Soils 35 



Often there is a distinct banding of the layers of the different- 

 sized particles. 



The particles of soil are designated according to size as clay, 

 silt, sand, gravel, and stones. Scientists, for the purpose of 

 making mechanical analyses of soils, have given arbitrary sizes 

 to the various groups of soil materials and have found ways of 

 determining the proportion of each of the different groups in soil 

 samples. The Bureau of Soils of the United States Depart- 

 ment of Agriculture uses the following sizes : 



Clay below .005 millimeter 



Silt .05 to .005 millimeter 



Very fine sand .10 to .05 millimeter 



Fine sand .25 to .10 millimeter 



Medium sand .5 to .25 millimeter 



Coarse sand 1 to .5 millimeter 



Very coarse sand 2 to 1 millimeter 



In classifying soil according to texture, the same general 

 terms are used as in designating the size of particles. Thus 

 soil composed of a large proportion of clay is known as a clay 

 soil, one made up largely of silt is a silty soil, one that is mostly 

 sand is a sandy soil. Gravelly soil and stony soil are those with 

 a predominance of these materials. The term loam is used 

 also in describing soil texture. A loam soil is one in which fine 

 and coarse particles are about equally distributed. If one 

 size of particle predominates, for example fine sand, this name is 

 prefixed. 



The texture of the soil cannot be modified by the farmer to 

 any marked extent. Of course by mixing particles of different 

 sizes together he can make a soil of different texture, but this 

 is not practicable. The farmer, however, should study soil 

 texture in order to determine the best utilization that can be 

 made of each kind of soil. For example, it has been found that 

 fine sandy loams are good trucking soils and that rich loam 

 soils are usually well adapted to corn. 



