176 THE CHEMICAL ANALYSIS OF SOILS [chap. 



chloric acid used in analysis, but is only liberated when 

 the soil is completely brought into solution by fusion 

 with other alkalis or by treatment with hydrofluoric 

 acid. As a rule the analysis extracts more potash from 

 the subsoil than from the surface soil, whence it has to 

 some extent been removed by greater weathering and 

 solution. 



The work, then, of soil analysis must be extended 

 to include some investigation of the condition of the 

 plant food in the soil, as well as its absolute quantity : 

 it is not enough to determine what constituents are 

 present with the view of making good the deficiencies, 

 because there is always more than enough for many 

 crops; inquiry must be rather directed towards finding 

 how much is likely to reach the crop. The attempt 

 to discriminate between the total and what may 

 be termed the available plant food in the soil, i.e., 

 that which is in a form the crop can immediately 

 utilise, has been made in two ways by using the 

 growing plant as an analytical agent, or by attacking 

 the soil with very dilute acids, whose action is akin 

 to the natural solvent agencies at work when the 

 plant is growing. The former process proceeds upon 

 the assumption that any given plant has a certain 

 average composition which it will acquire when freely 

 supplied with all the elements of nutrition ; if this 

 plant be grown upon a soil deficient in one particular, 

 that deficiency will be reflected in the analysis of the 

 plant when fully grown. It is thus necessary to select 

 a standard plant and grow it under normal conditions 

 of manuring to ascertain the proportion that nitrogen, 

 phosphoric acid, and potash usually bear to the ash. 

 The selected plant is then grown upon the soil in 

 question, gathered at the appropriate stage and ana- 

 lysed, when the composition of the ash, as compared 



