THE PHYSICO-CHEMICAL INVESTIGATION OF SOILS. 321 



The reserve food-store may then be placed under the fol- 

 lowing heads : 



Hydrous or " zeolitic '" silicates, from which dilute acids 

 can take up the bases potash, soda, lime and magnesia. These 

 silicates may be in either the gelatinous or powdery form ; in 

 the former case they may also occlude water-soluble sub- 

 stances. 



Carbonates of lime and magnesia, which are readily dis- 

 solved by carbonated water as well as by the vegetable acids. 



Phosphates of lime and magnesia, not very readily soluble 

 in carbonated water, but more readily attacked by the acids 

 of the soil and of plant roots; thus supplying phosphoric acid 

 to plants. The more finely divided they are the more readily 

 they are dissolved; some soils containing only crystalline 

 needles of apatite (see chap. 5, p. 63) only are nevertheless 

 poor in available phosphoric acid. 



The natural phosphates of iron and alumina are practically 

 insoluble in all solvents at the disposal of vegetation and 

 though present in considerable amounts in some soils, (see 

 chapter 19, page 355), may be considered as being permanently 

 inert, and therefore not to be counted among the soil resources 

 for plant nutrition. As yet no artificial process by which 

 their phosphoric acid can be made available within the soil, 

 has been discovered. 



Water-soluble Ingredients. As regards these it has already 

 been explained that they are largely retained in the condition 

 of purely physical adsorption, as in the case of charcoal or 

 quartz sand, through which sea water filters and is thereby 

 partially deprived of its salts. But these can be gradually 

 withdrawn by washing with pure water alone, and still more 

 easily when stronger solvents are used. Since the soil-water 

 is always more or less charged with carbonic acid, and the 

 roots themselves secrete carbonic as well as stronger acids in 

 their absorption of mineral plant-food, there is no difficulty 

 about explaining the manner in which such physically con- 

 densed ingredients are taken up. 1 



1 Whitney (Bull. 22, U. S. Bureau of Soils) claims on the basis of a large number 

 of (three-minute) extractions of soils made with distilled water, that these solutions 

 are essentially of the same composition in all soils ; that all soils contain enough 

 plant-food to produce crops indefinitely; and that the differences in production 



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