584 SCIENCE PROGRESS 



removed is determined, and defined as the Moisture Equivalent. 

 Briggs considers that this value gives a quick and reliable 

 basis for quantitative comparisons of soils superior to the 

 slower and more cumbersome methods, such as mechanical 

 analysis, and certainly of more value than measurements 

 such as the water-retaining capacity, which expresses the 

 amount of water retained by a previously saturated soil column 

 of given dimensions, after thorough draining under the force 

 of gravity. 



Concurrently with the development of these various co- 

 efficients of soil moisture, the feeling arose that the relations 

 of the soil to its moisture content could not be completely 

 expressed by dividing up the soil moisture into various divisions 

 and by the establishment of certain equilibrium coefficients. 

 Endeavours were made to link up in some way the various 

 soil constants, and a number of cross-relations between the 

 variables were experimentally worked out. Briggs gave a 

 series of ratios by which the wilting coefficient of a soil could 

 be calculated from any one of the following constants : 

 moisture equivalent, hygroscopic coefficient, water-holding 

 capacity, mechanical analysis. Alway has given a further 

 series of relations in a number of interesting papers, and has 

 also paid especial attention to the bearing of these constants 

 on the speed at which water moves from place to place in the 

 soil. His experiments were done with care, but the data 

 obtained only permitted of broad generalisations* 



It appears, from a critical survey of the literature, that the 

 physical significance of these various equilibrium points and 

 divisions of soil moisture has been stressed too much. Where 

 plants are growing in soil under natural conditions, physiological 

 and physical factors tend towards a kind of balance ; but, so 

 far as the physical relations between the soil and its water 

 content are concerned, it is necessary to regard the system 

 soil-soil moisture, as a whole, and to picture the physical 

 forces as acting in a continuous manner over a wide range 

 of moisture content. The experiments of the writer on the 

 evaporation of water from a thin layer of soil under controlled 

 laboratory conditions show that the course of the evaporation 

 can be expressed by an equation over a wide range of moisture 

 content, ranging, in the case of an average loamy soil, from 

 25 per cent, down to 2 per cent. — a range which includes all the 

 equilibrium points described above. The experimental curves 

 are smooth, and show no sign of any abrupt change of direction 

 in passing through the values of the soil constants discussed. 



Furthermore, the organic material in the soil is not mainly 

 responsible for the relationship, because a considerable portion 

 of it can be removed without affecting the characteristic 



