vi.] DORMANT AND ACTIVE PLANT FOOD 155 



It is not so easy to classify the various compounds 

 of potash existing in the soil : we know that as felspar 

 passes into kaolinite there are intermediate stages of 

 weathering in which the potash is gradually becoming 

 more soluble in soil water, but it is impossible to isolate 

 or classify the various hydrated silicates containing 

 potash that must exist. Potash, again, which has once 

 been dissolved, is caught and retained by the soil in 

 various ill-defined compounds, some of which must 

 reach the crop more rapidly than others. 



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. 



