SOIL FERTILITY 325 



hundred pounds to the acre, applied at intervals of from 

 one to three years, will, however, usually prove sufficient to 

 keep up the supply. 



The application of commercial fertilizers. Because 

 commercial fertilizers are so expensive, and also for the 

 reason that the application of a fertilizer when it is not 

 needed may do positive harm, the advice of your county 

 agricultural agent or some other agricultural leader of the 

 state should be sought before purchasing. An agriculturist, 

 after a survey of your soil, may be able not only to suggest 

 the best kind for local needs, but also the amount that should 

 be used. 



5. The Use of Lime on Soils 



Lime can hardly be called a fertilizer, since it does not 

 contain any of the elements in which the soil is generally 

 lacking. Yet because of its action on certain other elements 

 of the soil it is necessary to plant production. When lime 

 does not already exist in the soil it must be added in com- 

 mercial form if the fertility of the soil is to be maintained. 

 The purpose of lime is to cure the soil of its acid condition. 



Acid soils. All soils have a tendency to become sour, 

 or acid. This acidity comes about in several different ways : 

 in the decay of organic matter in the soil, certain acids are 

 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 



