EXPERIMENTS IN SOIL CHEMISTRY 273 



also is lacking. If fields are cultivated year after year the 

 plants are constantly removing these substances from the 

 soil. If nothing is done to return these substances of 

 course the soil becomes so deficient in these plant materials 

 that plants cannot thrive in it. It is very important that 

 farmers see to it that the lack is supplied in some way. 



There are several ways in which the soil may be kept 

 with a sufficient amount of plant materials. Among these 

 are the application of barn manures, the plowing under of 

 green crops, and the use of commercial fertilizers. 



Among the commercial fertilizers, the following are in 

 common use: 



For nitrogen, dried blood is used. This contains twelve 

 to fourteen per cent of nitrogen. Sodium nitrate is also 

 used, and this contains about fifteen per cent nitrogen. 



For phosphorus, steamed bone meal is used, which con- 

 tains about twelve or fourteen per cent of phosphorus. 

 Rock phosphate is also used. 



For potassium, potassium chloride is used, which has 

 forty to forty-two per cent of potassium. Kainit, which 

 contains twelve per cent of potassium, is also used. 

 Potassium carbonate in the form of wood ashes is also 

 used. 



Lime is used as a fertilizer either in the form of gypsum, 

 quicklime, or slaked lime. While most soils have enough 

 lime to supply the needs of the plants, lime is, nevertheless, 

 of considerable value to any soil that has become acid, as we 

 have already seen by experiment. It also helps to unlock 

 the unavailable potash, phosphorus, and nitrogen in some 

 soils, and changes them to a form available for plant use. 

 Why do soils become sour ? 



