TABLE SHOWING RELATIVE RAPIDITY OF PERCOLATION IN PURE SANDS AND SOILS, IN INCHES OF WATER DRAINED OFF. ===================+==================+==============+============ Diameter of Uniform| First | Second | Total Sand Grains. | 30 minutes. | 30 minutes. |in one hour. -------------------+------------------+--------------+------------ .475 mm. | 10.25 | 4.68 | 14.93 .155 “ | 5.67 | 4.52 | 10.19 .083 “ | 1.21 | .85 | 2.06 ===================+===========+======+===+==========+============ |First 21-23| First 10 |Second 10 | Total in Soils. | hours. | days | days | about | |following.|following.| 505 hours. -------------------+-----------+----------+----------+------------ Sandy loam | 2.64 | 5.07 | .91 | 8.62 Clay loam | 1.96 | 2.11 | .49 | 4.56 -------------------+-----------+----------+----------+------------ This table is very instructive in showing the great difference in the rapidity of percolation in materials of uniform, even-sized grains, as compared with such as contain particles of many different sizes, in which the interspaces of the larger ones are filled more or less closely by the smaller sizes of particles (see chapter 7, p. 109). While it is true that we have no definite physical analysis of the soils here used, the differences are so great as to be sufficiently striking. Compare the percolation through the sand of .155 mm. uniform grain-size (a fine sand), during the first half hour, with that through the sandy loam during the first 21 hours. Twice as much water has passed from the sand as from the soil in one forty-second part of the time. Comparing similarly the finest sand, .083 mm. in diameter, with the clay loam, we find the difference to be as one to seventy-three. It is thus evident that but for the variously assorted sizes of the soil-particles, water would not be held long enough to supply plant growth.