154 CHEMICAL COMPOSITION OF THE SOIL [chap. 



over, a certain proportion of it was found to have been 

 converted into clay. The solvent action, however, in 

 both these cases can be put down to the carbon dioxide 

 excreted by the roots. 



Many attempts have been made to devise processes of 

 analysis which should discriminate between the available 

 and the total plant food present in the soil : it has already 

 been pointed out that an ordinary straightforward 

 analysis which determines all the nitrogen in the soil 

 and all the phosphoric acid and potash which can be 

 dissolved out by strong hydrochloric acid, shows enough 

 food material for fifty and even one hundred ordinary 

 crops. Consequently, when such an excess is revealed 

 it is difficult to interpret the analysis, or to base upon 

 it any recommendations to the farmer regarding the kind 

 of manures he requires. Very dilute acids have, there- 

 fore, been suggested — a solution of citric acid that shall 

 be comparable to the acid sap of the root, or a solution 

 of carbon dioxide that shall be like the soil water — with 

 the idea that all of the phosphoric and potash which 

 these liquids would dissolve might be regarded as 

 available. It cannot be said that these methods of 

 analysis have provided infallible information ; it is not 

 possible, for example, to say that if a soil contains less 

 than a certain limited amount of phosphoric acid 

 determined by this method it follows that a phosphatic 

 manuring is required. The limit to be adopted will 

 vary with the texture of the soil, the water usually 

 present, the amount of carbonate of lime, the crop under 

 consideration, and other independent factors. For 

 example, a sandy soil may contain much less phosphoric 

 acid than a clay soil and yet feed the plant equally well, 

 simply because the open, aerated soil induces a far 

 greater root development. Again, a crop like wheat, 

 with its long period of growth and deep roots, is much 



