PHYSICAL, COMPOSITION AND CLASSES OF SOILS 85 



to the plow very tenaciously when too wet. They plough fairly 

 well when in right condition, but turn up in clods if ploughed 

 when dry. Loams are intermediate between sands and clays, in 

 physical character and in properties, and in general are good soils. 



The coarse soil particles, therefore, tend to make the soil more 

 open and porous and more easily tilled. The fine particles tend 

 to make the soil more compact and less easily tilled. The final 

 resultant depends upon the relative quantities of the different 

 kinds of particles, as well as their chemical composition or their 

 properties. The presence of a certain amount of clay in a sand 

 is desirable. If no clay is present, the soil is liable to pack on 

 wetting, but clay holds the particles into crumbs characteristic of 

 a well tilled soil. Further, sands containing less than 4 per cent, 

 clay have little power of retaining moisture and are particularly 

 liable to suffer from drought Sand particles in a clay will not 

 diminish its stickiness, while silt particles make the clay less 

 adhesive, though perhaps more heavy to work. 



Mechanical analysis is also to be interpreted with consideration 

 as to the amount and distribution of rainfall and the temperature 

 and the effect of the underground water. A light soil under 

 heavy rainfall may behave like a heavier soil under light rainfall. 

 A soil with ground-water at such distance that it may be brought 

 to the roots of plants by capillary action, is in better condition than 

 when the ground-water is deeper. 



Classes of Soil Related to Mechanical Analysis. Soils are 

 classed as sands, loams, clays, etc., according to their physical 

 characteristics. There is room for difference of opinion as to 

 exactly what characteristics should be signified by each term. The 

 classification is made partly by field observations, and partly by 

 mechanical analysis. 



As a result of the mechanical analysis of a great number of 

 soils, the Bureau of Soils 1 of the U. S. Department of Agriculture 

 finds different types to have the following average composition 

 (see table). 



The predominance of various grades of material is well brought 

 out in the table. For example, the coarse sands contain on an 



1 Bulletin No. 78. 



