94 THE ORIGIN AND NATURE OF SOILS [chap. 



tion of the soil extract. It is, however, not quite 

 sufficient to consider the soil materials that are soluble 

 in water as alone capable of feeding the plant ; we have 

 already seen reason to suppose that the carbon dioxide 

 excreted by the plant's roots and also present in the 

 soil from other sources, adds greatly to the solvent 

 action of the water, enabling it to attack substances 

 which are usually regarded as insoluble in water. In 

 consequence, we have come to regard any substance in 

 the soil that is soluble in acids as a potential plant food 

 which may, however slowly and in whatever small quan- 

 tities, be brought into solution and so reach the plant. 



If, then, we attack a soil with strong hydrochloric 

 acid, which we may be sure will dissolve everything that 

 is likely to reach the plant within the next century, 

 we shall find some 5 to 1 5 per cent, of the soil going 

 into solution, but by far the greater part of this consists 

 of oxides of iron and alumina, to which (except for a 

 trace of iron) the plant is indifferent. If we analyse the 

 soluble material we shall find exactly the elements 

 we have previously ascertained to be common 

 to all plants, i.e., chlorine, phosphoric acid, 

 sulphur, and some silica among non - metals, 

 with soda, potash, magnesia, lime, iron, and alu- 

 mina in excess ; but the elements that are necessary 

 to the plant rarely make up i per cent of even the 

 most fertile soils. This i per cent, however, is all- 

 important, constituting as it does a vast reservoir of 

 potential plant food. Nor is the quantity so small as 

 might at first sight appear ; a cubic foot of soil in situ 

 weighs from 90 to no lb., clays being the lighter and 

 coarse sands the heaviest of all ; at Rothamsted, again, 

 where the soil is a clay loam, the layer of soil, an acre in 

 extent and 9 inches deep, weighs about two and a half 

 million pounds, exclusive of stones, ix. rather more than 



