14 THE RELATION OF DESERT PLANTS TO 



It seemed possible that the rate of movement here found was 

 too high for the natural soil on account of the fact that the latter is 

 apt to be more firmly packed than was the soil in these columns. There- 

 fore a similar tumbler of soil was prepared, tamped as firmly as possible, 

 and the rate of water penetration into it studied in the same way. 

 The upper surface of the column was not packed as firmly as the soil 

 below, and the initial rate of advance of the water was nearly as rapid 

 as that in the previous experiment. In two minutes it had advanced 

 2.5 cm., during the third minute its increment was 0.2, during the 

 fourth it was the same, during the fifth it was 0.15, and this rate 

 decreased so that at the end of 3 hours the soil was moist to a depth of 

 only 4.8 cm. below the surface. It thus appears that the rate observed 

 in the case of the first set of six tumblers is perhaps about twice as 

 great as in the natural soil. It was deemed worth while, however, 

 to study the decrease in the rate of penetration in the case of a longer 

 soil column only moderately tamped. This column was 4.5 cm. in 

 diameter and 93 cm. high, a column of water being kept about 2 cm. 

 high above the soil. The experiment was continued for 30 hours, obser- 

 vations being taken from time to time and the hourly rates of water 

 movement being calculated from the observed increments. These 

 rates, in centimeters per hour, are expressed in the form of a curve in 

 figure 1 (p. 15). In this curve abscissas denote time, the numbers on 

 the horizontal axis representing hours. The ordinates denote the rates 

 and are plotted at the middle of the time periods to which they corre- 

 spond. These rates are placed adjacent to the points marked by crosses, 

 which determine the position of the curve. It will be observed that, 

 after the first five hours, the rate of downward movement decreased 

 quite uniformly to the end of the experiment. During the last three 

 hours the rate was about 1.3 cm. per hour, the record ending when the 

 water had reached a depth of 42.4 cm. below the soil surface. 



Data on the question as to the exact relation of these figures to the 

 natural conditions near the Desert Laboratory during the summer rains 

 were not obtained, but the fact was established that, as early as 

 August 1, the moisture of precipitation had penetrated to a depth of 

 from 20 to 30 cm., thus connecting, by means of moist soil, the surface 

 layers with the lower-lying ones, which were moist at the beginning 

 of the rains. 



RETAINING POWER OF THE SOIL FOR PERCOLATING WATER. 



The power of soils to absorb and retain water and prevent its down- 

 ward flow into lower layers varies exceedingly according to their nature. 

 The coarser the soil particles and the smaller the amount of organic 

 debris contained, the smaller will be its retaining power. Thus, in 



