82 PRINCIPLES OF GARDENING. [CH. II. 



Although plants ^^■ill uot grow u2:)on soils com- 

 posed of the earths only, yet these have a great in- 

 fluence over plants, not merely by their secondare 

 powers of regulating the amount of moisture, heat, &c.. 

 but by entering directly into the constitution of the 

 plant ; for it is a result of experience, to which I 

 know of no exception, that a plant contains more of 

 any given earth if gro^Mi in a soil where it predomi- 

 nates, than if grown in a soil where it is in less pro- 

 fusion. This fact was first pointed out by Saussure, 

 who found that the Rhododendron ferrugineum, when 

 growing on the calcareous formation of Mount Jura, 

 contained in its ashes 43.25 per cent, of carbonate of 

 lime, but only 0.75 of silica. On the other hand, 

 the ashes of the same plant, from the granitic dis- 

 trict of Mount Brevere, contained 2.0 per cent, of 

 silica, but only 16.75 of carbonate of lime. 



However vai'}ang in the proportions, yet every soil 

 is composed of silica, alumina, lime, magnesia, oxide 

 of iron, salts, and animal and vegetable remains. 

 The most important consideration is, what propor- 

 tions those are which constitute a fertile soil. 



The heau ideal of a fertile soil is one which con- 

 tains such a proportion of decomposing matter and of 

 moisture, as to keep the crop gro\^ing upon it always 

 supplied wdth food in a state fit for introsusception, 

 yet not so superabundantly as to render the plants 

 too luxuiiant, if the object in view is the production 



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