226 NITROGEN, PHOSPHORUS AND POTASSIUM 



16. In good soil management, is it necessary to sacrifice a crop (for green- 



manuring) to make soil improvement possible? Explain. 



17. What are commercial fertilizers? How are the fertilizing elements hi 



fertilizers commonly expressed? 



18. Name the four classes of commercial fertilizers. 



19. Name the common nitrogen fertilizers, and mention some facts about each. 



20. Are farmers to rely on nitrogen fertilizers as the source of nitrogen? Discuss 



this point. 



21. Name the common phosphate fertilizers. Tell of their phosphorus content, 



and availability. 



22. In what three ways may the fertilizing constituent of phosphate fertilizers 



be expressed? 



23. How is rock phosphate made? Discuss its use as a fertilizer. 



24. What is basic slag? TeU of its use. 



25. Compare ground steamed bone meal and acid phosphate as fertilizers 



26. Distinguish between phosphorus, acid phosphate and phosphoric acid. 



27. Discuss in general the necessity of fertilizing with phosphates. 



28. In what special ways do phosphates affect crops? 



29. A soil of low fertility was found to have lost one-half of its original phos- 



phorus supply through exhaustive cropping. Is it absolutely necessary 

 to add phosphates to raise the phosphorus content to the original amount 

 before the soil can produce maximum yields? Explain. What kind of 

 fertilizer should be used during the first few years of improvement? 

 Later on what substitution may be made? 



30. WTiat phosphate is best to use for top-dressing? When does bone meal 



give best results? Basic slag? Rock phosphate? 



31. Under what conditions do phosphates give best results on some soils? 



32. Which seems the more profitable, rock phosphate or acid phosphate? 



33. Name the common potash fertilizers, and give some facts about each 



34. Distinguish between potassium and "potash." 



35. Are potash fertilizers very generally used? What soils are particularly 



in need of potassium? Name some crops requiring large amounts of 

 this element. 



36. Discuss the use and value of wood ashes, muriate and sulfate of potash, 



and kainit. 



37. What are mixed and complete fertilizers? In practice, how are these 



fertilizers designated? 



38. What can be said of the use of mixed fertilizers? 



39. Discuss the relation of commercial fertilizers to the maintenance of 



soil fertility. 



40. What really determines whether or not a farmer should use, or continue 



to use, commercial fertilizers? 



41. What are the factors determining the kind of fertilizer to use? Discuss 



and illustrate. 



42. Compare high and low grade fertilizers as to meaning and value. 



43. What is to be said concerning home mixing of fertilizers? What materials 



should never be mixed? 



44. Discuss the ways in which fertilizers may be applied. 



45. Are there ever any beneficial after-effects produced by commercial fertil- 



izers? What makes this after-effect possible? 



