liat are the three chief sources of plant food that can be employed in 



vegetable growing? 



3. What valuable material, aside from plant food, do animal manures add 



to the soil? 



4. Upon what factors does the plant food content of animal manures depend? 



5. What is "compost"? How is it made? What are its special uses? 



6. What are "green manures"? 



7. Discuss the relative merits of different crops for green manuring under 



specified conditions. 



8. Under what circumstances is green manuring preferable to the use of 



animal manures, and vice versa? 



9. Wliat is a "complete fertilizer"? An "incomplete fertilizer"? 



10. What are the advantages of home mixing of commercial fertilizers? 

 IL Wliat are some of the leading commercial sources of nitrogen? Of phos- 

 phorus? Of potassium? 



12. What dangers attend the continued use of commercial fertilizers as the 



sole source of plant food? 



13. Wliat circumstances determine the time and method of application of 



manures and fertifizers? 



14. How much manure per acre can profitably be used for the production 



of vegetable crops? 



15. WTiat forms of fertilizer are used in market gardening in your section? 



