New York Agricultural Experiment Station. 49 



natural fertilizers, such as cotton-seed meal and tobacco stems, are 

 also ineJudcd among the materials of artificial fertilizers. 



Complete Fertilizers, known also as general fertilizers, are those 

 which contain nitrogen, phosphoric acid and potash. 



Incomplete Fertilizers, also called special fertilizers, are those 

 which contain oiilv one or two of the three constituents, nitrogen, 

 phosphoric acid and potash. 



There is a common practice among farmers and dealers of call- 

 ing all commercial fertilizers " phosphates," regardless of whether 

 thej contain any phosphates at all or not. The practice is clearly 

 objectionable, because a phosphate is not the only fertilizing con- 

 stitnent presented in commercial fertilizers, — in some cases it may 

 be entirel}'- absent. The term "superphosphates" applies truth- 

 fully to many commercial fertilizers, but even these can not be 

 correctly spoken of as simply •' phosphates." This common usage 

 of the term " phosphate " for any form of fertilizer emphasizes 

 the fact that there has been a tendency to overestimate the value 

 and importance of this constituent, resulting in large applications of 

 it without regard to the needs of soil or crop. 



3. The Relations of the Different Elements of Plants to 



Fertilizers. 



Carbon. — We know that carbon must be an important element in 

 plant-food, since it forms nearly one-half of the solid portions of 

 plants. Notwithstanding the fact that carbon forms so large a por- 

 tion of plants, it has no importance as an active food constituent of 

 direct fertilizers. This statement may appear strange and the ques- 

 tion may be asked, " Why is not carbon to be regarded as an essen- 

 tial constituent of direct fertilizers ?" The answer is that tlie car- 

 bon of plants comes from the carbon dioxide (carbonic acid gas) of 

 the air, and the air furnishes an inexhaustible and available supply 

 of this substance. We do not, therefore, need to add carbon to the 

 soil in order to supply the needs of plants. However, some forms 

 of carbon possess value as indirect fertilizers. When vegetable or 

 animal matter undergoes decomposition in the soil more or less car- 

 bon dioxide is formed. This is taken up by the soil-water and acts 

 as a solvent, changing unavail-able into available forms of plant-food. 



Hydrogen and Oxygen. — As already stated, water is formed by 

 the union of two gases, hydrogen and oxygen. These elements are 

 4 



