138 ON THE PRINCIPLES OF 



The same thing- happens, thoug-h vice versa, if we wish to 

 produce a rich crop of grain by means of animal excrements; 

 in this case, one part of giiano and five parts of animal ex- 

 crements produce the same effect as 13-15 parts of animal 

 excrements. 



To understand the proper meaning* of these numerical 

 proportions, it is sufficient to mention that 400 pounds of 

 bones contain as much phosphoric acid as 1,000 pounds of 

 wheat; these 400 pounds of bones can furnish sufficient 

 phosphoi'ic acid to 8 acres. 



If we take the importation of bones into Great Britain in 

 the last ten years to amount to one million of tons, enough 

 phosphoric acid has been supplied to the fields for 25 mil- 

 lions of tons of wheat ; but only a small proportion of the 

 phosphoric acid of the bones is in a state to be assimilated 

 by the plants, and applicable to the formation of the g-rain. 

 The plants, in order to apply the other far g-reater part of 

 that phosphoric acid to their formation, must find a certain 

 quantity of alkaline bases besides the bone earth, which are 

 not given to the plants in the bones, because they contain 

 neither potash nor soda. 



To have increased the fertility of the fields in the right 

 proportion, 800,000 tons of potash ought to have been 

 added to the one million of tons of bones, in a, suitable form. 



The same is the case with guano; 60 to 100 pounds of it 

 are sufficient to furnish phosi)horic acid to one acre of tur- 

 nips; but the four to eight fold quantity is required to 

 furnish the turnips with the necessary alkaline bases, and it 

 is still doubtful whether they can be at all provided with, 

 the latter by means of the salts with alkaline bases which 

 the guano contains. 



At a time when the necessity of the mineral substances 

 for the growth and develo]nnent of the plants, and the direct 

 relation which the effect of manure has to its amount of the 

 same substances, had not been ascertained, a prominent value 

 was ascribed to the organic matters which it contains. For 

 a long time it was thought that the produce of a field of 

 those substances, containing nitrogen, which serve as food 

 for man and animals, stood in a direct proportion to the 

 nitrogen contained in manure. It was believed that its 

 commercial value or its value as manure might be expressed 

 in per cents, by its proportion of nitrogen, but later and 

 more convincing observations have induced me to contradict 

 this o})inion. 



