SOME CONCEPTIONS OF THE SOIL 13 



tively large amounts. Its occurrence in complex and insoluble 

 silicates makes its availability of vital consideration. The 

 presence of abundant organic matter may have much to do 

 with the liberation of sufficient potash for vigorous plant 

 growth. 



The amount of lime (expressed as CaO) in soils is difficult 

 to state with any degree of satisfaction because of a very 

 wide range in composition. Some soils carry only a fraction 

 of a per cent., while others, especially those formed under 

 conditions where an originally high calcium content has been 

 maintained or where calcium has accumulated, show as much 

 as 10 or 12 per cent. The variability of the sulfur is much 

 less. A range from .02 to .30 per cent, of sulfur (expressed 

 as S0 3 ) will include most soils. 



It is interesting at this point to note the average composi- 

 tion of thirty-five representative American surface soils 1 , 

 which were studied by the United States Bureau of Soils dur- 

 ing a systematic investigation of the arable lands of the United 

 States east of the Rocky Mountains. A comparison of these 

 data with those setting forth the composition of the litho- 

 sphere 2 may be made with profit. (Table I, page 14.) 



It is immediately noticeable that silicon, aluminum, and 

 iron make up the greater portion of both soil and lithosphere 

 and that the nitrogen, sulfur and phosphorus are particu- 

 larly low in both cases. Magnesium, calcium, sodium, and 

 potassium occur in fair amounts, especially in the earth's 

 crust. It is noticeable also that the soil is much higher than 

 the lithosphere in silicon, nitrogen, organic matter, and car- 

 bon but much lower in all of the other constituents. These 

 differences have developed as a result of the losses and gains 

 during soil formation. 



1 Robinson, W. O. et ah, Variations in the Chemical Composition of 

 Soils; U. S. Dept. Agr., Bui. 551. June, 1917. 



3 The Lithosphere refers to the solid portion of the earth, in this case 

 to a depth of ten miles. Clarke, F. W., Data of Geochemistry; U. S. 

 Geol. Survey Bui. 695, p. 33. 1920. 



