MECHANICAL ANALYSIS OP SOIL. 



39 



It will be seen that none of the foregoing analyses showS more varia- 

 tion than those of the Bureau of Soils. 



Hilgard ^ has proposed as a graphic representation of the mechanical 

 structure of soils as determined by analysis, that photographs be pub- 

 lished showing glass tubes containing the separations arranged in order 

 of grain size on the assumption that the depth of material in each vial 

 is proportional to the amount of the sized material in the soil and that 

 the surfaces in the row of tubes describe a curve of the soil composition. 

 An objection to this scheme is that the clay and silt cake in drying and 

 fill their tubes largely with void space. The sands also have different 

 percentages of voids, varying with the sizes of the grains, so that repre- 

 sentation made by photograph is not accurate. 



A more accurate, complete and satisfactory way of showing the soil 

 character is that made apparent in the accompanying text figure, which 

 represents graphically the mechanical composition by weight of a surface 

 soil and its subsoil. Not only the relative proportion of the different 

 grain sizes is shown, but also the physical composition of the soil as a 

 whole. ^ By means of such curves the relation of a soil to its subsoil is 

 at once apparent and comparisons may readily be made with other soils. 

 The prospect of a crop may partially be interpreted by comparing with 

 the curve for the ideal soil for that crop. 



Often, a knowledge of the geologic features of a region will aid in 

 advance interpretation of soil character and taken together with the 

 physical analysis the two supplement each other. The reverse is also 

 true that one can often interpret certain geologic features from the soil 

 analysis, as shown by the following analyses of Philippine soils. 



Table of analyses' of Philippine soils. 



Soil. 



Diameter 

 of grains. 



Sample No.— 



1«. 



l^ 



2». 



2>'. 



3«. 



SK 



4». 



4b. 



Fine gravel 



Coarse sand 



Medium sand ^ 



Fine sand 



Very fine sand.._ 



Silt 



Clay 



mm. 

 2.0-1.0 

 1.0-0.5 

 0. 5 -0. 25 

 0. 25-0. 10 

 0. 10-0. 05 

 0. 05-0. 005 

 0.005 and 

 less. 



Perct. 

 12.3 

 6.6 

 13.7 

 12.8 

 12.2 

 30.0 

 12.7 



Perct. 

 41.8 



6.7 

 15.1 

 10.5 



6.0 

 13.4 



5.5 



Per ct. 

 0.0 

 0.1 

 1.0 

 12.2 

 15.4 

 52.0 

 19.2 



Per ct. 

 0.1 

 0.0 

 0.4 

 13.7 

 12.8 

 51.7 

 20.4 



Per ct. 



14.2 



1.4 



5.0 



7.8 



12.9 



40.0 



18.7 



Per ct. 



20.1 



1.0 



3.5 



7.3 



13.4 



40.5 



14.4 



Per ct. 

 0.8 

 5.9 

 30.1 

 26.4 

 11.6 

 12.2 

 13.2 



Per ct. 

 0.8 

 3.9 

 26.3 

 26.1 

 10.4 

 15.1 

 17.4 



Total 



10O.3 



100.0 



99.9 



99.1 



100.0 



100.2 



100.2 1 100.0 1 









1 



" Surface soil. 



•> Subsoil. 



Soils, New York (1907), 94. 

 ' This would only be approximated by a photograph of the tubes. 



