146 SOIL 



than 1 millimeter in diameter may be classed as gravel; those from 

 1 to 0.05 millimeter as sand; those from 0.05 to 0.005 millimeter as 

 silt; and those less than 0.005 millimeter as clay. The texture of a 

 soil depends upon the proportions of the different sizes of soil par- 

 ticles that it contains. 



A colloid is any substance that is made up of insoluble particles 

 that are so small that they will remain permanently in suspension 

 in a solution instead of settling to the bottom. Such a substance 

 exhibits certain characteristics such as great ability to absorb water, 

 ability to swell and to become sticky when wet, and to shrink and 

 become hard when dry. The colloidal properties of any soil are 

 due in part to the smallest clay particles and in part to the organic 

 matter of the soil which is collectively called humus and which is 

 made up of particles that are as small if not smaller than those of 

 clay. Therefore, the colloidal characteristics of a soil depend upon 

 the amounts of clay and of humus that it contains. 



Soils may be classified in various ways but on the basis of texture 

 they may be classified simply as sand, sandy loam, clay loam, and 

 clay. These classes are determined by the amounts of sand, silt, 

 clay, and humus that they contain. A soil consisting entirely of sand 

 is likely to be poor in soluble substances and too loose and dry for 

 best plant growth. One composed entirely of clay easily becomes 

 waterlogged when wet and hard and badly cracked when dry. A 

 loam soil is usually most suitable for plant growth because it has 

 some large particles that remain as individuals and tend to keep the 

 soil porous and also smaller particles which tend to become cemented 

 together by colloidal material into crumbs which have great water 

 absorbing capacity. 



94. Soil Water.— The water-holding capacity of a soil is one of the 

 most important of all soil factors and is dependent upon the size 

 of the soil particles. The chief, and usually the only, source of soil 

 water is rain. A considerable percentage of the rainfall in any year 

 flows off from the surface of the soil and is of no use to plants. This 

 part is called the run-off and it may do a great amount of damage by 

 w^ashing away portions of the soil. The remainder of the rain enters 

 the soil and the soil becomes wet. If it continues to rain after the 

 soil is thoroughly wet the water percolates downward under the 

 influence of gravity until it reaches the level of standing water where 

 all the soil spaces are completely filled. This level is called the 

 water table and its depth varies greatly from place to place and from 

 season to season. 



