T. Deighton 229 



Summary. 



In this paper an examination is made of the processes operative and 

 the limits of accuracy of the electrical method of determining soil moisture. 



The resistance over a small plot is found to vary under similar 

 conditions and it is concluded that these differences are mo.st probably 

 due to actual differences in moisture or other factors. 



The effect of the distance apart of the electrodes is investigated and 

 a probability of a minimum resistance between two electrodes being 

 observed under certain conditions is indicated. 



The use of electrode values in computing soil resistances is discussed 

 and criticised. 



The path of the current in the soil is investigated mathematically 

 and it is shown that the results obtained accord well with the observed 

 facts. 



It is concluded that the method gives the mean water content of a 

 volume of soil somewhat greater than a sphere whose poles are the 

 electrodes. The practical limits of the method are indicated. 



Certain resistance-moisture curves obtained in the laboratory are 

 discussed and it is concluded that while at water contents above 

 10 per cent, the relation found by the American investigators holds good 

 — viz. that the resistance varies inversely as the square of the moisture 

 content; at lower water contents than this one and possibly two dis- 

 continuities appear in the curve. 



These discontinuities are reversed in the case of artificial mi.xtures 

 not containing colloids. 



A tentative explanation of these phenomena is given. 



REFERENCES. 



(1) Whitney, M. Some Physical Properties of Soil in their Relation to Moisture 



and Crop Distribution. U.S. Dept. Agric, Weather Bureau Bull. No. 4, 1892. 



(2) Whitney, M. and Means, T. H. An Electrical Method of determining the 



Soluble Salt Content of Soils. U.S. Dept. Agric, Dimsion of Soils, Bull. 

 No. 8, 1897. 



(3) Gardner, F. D. The Electrical Method of Moisture Determination in Soils; 



Results and Modifications in 1897. U.S. Dept. Agric, Division of Soils, 

 Bull. No. 12, 1898. 



(4) Beigqs, L. J. Electrical Instruments for Determining the Moisture, Tem- 



perature, and Soluble Salt Content of Soils. U.S. Dept. Agric, Division of 

 Soils, Bull. No. 15, 1899. 



