PHYSIOGRAPHIC FACTORS 153 



sible by using the names for the fractions singly or in combina- 

 tion. The soil classes are named primarily for the predominating 

 size fraction, 87 but when many sizes are present, the term, loam, 

 is introduced. Thus a soil may be termed gravel or clay if either 

 of these sizes is present almost exclusively, but if gravel or clay 

 merely predominates and is mixed with several other size classes, 

 the soil is called gravelly loam or clay loam. 



A knowledge of the textural grade of a soil suggests numerous 

 other characteristics of that soil. With experience, even a rough 

 estimate determined by "feel" is useful, for texture indicates other 

 physical properties, particularly those affecting moisture, aera- 

 tion, and workability. 



Soil Structure— The arrangement of soil particles becomes es- 

 pecially important when small size classes are involved. Sands have 

 single-grain structure, but silts, and more particularly clays, tend 

 to have particles aggregated in clumps. Aggregation is largely 

 caused by the colloidal portion, less than 0.001 mm., of the clay. 

 Just as clay soils with their tremendous internal surface swell when 

 wet, they also contract as they dry. The minute particles are 

 drawn together by cohesive forces in large or small aggregates 

 whose size and shape affect drainage, percolation, erosion, and 

 aeration (Fig. 79). 



If the granular structure is lacking or destroyed by mismanage- 

 ment, as when trampled by livestock or worked too wet, the soil 

 puddles or bakes into hard solid masses, and shrinkage results in 

 the formation of deep cracks. In a loam soil or one with a high 

 organic content, these undesirable features are reduced while the 

 desirable characteristics produced by colloids are retained. 



Organic Content— The amount of organic material in soil mav 

 greatly modify its physical characteristics as determined by the 

 mineral components. In addition, organic material is the major 

 source of certain plant nutrients, especially nitrogen, so that fer- 

 tility and productiveness are usually correlated with it. 



Under natural conditions, organic matter in soil is derived from 

 remains of plants and animals. Mostly these remains accumulate 

 on the surface of the mineral soil to form a layer of litter, which, 

 if sufficiently thick, may reduce the effects of insolation, check 

 erosion, and prevent compacting resulting from precipitation. 



