THE SOIL. 9 



whether they are acid, neutral, or alkaline in character. 

 It is found that blue litmus paper, used for testing sub- 

 stances to determine whether they are acid or not, 

 when applied to many soils will turn pink or red, indi- 

 cating acidity. 



Acidity and Liming. This acidity is not in itself 

 injurious to most plants, but it is unfavorable to the 

 maintenance of fertility. This is especially true with 

 reference to the availability of phosphorus and accu- 

 mulation of nitrogen by legumes. It is, therefore, 

 usually desirable to correct this acidity by adding either 

 fresh slaked lime or simple ground limestone or marl. 

 The use of fresh lime or of simply water-slaked lime 

 is somewhat dangerous, since it is apt to hasten the 

 burning out of the organic matter or humus. It is 

 usually better, therefore, to use old thoroughly slaked 

 lime or finely ground limestone or marl. 



Maintaining Fertility with Legumes. The accu- 

 mulation of nitrogen by legumes, which has been re- 

 ferred to above, is a matter of the utmost importance 



A FARM WHERE THE MANURE IS NOT PRESERVED AND THE SOIL IS 

 ALLOWED TO RUN DOWN. 



in maintaining the fertility of the soil. Nitrogen con- 

 stitutes four-fifths of the atmosphere and, of course, 

 exists in large amounts in the air all through the soil. 



