THE CHEMISTRY OF GARDEN CROPS 193 



theless his aim will be, and must be, of a somewhat universal kind, 

 and a clear idea of the relations of plants to the soil in which they 

 grow will be of constant and incalculable value to him. 



We are bound to say at the outset that a complete essay on the 

 chemistry of vegetation is not our purpose. We are anxious to 

 convey some useful information, and to kindle sufficient interest to 

 lead those who have hitherto given but slight attention to this subject 

 to inquire further, with a view to get far beyond the point at which we 

 shall have to quit the subject. 



Plants consist of two classes of constituents the Inorganic, which 

 may be called the foundation ; and the Organic, which may be con- 

 sidered the superstructure. The first alone concerns us now. A 

 plant must derive from the soil certain proportions of silica, lime, 

 sulphur, salt, phosphates, alkalies, and other minerals, or it cannot 

 exist at all ; but given these, and its manufacture of fibre, starch, gum, 

 sugar, and other organic products will depend very much upon the 

 action of light, heat, atmospheric air, and moisture upon it, for 

 these have to be produced by chemical (or vital) action within the 

 structure, or, as we sometimes say, the tissues of the plant itself. To 

 a very great extent the agencies that conduce to the elaboration of 

 organic products are beyond our control (though not entirely so), 

 whereas we can directly, and to a considerable degree, provide 

 the plant with the minerals it more particularly requires ; first, by 

 choosing the ground for it, and next by tilling and manuring in a suit- 

 able manner. A clay soil, in which, in addition to the predominating 

 alumina, there is a fair proportion of lime and silica, may be regarded 

 as the most fertile for all purposes ; but we have few such in Britain, 

 our clays being mostly of an obdurate texture, retentive of moisture, 

 and requiring much cultivation, and containing moreover salts of iron 

 in proportions almost poisonous to plants. But there are profound 

 resources in most clays, so that if it is difficult to tame them, it is also 

 difficult to exhaust them. Hence a clay that has been well cultivated 

 through several generations will generally produce a fair return for 

 whatever crop may be put upon it. Limestone soils are usually very 

 porous and deficient of clay, and therefore have no sustaining 

 power. Many of our great tracts of mountain limestone are mere 

 sheep-walks, and would be comparatively worthless except for the 

 metals that may be extracted from them, or the lime that may be 

 obtained by burning. On the other hand, chalk, which is a more 

 recent form of lime, is often highly productive, more especially where, 

 through long cultivation, it has been much broken up, and has 



