SOIL FERTILITY AND PLANT GROWTH 247 



produced; hence the soils rich in humus are likely to be 

 acid. Plant roots give off acid in the process of their 

 growth, and this acid remains in the soil. The action of 

 the nitrifying bacteria also adds to the acidity of the soil. 

 Heavy, non-porous clay soils which do not allow the en- 

 trance of fresh air are usually sour. 



The degree of acidity of soils can be judged (1) by the 

 refusal of certain plants to grow in them ; for example, the 

 legumes will not thrive in acid soils, and the failure of clover 

 or alfalfa to do well should arouse a suspicion of too much 

 acid. (2) The presence of such weeds as sheep-sorrel, 

 horsetail rush, corn spurry, and wood horsetail indicate 

 acid. (3) Blue litmus paper turns red when placed in con- 

 tact with a soil containing acid. 



Liming acid soils. Lime is a certain remedy for acid 

 soils. In some regions, especially where limestone abounds, 

 the natural supply of lime in the soil is sufficient to overcome 

 the surplus acid. In other regions, lime needs to be applied 

 in commercial form. This is the only cure for acid soil 

 within reach of the farmer. 



Nearly all prairie soil is in some degree acid, especially 

 on slopes where leaching of the soil has carried away the 

 original deposits of lime, and wherever large supplies of 

 humus have formed from the decay of organic matter. 

 Thousands of acres of acid land would well repay the cost 

 of liming by increased yields. Many farmers now look on 

 liming as a regular and necessary requirement. Of course 

 lime should not be applied unless needed, but the tests are 

 so simple that this is easily determined. 



Forms of lime used. Lime is available for applica- 

 tion to the soil in several forms: (1) Quicklime, or lime 

 ready for use in making plaster, when finely ground may be 

 applied at the rate of about one ton to the acre. (2) Air- 

 slacked lime, or ordinary lime that has been exposed to the 



