EXERCISE 25 



THE ACIDITY OF SOILS 



Statement. In humid regions there is likely to be a loss of basic, or alkaline, materials in soils that 

 have been long under cultivation, and an accumulation of acids due to the decay of humus and the 

 excretion of acids by the roots of growing plants. As a consequence, such soils may become sour, or 

 acid. Many plants are directly affected in their ability to grow by the acid or the alkaline content of 

 the soil. Among the important plants which are more or less sensitive to acid, and consequently thrive 

 best on alkaline or neutral soils, are alfalfa, red clover, Kentucky bluegrass, tobacco, barley, wheat, and 



oats. Some of the plants which pre- 

 fer a slightly acid soil are rye, redtop, 

 millet, and carrots. 



Object. To test the acidity of 

 soils of the neighborhood and to 

 study the methods of correcting this 

 condition. 



Materials. Samples of surface 

 soil chosen from different soil types 

 of the neighborhood and of subsoil ; 

 two tumblers ; an evaporating dish ; 

 red and blue litmus papers ; blotting 

 paper; distilled water or clear rain 

 water; ammonia; hydrochloric acid. 



Directions. Collect samples of 

 surface soil and subsoil. Secure the 

 surface samples at a depth of seven 

 inches, and the subsoil samples at 

 a depth of about fourteen inches, 

 i. Cut a piece of filter paper to 

 fit the bottom of the glass tumbler. 



Place in the bottom of the tumbler a piece of red and a piece of blue litmus paper, and over them 



place the disk of filter paper. Place soil to be tested in the tumbler and cover with distilled water. 



Observe any changes that may take place in the color of the litmus papers and record results at the 



end of thirty minutes. 



2. Fill two tumblers three-quarters full of distilled or rain water. To the first add a few drops of 

 ammonia. Then into each tumbler stir for three minutes a tablespoonful of the soil to be tested, being 

 careful to use two spoons and to keep each in its respective glass. At the end of two hours examine 

 the contents of each glass. If the soils need lime, the water standing above the soil in the glass in 

 which the ammonia has been added will have a dark, reddish-brown, or black appearance, while the 

 water in the other glass will be very nearly clear. On the other hand, if the soil is well stocked with 

 carbonates of lime or magnesia, the soil water in both glasses will be entirely clear. 



3. Make a mud ball, mold it cup-shaped, and pour several drops of hydrochloric acid on it. 

 If bubbles escape, it is a sign that there is an abundance of carbonates. If there is no effervescence, 

 the soil lacks carbonates and is probably acid. 



Questions. What are some of the conditions which cause a soil to become sour? What are the best 

 ways to correct acidity in a soil? What effect does acidity have upon plant growth? The presence 

 of what plants indicates an acid soil ? What an alkaline soil ? What does ground limestone cost a ton in 



[SO] 



Fig. 30. Clover on an alkaline and an acid soil. 



The growth of clover on an alkaline soil is represented by the pile of hay on the left. That 



produced on an equal area of acid soil is contained in the straw hat on the right. (Rhode 



Island Experiment Station.) 



