20 BULLETIN 521, U. S. DEPARTMENT OF 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. 
3. 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 of the sample. The following questions 
are suggestive: What are indications of an acid soil? What is the 
