TEXTURE AND STRUCTURE OF SOILS 



35 



ci^r 



34. Some properties of the separates. — In addition to 

 differences in their size, there are other distinctions that are 

 more or less characteristic of these separates. A mechanical 

 analysis, therefore, tells us something about several of the 

 properties of a soil. 

 Clay particles, by 

 reason of their mi- 

 nute size, tend to 

 make a soil plastic 

 and may cause it to 

 become hard, com- 

 pact and cloddy 

 when dry. Silt 

 does this to a much 

 less degree. The 

 extent to which a 

 soil exhibits these 

 properties depends 

 on its content of 

 clay or silt. Soils 

 containing much 

 clay or silt must not be plowed when wet or they will puddle. 

 Both clay and silt serve to increase the water-holding power 

 of a soil, and clay especially increases the difficulty of tillage. 



The sand separates have the opposite properties of 

 clay, and in the order of their greater size of particles. 

 Sandy soils are more easily worked, are not likely to puddle 

 or to form clods, and do not hold a large amount of water, 

 but on the contrary have a tendency to become dry. Sandy 

 soils are termed " light " soils because they are easy to till ; 

 clay soils are called " heavy " because they make a heavy 

 draft on the plow. 



The absolute specific gravity, or weight of the particles as 

 compared with the weight of the volume of water which 



Fig. 4. — Plan by which the soil class may be 

 ascertained from a mechanical analysis. 



