242 HANDY-BOOK OF HUSBANDRY. 



In nine cases out of ten, that which is most needed, and whose 

 return produces the best result, is undoubtedly phosphoric acid. 

 Such lands, however, as have been long devoted to the cultivation 

 of tobacco, potatoes, etc, most need additions of potash ; and in 

 almost all cases it will be found advantageous to apply both 

 potash and phosphoric acid. It should be borne in mind that we 

 are now speaking only of the requirement of manure for the 

 actual feeding of plants. The solvent action of certain sub- 

 stances makes it frequently profitable to apply fertilizers whose 

 constituents belong to the list given above of matters which the 

 soil always supplies in sufficient quantity. This subject will be 

 discussed hereafter. 



Phosphoric acid being, then, the most important mineral element 

 of foreign as well as of home-made manures, it will be well for 

 us to examine the sources from which it may be most cheaply 

 and most advantageously procured, and the best method for its 

 application to the soil. 



The bones of animals consist, vC^hen thoroughly dried, of about 

 two-thirds earthy matter and one-third organic or combustible 

 matter. The earthy part is almost entirely phosphate of lime, 

 which is also called bone earth, and this consists of about forty- 

 six per cent, of phosphoric acid and about fifty-four per cent. 

 of lime. Bones, therefore, are the most common and most pro- 

 lific source of the phosphoric acid used in agriculture ; although 

 it is also a very important element of Peruvian guano, and still 

 more largely of what are called phosphatic guanos, and of the 

 phosphatic deposits recently discovered near Charleston, South 

 Carolina. 



The manner in which phosphate of lime is used as a manure 

 affects in very great degree its efficiency, and consequently the 

 economy of the application. To state the case in a single sen- 

 tence, the finer the particles of the manure the more active and 

 the more valuable it will be. In order to attain the greatest degree 

 of fineness, it is found best to manufacture it into what is called 

 superphosphate of lime ; — that is, a compound containing more 

 phosphoric acid and less lime than the simple phosphate does. 



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