MANURES. 131 



These manures have four modes of action when 

 applied to the soil. 



1st. They furnish food for the mineral part of 

 plants. 



iM. They prepare matters already in the soil for 

 assimilation by routs. 



3d. They improve the mechanical condition of the 

 soil. 



4th. They absorb ammonia. 



Some of the mineral manures produce in the soil 

 only one of these effects, and others are efficient in 

 two or more of them. 



The principles to be considered in the use of 

 mineral manures are essentially given in the first 

 two sections of this book. It may be well, however, 

 to repeat them briefly in this connection, and to give 

 the reasons why any of these manures are needed, 

 from which we may learn what rules are to be ob- 

 served in their application. 



1st. Those which are used as food by plants. It 

 will be recollected that the ash left after burning 

 plants, and which formed a part of their structures, 

 has a certain chemical composition ; that is, it con- 

 sists of alkalies, acids, and neutrals. It was also 

 stated that the ashes of plants of the same kind are 

 always of about the same composition, while the 

 ashes of different kinds of plants may vary mate- 

 rially. Different parts of the same plant too, as we 

 learned, are supplied with different kinds of ash. 



For instance, clover, on being burned, leaves an 

 ash containing lime, as one of its principal ingre- 



