New York Agricultural Experiment Station. 61 



question often raised now as to why the application of land-plaster 

 does not give such results in crop yields at present as in former 

 days. When land-plaster was the only fertilizing 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 nitro- 

 gen, 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. 



" 3. 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 phosphoric 

 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. 



(b) Commercial fertilizing materials containing phosphoric acid 

 compounds. 



(c) Commercial fertilizing materials containing potash compounds. 



(d) Farm-produced fertilizing materials. 



This division is not sharply defined, since one material may con- 

 tain more than one form of nutriment ; however, in most cases, 

 each kind of material contains some one of the three forms of plant- 

 food in much larger proportions than any other form. Owing to 

 the value of farm-produced manures and to the importance of calling 

 special attention to their value, we treat this subject under a separate 

 head in connection with the forms of materials used in commercial 

 fertilizers. 



(«) Fertilizing Materials Containing Nitrogen Compc^rnds. 



The vaiious materials which are used to furnish nitrogen in com- 

 mercial fertilizers are derived from three general sources, which we 

 can indicate as follows: 



(1) Mineral nitrogen compounds. 



