ACID SOILS AND ALKALI SOILS 113 



by saturation or compaction of the soil, or a lack of lime, may 

 lead to the formation of these acids. Acid soils to which 

 certain organic acids have been added were found to be 

 unfavorable to the growth of plants like wheat, while the 

 same soil, to which lime had been applied, produced a much 

 better growth. Lime overcomes the injurious effect of this 

 kind of acidity. 



135. Negative acidity. — When a soil contains no free 

 acids but is sour in its relations to plant growth, it may be 

 said to possess negative acidity. Negative acidity is coun- 

 teracted by the application of lime just as is positive acidity. 

 The condition that renders the soil acid is a lack of sub- 

 stances like lime, magnesia, soda and potash. Any one of 

 these four substances is called a base. Lime, being the cheap- 

 est of these to apply, is the usual corrective. The injurious 

 action of soil acidity on plant growth has been attributed 

 to one or more of the following causes : (1) lack of lime to 

 overcome organic acids when they are formed ; (2) absence 

 of sufficient carbonate of lime ; (3) great absorbent properties 

 that cause the soil to compete with plants in their attempt 

 to draw plant-food materials from the soil. 



136. Ways by which soils become sour. — In regions of 

 ample rainfall there is always a tendency for soils to become 

 sour, and unless they originally contain large quantities 

 of lime, or are of recent formation, they are likely to be in 

 need of lime. This tendency may be due to any one or more 

 of the following causes: (1) removal of lime and similar 

 substances in drainage water; (2) removal of these sub- 

 stances by plants; (3) accumulation of acids contained in 

 fertilizers applied to the soil ; (4) formation of organic acids 

 from plant remains. 



137. Drainage as a cause of acidity. — The chief cause of 

 soil acidity is doubtless the removal of lime, magnesia, soda 

 and potash from soil by the water that percolates through 



