RAINFALL AND SOIL MOISTURE 15 



SOIL MOLSTURE 



A series oi weekly determinations of soil moisture was made 

 from August", 1910, to July, 1911, at a single locality on the 

 slopes of Tumamoc Hill, near the Desert Laboratory.- The soil 

 concerned is a very fine brown clay, derived from the weather- 

 ing of basaltic rock. The surface of the soil is covered with a 

 rock mulch of pebbles and larger stones, and throughout the soil 

 rock fragments are extremely numerous. At a depth varying 

 from 8 to 16 in. (20 to 40 cm.) the larger stones are cemented 

 together witli hardpan, or ''caliche," beneath which is found 

 either the rock in situ, or else lower pockets of soil. Throughout 

 the deeper portions of the soil are still deeper extensions which 

 fill cracks in the bed rock. It is mainly in these deep pockets 

 of soil that the root systems of the adult desert perennials are 

 located, and the successful establishment of their seedlings is 

 largely a matter of the chance finding of such pockets by their 

 extending root systems. 



The weekly samples of soil for moisture determination were 

 taken at depths of 3, 15 and 30 cm. (1.2, 5.9 and 11.8 in.), placed 

 at once in bottles with tight stoppers, weighed, unstopped, and 

 dried in a gas oven until successive weighings showed no further 

 loss of weight. The temperature at which the soils were dried 

 was not allowed to exceed 100°C., as higher temperatures are cal- 

 culated to destroy the organic material of the soil and to occasion 

 losses of weight not due to the escape of soil moisture. The per- 

 centage of moisture was calculated on the dry weight. A single 

 series of samples was taken each week, each from immediately 

 under the preceding sample. On the following week the series 

 was taken from a spot about 2 m. distant. At no time were the 

 three samples which formed a weekly reading taken from differ- 

 ent holes, and at no time were successive weekly readings taken 

 in widelj' separated spots. The taking of four series of samples 

 on the same day, from the corners of a square of 2 m. diameter 



- The annual march of soil moisture in four soils in the vicinity of the Desert 

 Laboratory, including the clay of Tumamoc Hill, has been described by: Living- 

 ston, B. E., Relation of Soil Moisture to Desert Vegetation. Bot. Gaz. 50: 241- 

 2.56. 1910. 



