No. 4.] 



SOIL VARIATION. 



15 



Scheme of Soil Classification, based upon the Mechanical Composition 

 of Soils — Concluded. 



[Millimeters.] 



From this broad generalization it will be seen that tlie 

 surface soil should contain an apjn-eciable amount of sand. 

 The sands, moreover, should not be all of one grade ; that 

 is, a high percentage of coarse sand would give a poor soil, 

 whereas a moderate admixture of it with the finer grades of 

 sand, together with sufficient clay and silt, would work no 

 harm. In general, the sand content should be of the finer 

 grades, but soils also occur, though comparatively rare, which 

 would be too heavy for this variety were it not for a marked 

 content of the coarse sands, the effect of which is to make the 

 soil mass much more friable and open than would be expected 

 with the presence of so much clay. Such soil dries quickly 

 after a rain, and is not to be classed as a moist soil. It will 

 never clod if worked under conditions at all reasonable. The 

 subsoil, on the other hand, must never be heavy enough to 

 impede ready drainage of excess moisture, yet sufficiently 

 clayey to retain a good moisture supi)ly; that is, plastic, not 



