ARID PORTIONS OF SOUTH AUSTRALIA. 25 



stricted movement of the air within it, however, it contains less oxygen, 

 but a greater percentage of nitrogen and carbon dioxide. Where 

 such conditions obtain, molecular and not molar gas-movement takes 

 place, gaseous exchange is relatively slow, and soil aeration is least 

 favorable for aerobic organisms. So far as the soils of arid regions 

 are concerned, possibly the most usual cause of poor aeration, in 

 both meanings as above presented, lies in the puddling following rains. 

 Under such conditions the surface is compacted, the soil spaces are 

 filled with water, and mass air-movement ceases. Where this is ac- 

 companied by relatively high soil-temperatures the amount of oxygen 

 in the soil atmosphere rapidly decreases and that of carbon dioxide 

 rapidly increases, following the respirational activities of soil organisms 

 of all kinds. There follow differential reactions by which the course 

 of development may be and in certain species certainly is determined. 

 Under extreme conditions of poor aeration such may become a factor 

 limiting the survival of a species in a given habitat. 



Temperature of the Soil. 



Few data on the temperature of the soil, particularly as to the more 

 arid regions of Australia, are to be had. However, the general fea- 

 tures of the course of the soil-temperature in arid regions are fairly 

 well known. The temperature of the soil varies with a variation in 

 the physical character of the soil, with its moisture-content, and with 

 the depth. Possibly no environmental factor is of greater importance 

 to plant life than this one. 



Observations on the temperature of the soil, made at the Desert 

 Laboratory (Cannon, 1911 : 20),:will illustrate the course of the tempera- 

 ture of the soil in a semi-arid climate. The records referred to relate 

 to the temperature taken at two depths, 15 and 30 cm., by means of 

 thermographs. At the lesser depth the daily variation in temperature 

 is about 8° to 12° F. and the maximum annual variation is about 69° F. 

 The period of maximum temperature coincides with that of the highest 

 summer temperature and immediately precedes the rains of that sea- 

 son. With the coming of the rains of summer the soil-temperature 

 immediately falls and it continues to drop gradually until somewhat 

 past midwinter, when the upward movement begins. The rise is 

 gradual until the last of March, when it is somewhat accelerated, and by 

 May the temperature of the soil is nearly that of midsummer. 



There are, therefore, approximately 3 months each year, the arid 

 foresummer, in which high soil-temperatures occur at a depth of 

 15 cm. The maximum temperature, depth 15 cm., observed was 

 105° F. (40.56° C.) and the minimum temperature at that depth was 

 found to be 34° F. (1.11° C). The temperatures of the soil at a depth 

 of 30 cm. have many features characteristically different from those of 

 the lesser depth. Thus the daily range in temperature is 2° to 4° F. 

 and the annual variation is about 30° F. The ma^iimum temperature 



