304 THE PRINCIPLES OF AGRICULTURE 



in the older states, and it will be of increasing 

 importance as the country grows older. It is a 

 technical subject, for the complete understanding 

 of which much chemical knowledge is needed. 

 Persons who desire to study the subject in detail 

 should consult special works and bulletins. 

 However, the general philosophy of the applica- 

 tion of commercial plant -foods may be under- 

 stood from this brief chapter. 



It will be noticed that the chapter has six 

 coordinate parts: (1) what the elements of plant- 

 food are in the soil, and which ones are most 

 likely to be exhausted; (2) the nitrogen supply; 

 (3) phosphoric acid supply; (4) potash; (5) 

 amendments, or those substances that act bene- 

 ficially on the structure or physical condition of 

 the soil; (6) discussion of commercial fertilizers. 



What is a chemically fertile soil? What is an element ( l'27a) 1 

 How many elements are supposed to be necessary to the plants 

 (127; pages 115-117)? Which of these elements are most likely to 

 be depleted by the growing of crops? In order that these elements 

 may be useful to the plants, what must be their relation to water? 

 Do plants use these elements in their original or uncombined 

 forms? What is meant by a compound in the chemical sense 

 (130a)? What is meant by "available" plant-food? Does the 

 soil contain much unavailable food of the elements that plants 

 need? What makes plant-food available? What is the influence 

 of tillage in this respect? Do roots themselves make plant-foods 

 available (131o, review also paragraphs 30 and 30a)? What are 

 the disadvantages in the use of barn manures? 



What is the office of nitrogen? How does it affect the plant? 

 How may the lack of nitrogen be discovered? Explain what 



