20 ANALYSIS OF SOILS. 



mon sand, but it was an expensive article ! Tem- 

 porary food for a crop may be found in animal, 

 vegetable, or earthy manures, but these are ex- 

 haustible ; and when aliment ceases, the crop pro- 

 portionabfy diminishes. In one respect, chemical 

 investigation may importantly aid the agriculturist, 

 by pointing out the proportion of magnesian earth 

 in certain limes used for manure, and thus indicate 

 its beneficial or injurious effects on vegetation. I 

 should not like lime containing 20 per cent, of this 

 earth ; but when it contains a much smaller pro- 

 portion, I should not think it very deleterious. 

 This earth acts as a caustic to vegetation, and, 

 neither being soluble in water, nor possessing the 

 other virtues of lime, diminishes the number of 

 bushels used according to its existence, and thus 

 deprives the crop of that portion of benefit : but 

 after all, as Kirwan says, the secret processes of 

 vegetation take place in the dark, exposed to the 

 various and indeterminable influences of the at- 

 mosphere ; and hence the difficulty of determining 

 on what peculiar circumstance success or failure 

 depends, for the diversified experience of years 

 alone can afford a rational foundation for solid and 

 specific conclusions. 



The real goodness of a soil consists principally, 

 perhaps, in the power it possesses of maintaining 

 a certain degree of moisture ; for without this, the 

 plant could have no power of deriving nutriment 



