The Composition and Use of Fertilizers. 223 



the crop in the soil. Gas-lime may contain a small amount of 

 nitrogen. 



Common Salt has an indirect fertilizing value which is due to 

 the fact that it has the power of changing unavailable forms of 

 plant food into available forms, and increasing the power of soils 

 to absorb water. Salt probably furnishes no needed form of plant 

 food. All the salt that can probably be utilized to advantage is 

 contained as impurities in some of the potash salts, such as kainit. 



Danger in using Stimulant Fertilizers. — It should be kept in 

 mind that these stimulant fertilizers are not used for the plant 

 food contained in them; hence, as used, they do not furnish needed 

 plant food. The chief value of their use lies in the fact that 

 they can change unavailable into available forms of plant food. 

 It can readily be seen that, when stimulant fertilizers are used 

 exclusively for a term of years, the soil each year loses nitrogen, 

 potassium and phosphorus compounds, which are not replaced. 

 The inevitable result of such treatment is the exhaustion of these 

 important food constituents from the soil. This affords an ex- 

 planation of the question often raised now as to why the applica- 

 tion of land-plaster does not give such results in crop yields at 

 present as in former days. When land-plaster was the only fer- 

 tilizing material added to soils for years in succession, it was 

 possible to produce increased crops, so long as there were in the 

 soil enough compounds of nitrogen, potassium and phosphorus to 

 be rendered available by the action of the land-plaster. When, 

 therefore, these forms of plant food were largely removed, there 

 was nothing for the land-plaster to act upon, in order to increase 

 the supply of available food material. The land plaster furnished 

 no needed food, but simply helped the crops to use up more 

 rapidly the store of plant food present in the soil. 



4. Nutritive or Direct Fertilizing Materials. 



Nutritive or Direct Fertilizers contain forms of plant food, 

 which contribute directly to the growth and substance of plants. 

 Such materials may contain either nitrogen or potash or phos- 

 phoric acid compounds, or any two, or all three of these forms of 

 nutriment. We shall consider these various materials under the 

 following heads: 



(a) Commercial fertilizing materials containing nitrogen com- 

 pounds. 



