CHEMISTRY OF SOILS 127 



of phosphoric acid and 0'005 per cent, of potash, soluble 

 in the dilute citric acid reagent, stand in need of phos- 

 phatic and potash manures. 



The method has attracted the attention of agricultural 

 chemists in all parts of the world, and has been very exten- 

 sively employed in this country. It has been applied to 

 many different types of soil, and has been compared with 

 other methods. In general, the result has been to confirm 

 Dyer's views; the 1 per cent, solution of citric acid 

 gives more trustworthy information regarding the manurial 

 requirements of soils than any other solvent yet examined, 

 but probably the standards proposed are not generally 

 applicable to all soils alike. 



CHEMICAL CHANGES. 



Chemical changes of one kind or another are constantly 

 taking place in the soil, and sooner or later most of the 

 constituents are affected. Many undergo slow but con- 

 tinuous modification as a result of the operation of the 

 various forces mechanical, chemical and biological to 

 which ;they are exposed. Soluble salts are produced by the 

 decomposition of insoluble minerals ; compounds are 

 oxidised, reduced and dehydrated ; pans are formed and 

 disintegrated ; humus accumulates and disappears, and 

 various constituents of the soil react upon each other and 

 upon substances applied to it. 



A few of the changes are simple, direct and easily under- 

 stood, but some are reversible, and most of them are very 

 complex and involved in obscurity. Those which take place 

 slowly are all the more difficult to follow by reason of the 

 numerous intermediate stages which can sometimes be 

 recognised but cannot be determined. Final products can 

 be traced to original compounds and the character and 

 direction of the change made clear, but the number of 



