86 PRODUCTIVE FARMING 



16. By what plan is manure used to the best advantage? 



17. Describe the making of a compost. Of what use is it? 



18. What three important plant-foods are found in commercial 

 fertilizers? Which is the most important? 



19. Name five or more substances from which nitrogen is derived. 



20. Give three sources of phosphoric acid. 



21. What are superphosphates? 



22. Give the sources of potash in commercial fertilizers. 



References. — United States Farmers' Bulletins: 44, Commercial 

 Fertilizers; 48, The Manuring of Cotton; 77, Liming of Soils; 192, 

 Barnyard Manure; 245, Renovation of Worn-out Soils; 278, Leguminous 

 Crops for Green Manuring; 286, Comparative Value of Whole Cotton 

 Seed and Cotton-seed Meal in Fertilizing Cotton; 326, Building up a 

 Run-down Cotton Plantation; 406, Soil Conservation. 



CHAPTER IX. 

 SYSTEMS OF CROPPING. 



By rotation of crops is meant the succession of different 

 crops, in contrast with the bad practice of having the same 

 kind of crop grow year after year on the same field without 

 any cover crop intervening. 



Special and General Farming. — In many of the Eastern 

 States special farming is more profitable than general farm- 

 ing. The raising of dairy produce, poultry products, hay, 

 and seed-corn are some of the distinct lines of special farm- 

 ing. In the South, cotton, cane, corn, and tobacco are 

 among the special crops. In the Northwest and Middle 

 West grain is the only crop on many farms. 



General farming is practised in nearly all parts of the 

 country. In such cases it is found much easier to carry out 

 good systems of rotation. 



Why We Rotate Crops. — There are a number of advan- 

 tages gained when crops are raised in rotation. 



1. Some crops have deep roots; others shallow roots. 

 The growth of a variety of crops on a certain field will pre- 

 vent it from being exhausted so soon. 



