VEGETABLE MANURES. 159 



require an addition of 48 tons of quicklime. To undrained 

 soils it must be applied in greater quantities than when the 

 excess of water has been removed, as the moisture present 

 not merely produces a greater proportion of acid compounds, 

 which it is necessary to neutralise, but prevents its full 

 operation. In determining the quantity of lime required by 

 any particular soil, it is therefore evident we must be guided 

 both by a consideration of its physical qualities, and its chemi- 

 cal composition as determined by analysis.* 



249. The state in which lime is applied must materially in- 

 fluence its effects upon the soil. It is, as you are aware, used 

 by farmers : — 



C Shell lime applied directly from 

 1 . In the caustic state. < the kiln. 



( Slaked by the addition of water. 



CAs slaked by absorption of 



2. In the partially mild ) moisture from the air, and in 

 state. ^ part combined with carbonic 



(^ acid. 



{As chalk, marl, shell-sand, coral- 

 sand, limestone-gravel, and 

 as burned lime rendered mUd 

 by long exposure to the air. 

 250. For full directions as to the best methods of applying 

 lime in the various states in which it is used, I must refer you 

 to the works of writers on the practice of agriculture. The 

 following general rules will, however, serve to direct you in 

 regulating its application : — 



I. That in reclaiming peaty soils, or those composed of 

 intractable clay, or which contain substances in- 

 jurious to plants (246), it should be applied as 

 hot as possible from the kiln. 



• It is only lately that the attention of chemists has been directed to 

 the changes which limestone may undergo by burning; thus, it has been 

 found by analysis (Johnston), that when burned with coal several new 

 compounds are produced, the sulphur which the coal contains uniting 

 with the lime so as to form gypsum, while the silica contained in the 

 earthy matter of the limestone and in the coal, upon the expulsion of 

 the carbonic acid, enters into combination with some of the caustic lime, 

 producing a silicate of lime, thus converting the silica into a state in 

 which it may, when pkced on the soil, contribute to the support of 

 plants. 



