47 



did not give a complete picture of the continuous processes operat- 

 ing between soil and its moisture content when the latter varied 

 over wide limits. Endeavours were therefore made to link up the 

 sub-divisions by means of cross-relations between the variables, 

 but they were mainly applicable over some small range of moisture 

 content or to some approximate equilibrium values. 



The development of the study of colloids rendered it possible 

 to consider the relations between soil and its moisture content by 

 an alternative hypothesis which would stress their continuous 

 nature. It is now considered that the soil particles are coated with 

 a colloidal complex, derix ed from the v\ny and the organic matter. 

 In the concluding section of the paper a number of investigations 

 are considered and interpreted on this h\ pothesis, and some of the 

 most promising future lines of work are indicated. 



XXVI. B. A. Keen. '" The Physical Investigation of 



SoiV Science Progress,' 1921. \^ol. XV. pp. 

 574-589. 



This is a general account of the st ope of physical science in 

 investigations on soil. It deals with the dimensions of soil parti- 

 cles and the manner of their arrangement in the soil, the tempera- 

 ture, moisture, and atmospheric relations in the soil, and indicates 

 also the great need for research on methods of cultivation and the 

 effect on the soil of the form of implement used, in view of the 

 important changes in farming practi<x" bnnight about by the intro- 

 duction of the tractor. 



XXVII. B. A. Keen and E. J. Russell. '' The Factors 



determining Soil Temperature." Journal of 

 Agricultural Science, 1921. Vol. XL 



The purpose of this paper is to discuss the factors influencing 

 soil temperature and the extent to which other measurements (air, 

 temperature, hours of sunshine, etc.) can be utilised in cases where 

 direct determinations of soil temperature are not made. 



An analysis has been made of one year's records given by a 

 special recording thermometer embedded at the 6in. depth in bare 

 soil, together with continuous records of air temperature and 

 hours of sunshine ; these ha\e been supplemented bv daily readings 

 of rainfall, radiation, and soil temperature at the 12in. depth. The 

 extent of the temperature rise at the 6in. depth is largely deter- 

 mined by the amount of solar radiation reaching the earth's sur- 

 face (correlation co-efficient .877^.009). As would be expected, 

 the hours of sunshine also provide a good measure of this radia- 

 tion. 



The maximum temperature at the 6in. depth during the 

 summer months is about equal to that of the air, and the minimum 

 temperature is from 6° — 8° C. higher than the air minimum. 



During this period, the conditions therefore resemble those in 

 a 20« C. incubator. 



In the winter months the minimum temperature at the 

 6in. depth is usually about 2° — 3° C. higher than that 

 in the air, and the maximum temperature is a little below the 

 "I'T^^imum air temperature. The effect of rainfall is generally to 

 diminish the rise of temperature, but the relation is by no means 

 exact. No evidence was found supporting the belief that spring 



