vi.] DORMANT AND ACTIVE PLANT FOOD 175 



it is found in nuclein and lecithin, which in a more 

 or less humified condition are found among the plant 

 and animal residues : it also occurs as phosphate of 

 the sesquioxides of iron and alumina; as tribasic, and 

 probably also as dibasic phosphate of lime. Of these 

 compounds the latter are undoubtedly the most soluble 

 in either pure water or the carbonic-acid-charged water of 

 the soil, but much must depend on the physical condition, 

 as well as on the chemical combination, in which the 

 material exists. For example, when using tribasic phos- 

 phate of lime as a manure, the softer phosphates, such as 

 steamed bone flour, are more effective than the chemically 

 similar but harder material in ground rock phosphate. 

 Almost invariably phosphoric acid is less abundant in 

 the subsoil than in the surface, where it has been 

 accumulated as a residue from the decay of previous 

 vegetation. 



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 amount 

 of potash found bears a fairly close relationship to the 

 amount of the clay fraction and the proportion of 

 alumina obtainable from the soil. As already shown 

 (p. 42), the percentage of potash increases rapidly in 

 the finest fractions of the soil, which would seem to 

 show that the clay contains a definite group of silicates 

 attacked by the acid. There is, however, always potash 

 in the soil that is not extracted by the strong hydro- 



