ACID SOILS AND ALKALI SOILS 117 



Cauliflower Cabbage. 



Some of these plants will grow well on soil that is too sour 

 for other crops. For example, red clover will grow fairly 

 well on soil that is too acid to raise alfalfa. 



144. Litmus paper test for soil acidity. — This test is 

 made with blue litmus paper, which is brought in imme- 

 diate contact with wet soil. A rapid and decided change 

 to red is taken to indicate an acid condition of the soil. 

 Carbonic acid, which is always present in soils, but 

 which is not injurious to plant growth, is supposed to give 

 only a faint pink color to the litmus paper. Various ways 

 of bringing the paper into contact with the soil have 

 been proposed, among others the placing of filter paper or 

 blotting paper between the soil and the litmus paper. 

 It has also been pointed out that the acid perspiration of 

 the fingers may lead to a mistaken conclusion that the soil 

 is acid. 



Much litmus paper is sold that is of very poor quality, 

 and an effort should be made to obtain a good article. When 

 good paper is used and the test is carefully made, the general 

 experience has been that it is a fairly good, although not an 

 infallible, guide to the need of a soil for lime. 



145. Litmus paper and potassium nitrate. — This is per- 

 formed in the same manner as the former litmus paper test, 

 except for the substitution of a saturated solution of potas- 

 sium nitrate instead of water for moistening the soil. It is 

 a more delicate test than the one with litmus paper alone. 

 The operation consists in working a small soil sample to a 

 thick paste with a saturated solution of potassium nitrate 



