122 SOILS AND FERTILIZERS 



5. If green-manures are found to produce acidity on a particular 

 soil, what precaution should be taken in using them ? 



6. Name three or four weeds whose presence in large numbers 

 indicates that a soil is acid. 



7. Name six or eight crops that are adapted to growth on sour 

 soils, and an equal number that are injured by a sour soil. 



8. Describe the litmus paper test for the detection of a sour soil. 



9. Describe the test with litmus paper and potassium nitrate 

 solution. 



10. State what is meant by an alkali soil. 



11. Explain the difference between white and black alkali, and 

 the effect of each on crops. 



12. Name some of the crops most tolerant of alkali. 



13. Describe four ways by which alkali may be removed from soil. 



LABORATORY EXERCISES 



Exercise I. — Acid soils in the field. 



Plan a field trip to a soil known to be distinctly acid. Observe 

 structure of soil, organic content, character of crop and, particularly, 

 character of other vegetation. It might be well to make a collec- 

 tion of the plants which are supposed to indicate acidity. Take 

 samples of this soil for future tests for acidity in the laboratory. 



Exercise II. — Litmus paper with and without potassium ni- 

 trate. 



Materials. — Litmus paper, acid soil, evaporating dish, a neutral 

 potassium nitrate solution. 



To prepare litmus paper boil litmus powder (1 part) with alcohol 

 (2 parts) for five minutes. Allow to settle ana* pour off the alcohol, 

 thus carrying away certain dyes of low sensitiveness. To the 

 powder now add five parts of water. Boil 10 minutes and allow 

 to stand overnight. Decant liquid and filter it. This gets rid of 

 most of the carbonates. Now make acid with sulfuric acid and bring 

 back to required tint with barium hydrate. Dip narrow strips of 

 filter paper into the solution and dry on glass. When dry cut into 

 strips of the required size. 



Procedure. — Mix one portion of & distinctly acid soil to a thick 

 paste in an evaporating dish with distilled or rain water. Allow 

 to stand for a few minutes, then pat to a smooth surface and apply 

 to it one end of a strip of litmus paper, leaving the other end free for 

 comparison. Press paper closely in contact with soil. 



