20 BULLETIN 521, U. S. DEPARTMENT OP AGRICULTURE. 



Lesson 29. — Green Manures and Cover Crops. 



1. Value as source of humus. 



2. Value as protection from erosion. 



3. Legumes as a source of nitrogen. 



Lesson 30. — Green Manures and Cover Crops — Continued. 



1. Legumes suitable for green manuring. 



2. Cereals suitable for green manuring. 



8. Management of green manures and cover crops. 



Special reference : Leguminous crops for green manuring, Farmers' 

 Bulletin 278. 



Lesson 31. — Renovation of Worn-out Soils. 



1. Why farms are abandoned. 



2. Effects of one-crop system. 



3. Application of previous lessons. 



Special reference : Renovation of worn-out soil, Farmers' Bulletin 245. 



Lesson 32. — Acid Soils. 



1. Causes of acidity. 



2. Testing for acidity. 



3. Remedies. 



4. Crop adaptation to acid soils. 



Illustrative material : Chemicals to demonstrate acids, bases, and salts. 



Lesson 33. — Lime and Other Amendments. 



1. Benefits of liming. 



2. Soils that need liming. 



3. Forms of lime and their use. 



4. Gypsum and other amendments. 



Illustrative material : Samples of different forms of lime. 

 Special reference : The Liming of Soils, Farmers' Bulletin 77. 



Exercise 19. — Testing Soil for Acidity. 



Purpose : To give students practice in the- use of the litmus test. 



Directions : Samples of soil should be taken from a field known to 

 be acid. In applying the litmus test care should be taken to avoid 

 handling the litmus paper with sweaty hands. Clean dishes should 

 be used in mixing the samples of soil into a paste. Use distilled 

 water if obtainable. After the soil has been moistened and the sur- 

 face made smooth, pieces of blue litmus paper should be pressed 

 against the smooth surface with a clean knife. The degree of acidity 

 may be determined to some extent by the time required for the paper 

 to turn red and the degree of coloring. After soil known to be acid 

 is tested, soils of origin unknown to the students should be tested, 

 each student having an opportunity to apply the test. In regions 

 where alkali soils abound, red litmus paper should be used to test 

 such soils. 



Record and report: Each student should make a written report of 

 the test, including the taking ©f the sample. The following questions 

 are suggestive: What are indications of an acid soil? What is the 



