88 THE PRINCIPLES OF AGRICULTURE. 



and chlorine. As plants contain both these elements, in 

 small quantities, salt is of some value as a fertilizer in 

 soils where they are lacking. It is also of some benefit 

 in preparing plant food by exerting chemical action upon 

 other substances. It must be used with caution, as in 

 too large quantities it is fatal to vegetation. 



An intelligent and economical use of fertilizers re- 

 quires a knowledge of four points : 

 ,., 1. What the Fertilizer Contains. As the composition 

 of substances used as fertilizers is determined by chemi- 

 cal analysis, this is beyond the power of the ordinary 

 farmer. The government generally affords protection 

 against the sale of worthless compounds by requiring 

 that the composition of the substance exposed for sale 

 shall be printed upon the package, and by providing for 

 chemical tests of the goods as they are found in the 

 market. 



2. What the Soil Requires. The composition and con- 

 dition of soil in different localities, and at different times, 

 are so varied, that no general rule for the use of fertil- 

 izers can be safely followed. 



One field may require nitrogen, while another may be 

 well supplied with this, but may lack phosphoric acid, 

 or potash, or both. The use of a fertilizer in each field, 

 containing an average quantity of each element, might 

 be a wasteful practice. 



The only method available to farmers for determining 

 what their soil needs is to conduct " field experiments," 

 applying different fertilizers to different sections, and 

 noting the results. For such a purpose, the three ele- 

 ments may be obtained singly in " chemicals." 



Nitrogen, for instance, may be obtained in sulphate of 

 ammonia or nitrate of soda ; phosphoric acid, in ground 



