THE NATURE OF SOIL 23 



THE FINENESS OF SOIL 



It was stated in Chapter I that the basis of most 

 farm soils is rock, ground into "rock-meal" by 

 Nature's millstones, the air, water, frost, ice and 

 other elemental forces. At first the soil particles 

 are very large, mere fragments of rock at the base 

 of a cliff, but upon these wild morning-glories or 

 mulleins may be able to grow. Some hundreds of 

 years later these small rocks will be finer; perhaps 

 they will average less than one-quarter r inch in dia- 

 meter, and they will be mixed with humus. The fin- 

 ing process goes on a few generations or centuries 

 more, until trie pieces of rock have been broken into 

 such small particles that farm crops thrive upon 

 them. Nearly every soil is constantly becoming 

 finer. All soils that contain small rocks or pebbles 

 receive from them each year many particles of soil 

 by weathering, and the size of the rocks and 

 pebbles is reduced that much. Even the rich 

 prairie loam or alluvial clay, which is apparently 

 all soil and contains no rocks or pebbles at all, is 

 becoming finer. Weathering is much less active 

 on such soil, however, than on gravelly and stony 

 soils. 



The number of individual particles in a fertile 

 soil is astonishing to those wno have not tried to 

 count them under a microscope. A good corn 

 soil has about 280,000,000,000 particles in an 

 ounce, while the clay loams that are preferred for 

 grass often have 400,000,000,000 particles in an 

 ounce. These particles are of varying sizes and 

 shapes, even in the same soil. Sometimes they 

 are uniform and rounded, and pack together 

 poorly, leaving large spaces between them, like 

 marbles piled together. Sometimes they are 



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