VARIATIONS OF SOIL PARTICLES 



95 



tides of different sizes, and each particle looks much the 

 same, under the glass, as do the larger ones to the naked 

 eye. The spaces separating the minute fragments are, of 

 course, filled by air or water ; and we know that soil con- 

 tains moisture, because so often it is damp. Soil, then, is 

 surface material, made up of waste or fragments of rocks, 

 of organic matter, and 

 of small particles of air 

 and water. 



56. The varying de- 

 grees of fineness or coarse- 

 ness of the soil particles 

 are expressed by the 

 names clay, silt, sand, 

 gravel, and stone. 



a. Clay is formed of 

 very small soil particles. 

 They are so small that 

 it would take at least 

 5000 of them laid side 

 by side to reach one 

 inch. The largest par- 

 ticles of clay, the red 



corpuscles of our blood, and the green corpuscles (chlo- 

 rophyll bodies) of plants are all about the same size. 

 If a bit of moist clay is rubbed between the fingers, it 

 is found to lack grit. A clay soil is hard to drain be- 

 cause its very fine grains will not allow the water to pass 

 through. In drying, clay tends to form a hard, compact 

 mass, through which it is difficult for plant roots to pene- 

 trate. If plowed when wet, it becomes puddled, as when 

 a small boy makes marbles out of wet clay. Clay lands 

 are cold, stiff, heavy, and difficult to work, although they 

 hold water well and are unusually rich in plant food. 



CLAY. 



The largest particles of clay are about 

 sii f an i nc h ' n diameter. 



