MOISTURE IN THE SOIL. 1 9 



soil, as, for example, 2 meters beneath the surface, a much higher water 

 content would probably have been found. 



The relation of the roots of the plants of the area will be spoken of later 

 in the paper, but it may be mentioned here that of non-fleshy forms only 

 those with deeply penetrating roots are to be found, and that only here 

 does a deeply penetrating root develop in a thoroughly normal and typical 

 manner. 



FLOOD-PLAIN OF THE SANTA CRUZ RIVER. 



The conditions of soil moisture of the river flood-plain are relatively 

 favorable for plants, perhaps the most so of any area under discussion, 

 which is partly owing to the character of the soil and partly to the com- 

 paratively level surface. In earlier times the flood waters covered the 

 plain, making of it a cienega ; but comparatively recently the river has cut 

 a channel and the only flood waters that go over the plain, at times not 

 inconsiderable, are derived from the adjoining bajada. The river channel 

 carries water only part of the year, and yet during seasons of storms it may 

 be a yellow torrent with great erosive power. The water table of the 

 flood-plain lies from about 5 meters, near the channel of the river, to about 

 12 meters at the western edge. 



The top soil of the plain is an adobe loam with a water-retaining capacity 

 of about 38.5 per cent its dry weight, the largest retaining capacity of any 

 soils of the different habitats so far examined. 



The digging of a well on the western side of the flood plain afforded an 

 opportunity of determining the water content of soil to a depth of 5.25 

 meters, although the upper adobe stratum did not attain a greater depth 

 than 5.10 meters. It was found that the water content increased with 

 depth to the limits of the adobe. Following are the determinations: At 

 the depths of 0.20, 3.30, 4.00, and 5.10 meters the water contents per dry 

 weight of soil were 12.9, 19.0, 22.6, and 23.1 per cent respectively. At the 

 depth of 5.25 meters, where sand was encountered, the water content was 

 7.1 per cent. It would appear from these determinations, therefore, that 

 sufficient water is present in the adobe, at whatever depth examined, to be 

 of use to plants, probably throughout the year. But, as will appear repeat- 

 edly in later portions of the study, the plants at the present time do not as 

 a rule actually penetrate to the deeper levels except only close by the river ; 

 for the most part, the roots are confined to the upper two meters of soil. 



Determinations of the water in the plain soils were made from October 

 3, 1907, to April II, 1908, and revealed the fact that the moisture content 

 of the soil at both depths taken for study, 15 and 30 cm., was greater most 

 of the time than 10 per cent its dry weight. The sample taken February 

 II showed a moisture content of 26.00 per cent at the depth of 15 cm. 

 From this date until April 1 1 the water content gradually fell away until 

 it became 10.9 and 11.6 per cent for the two depths respectively. 



