10 THE RELATION OF DESERT PLANTS TO 



the actual amount of water contained in this volume. The answer to the 

 second question depends upon the degree to which the soil holds its 

 water as related to the amount of absorptive power exerted by the plant. 

 Thus the availability for any species of any given volume percentage of 

 water in non-alkali soils is determined largely by the fineness of the soil 

 particles and by the physiological properties of the roots. It is only a 

 coincidence that extremely light soils, being mainly organic in their 

 nature, have a comparatively high power to withhold water from plants 

 growing therein. 



In order to secure uniformity in compactness the wet volume of the 

 samples was used for this purpose. To obtain the wet volume, a suffi- 

 ciently large graduate was partially filled with water and the soil poured 

 in and thoroughly stirred to allow inclosed air to escape. The thin 

 paste thus formed was allowed to stand until settling was complete, 

 when the volume of the saturated soil was read directly on the graduate 

 scale. It was found that the amount of soil which would occupy, on 

 settling in water, a volume of 100 cc., weighed, in its oven-dry state, 

 85.0 grams. The same amount of soil, when merely poured into a 

 graduate without tamping, occupied a volume of 78.9 cc., and when 

 thoroughly tamped as it was poured in it occupied 68.4 cc. Thus the 

 percentage figures of moisture content obtained on the basis of volume 

 when allowed to settle in water are considerably lower than would have 

 been the case had they been computed on the dry volume when either 

 poured into the graduate or tamped. Since uniformity in tamping is 

 very difficult to obtain, the method of tamping could not well be used. 

 The dry surface layers of Tumamoc Hill usually crumble and com- 

 press beneath the foot, indicating that the soil of these layers, as it 

 dries out after being wet, occupies a greater volume than it would if it 

 were pulverized. Therefore it seemed that the natural volume would 

 be more nearly approximated by the method here used than by any 

 other, and at the same time a uniform treatment of the different 

 samples could be secured. From the data given above it is clear that 

 the moisture contents here given would have been if> or 17.6 per cent 

 larger had they been computed on dry weight, as is usually done in such 

 measurements. 



By the method just described it was found that the soil samples air 

 dried in the laboratory contained from 2 to 3 per cent of moisture. 

 Samples of the upper 2 or 3 cm. of the natural soil, taken in the burn- 

 ing sunshine between July 1 and July 14, contained somewhat less 

 water, about 2 per cent. In most places on the hill it was impossible, 

 on account of rock fragments or caliche, to make small excavations to 

 a depth greater than 10 or 12 cm. Samples at this depth, lying against 



