ARID PORTIONS OF SOUTH AUSTRALIA. 23 



Tempebatube, Moistuee, and Aebation Conditions of the Soil. 



The course of the moisture, aeration, and temperature conditions of the 

 soil are of very great biolgical importance, but, unfortunately, so far as 

 Australia is concerned, only meager data are to be had respecting them. 

 From studies made elsewhere (which are also few) something of their 

 relation to the general problems with which this study in part deals may 

 be drawn; and the interest here Ues mainly in the results touching the 

 soils of r^ons having a small rainfall. As is weU known, the three 

 conditions above mentioned are intimately related and it may be 

 remarked that in consequence a modification of one brings changes in 

 the rest. It is also possibly true that, as to the moisture of the soil 

 and its temperature, the maximum of variability finds its apex in dry 

 climates. 



The moisture conditions of the soil are dependent on a great variety 

 of factors, among which may be mentioned the amount of rainfall, the 

 physical nature of the soil, atmospheric conditions relative to evapora- 

 tion, and the plant cover. 



The amount of water which a given soil is capable of holding is 

 related to the physical nature of the soil and according to Briggs and 

 Shantz (1912:31) varies from 23.2 to 69.5 per cent of the dry weight of 

 the soil. The smaller amount is that retained in coarse sand and the 

 larger amount is that retained in clay loam, in both instances in op- 

 position to the force of gravity when free water drainage is provided. 

 Not all of the water of such saturated soils, however, is available for the 

 use of plants. Thus the same authors (1911:217) show that, as con- 

 cerned the species experimented with, the amount of water possible of 

 absorption previous to wilting varied with the character of the soil, but 

 was considerably less than the maximum water-content of the soil. 

 Thus, in fine sand the plant used, Kubanka wheat, absorbed 97.01 per 

 cent; in fine sandy loam it absorbed 90.34 per cent; in clay loam it 

 absorbed 83.7 per cent of the water held by these soils when in a good 

 state of tilth. At the time of wilting, therefore, there is in the soil a 

 certain water-residue which varies with the nature of the soil. 



It would be of interest to know for how long a period, in dry central 

 Australia especially, there is sufl&cient water in the soils for the use of 

 plants. In Southern Arizona some attention has been paid to this 

 phase of the problem. At Tucson, for example, Livingston (1906:72) 

 has found that at a depth approximating 0.5 meter there is possibly 

 always sufficient moisture for absorption by roots. At least the upper 

 soils, on the other hand, are air-dry in the arid foresummer, when they 

 may contain not over 6.5 per cent of their dry weight of water (Shreve, 

 1914 : 21) . The soil referred to is a fine, brown clay and, from the work 

 of Briggs and Shantz, it would not be expected that the plants could 

 extract from it more than 85 to 90 per cent of its water-content, leaving 

 a non-available moisture content of 10 to 15 per cent. Therefore, the 



