COMPOSITION OF SOILS. 31 



The immediate fertility of a soil depends, however, 

 not so much upon the quantity of the constituents con- 

 tained in it as upon the amount of each that may be 

 available to the plant. 



Analysis of Soils. The composition of a rich wheat 

 soil and of a wheat plant, as shown by analysis, in 

 the diagrams 1 (page 32), indicates the relation of the 

 composition of the plant to the composition of the soil. 



The chemical analysis of a soil shows the percentages 

 of the different constituents contained in it. It is ob- 

 served that the constituents which the soil possesses 

 to only a limited extent are contained in the plant in 

 relatively large amounts; these are, therefore, termed 

 essential plant-food constituents, because of their greater 

 liability to exhaustion. 



Weight of Soils. In studying a soil from a state- 

 ment of its analysis, regard must be had to the weight 

 of soils. The constituents of the analysis are expressed 

 in per cent or pounds per hundred ; it is evident, there- 

 fore, that a soil weighing one hundred pounds per cubic 

 foot, and containing four-tenths of a per cent of phos- 

 phoric acid, would contain a much greater amount of 

 phosphoric acid per acre than a soil showing the same 

 percentage, but weighing fifty pounds per cubic foot. 



It is estimated that dry sand weighs from one hun- 

 dred to one hundred and twenty pounds per cubic foot; 

 loam, from ninety to one hundred pounds ; clay, seventy 

 to eighty pounds; and peat, thirty to fifty pounds. An 

 analysis, therefore, which shows the same per cent 

 of the constituents in one soil may not indicate its 



i Adapted from Villo. 



