Chemical Nature of Soils. 71 



the salts of magnesium except that they must be present in 

 the seed. 



The action of lime is held to be medicinal, its function 

 being to neutralize the poisonous oxalic acid liberated as 

 an intermediate product in the oxidation of carbohydrates. 



Large amounts of silica and alumina and smaller 

 amounts of many other substances are found in the ash of 

 plants but their presence there is regarded as accidental, 

 growing out of the simple fact that they chanced to be dis- 

 solved in the soil-water and passed into the tissues with it 

 during growth. / 



81. Chemical Composition of Soils. From what has been 

 said regarding the origin of soils and the manner in which 

 their particles have been moved from place to place, it is 

 evident that there must necessarily be a strong similarity 

 among them, of both chemical and mineral composition, 

 wherever found. It has been customary in analyzing soils 

 to digest a certain weight of dry soil for a stated time in a 

 certain strength of hot hydrochloric acid and to examine 

 the solution for the compounds it might contain, calling the 

 part not dissolved the insoluble residue. The tables on 

 pages 74-75 show the results of some of these analyses, 

 taken from the papers of Hilgard in the Tenth Census of 

 the United States. 



82. Chemical Difference Between Clayey and Sandy Soils. 

 Studying the table of clayey and sandy soils it will be 

 noted that out of every 100 pounds of the clayey soil there 

 were, as an average, 31.791 pounds which dissolved in hot 

 hydrochloric acid, while only G.79 pounds were soluble in 

 like weight of the sandy soil. In other words, a quarter 

 of the weight of the clayey soils more than of the sandy^soils 

 is soluble in a unit of time in hot hydrochloric acid. There 

 is about 2.5 times as much potash and organic matter, 

 nearly twice as much phosphoric acid, 7 times as much 

 lime, 9 times as much magnesia and 1.4 times as much 

 sulphuric acid in the clayey as in the sandy soil, which 

 may be dissolved out in equal times by the solvent used. 



These ratios, however, are sometimes a long ways from 



