10 



IRRIGATION PRACTICE 



would make an inch. The finest are about .00001 milli- 

 meter in diameter; and it would require about 25,000 of 

 them, placed side by side, to make an inch. Most of the 

 particles in an ordinary soil are of a size intermediate 

 between these extremes. 



The smallness of the soil particles means that the 

 number of them, in an acre of ground to a depth of one 

 foot, fairly transcends the human mind. If a soil were 

 made up entirely of the coarsest particles above men- 

 tioned, there would be, in 1 cubic inch, 12,167; if of the 

 finest, there would be in 1 cubic inch, 15,625,000,000,000 

 particles. These vast numbers of soil grains of all sizes 

 between the extremes given, are jumbled together in the 



soil in every conceiv- 

 able manner. Groups 

 and clusters of them 

 are formed; the larger 

 ones touch in few 

 points, while the 

 smaller ones fall into 

 the spaces between, 

 and lime and other 

 substances often cause the cementing together of parti- 

 cles. The relatively large air spaces between the particles 

 and groups form from 30 to 60 per cent of the whole 

 soil volume. These open or air spaces are sometimes 

 spoken of as pores or tubes. They are of infinite com- 

 plexity of shape and direction as they wriggle through 

 the soil mass. In spite of the immensity of the numbers 

 and variety of the sizes of the particles, the whole 

 porous sytem is held together as one, and possesses 

 definite properties. 



Of chief agricultural interest is the surface exposed 







FIG. 4. Soil is a mixture of particles of very 

 varying size. 



