14 SOIL COMPOSITION, CLASSES AND TYPES 



cementing nature of clay and much of the organic matter, group 

 themselves into compound particles and granules, thus developing a 

 "crummy" or ^'granular" structure. When a "heavy'' soil does 

 not crumble, but is a hard, compact mass instead, it may be said to 

 have a " compact " structure. A sand may be described as having a 

 "loose" or "sandy" structure. 



Soils Classified According to Texture. — ^Eight principal soil 

 classes based on texture are recognized. They are here given in an 

 order ranging from the coarsest to the finest textured soils. 



Soil classes How distinguished 



1. Medium sand Composed of 80 to 100 per cent sand (much medium 



and coarse sand). 



2. Fine sand Composed mostly of fine sand. 



3. Medium sandy loam. Having 50 to 80 per cent sand. 



4- Fine sandy loam Having 50 to 80 per cent fine sand. 



5. Loam Composed of 30 to 50 per cent sand and 50 to 70 per 



cent silt and clay. 



6. Silt loam Containing 50 per cent or more of silt. 



7. Clay loam Containing 20 to 50 per cent sand, 20 to 50 per cent 



silt, and 20 to 30 per cent clay. 



8. Clay Having 30 per cent or more clay. 



A soil class is understood to mean all soil of the same texture. 

 For example, soils consisting of about 50 per cent sand and about 

 50 per cent silt and clay belong to one and the same class regardless 

 of where they may be found. By virtue of its textmre this class of 

 soil is named "loam." 



A loam is to be defined as a class of soil composed of about haK 

 sand and the remaining half silt and clay (more silt than clay). 



Again, a silt loam is to be defined as a class of soil, or a soil class, 

 containing 50 per cent or more of silt. 



Of these classes of soils, silt loams are most widely distributed, 

 loams take second place, and fine sandy loams third. 



In this classification no consideration is given organic matter. 

 Soils containing much stone or gravel may be described as stony silt 

 loam or gravelly loam, as the case may be. Gradations may also 

 occur; such as, silty clay loam, loamy fine sand, etc. 



"Marsh" and "swamp" soils are to be regarded as class names 

 though they are not included in the classification based on texture. 



Marsh is commonly interpreted to mean a wet level area 

 covered with grasses. There are salt and fresh-water marshes. 



Swamp is usually understood to mean low, wet areas of fresh 

 water formation covered with tree growth. Tamarack swamp, 

 cedar swamp, and cypress swamp are familiar expressions. 



