212 The Electrical Method of Soil Moisture Detenu! nation 



]"2" apart at shallow depths while in the case of depths about 0" the 

 minimum is found when a distance apart of about 18" is reached. 



The Effect of Depth. The following experiment was made to determine 

 the effect of depth unhampered by other considerations. A very dry 

 soil was sifted through a 3 mm. sieve to extract .stones and packed as 

 uniformly as possible into a pot. Two electrodes 4" apart were then 

 sunk to different depths in it and the resistance measured. It was thus 

 shown that tlie resistance between them was approximately halved by 

 sinking to a depth of 4". A repetition of the experiment after .saturating 

 the soil gave a result quite in accordance since in this case there must 

 have been some moisture gradient due to the action of gravity on the 

 water in the pores of the soil. The observations are sho\\Ti in Table III. 



Table 111. Effect of Depth in Uniform Soil. 



Depth of centre of Resistance in oiims 



elei^trode below , ■ -^ ^ 



.soil surface Dry Saturated 



1" 12.5.000 550 



2' 102,000 460 



3" 72,000 260 



4" 66,000 200 



Electrode Values. A further series of experiments was undertaken to 

 determine whether it was really pos.sible to assign values to the electrodes 

 on a system similar to that employed by Gardner. For this purpose 

 equal sized electrodes were set out at the four corners of a six-inch 

 square and at a depth of G". The distance apart agrees witli Briggs' 

 figure for the volume of soil concerned in the resistance measurements. 

 Observations were then taken in all po.ssible ways between them. In 

 Table IV the results are shown, — in the last two columns we have the 

 resistances observed and parallel with these the resistances as calculated 

 from the first four observed resistances by Gardner's method, e.g. the 

 electrodes being equal, if R^ is the resistance due to the electrode A, 

 Ri, that due to the electrode B, and R^ that due to the electrode C; and 

 if Rat,. Rac ^ifl Rbc are the resistances observed between A and B, A and C, 

 and B and C respectively, we have 



.'. -Ra + ^6 + ^c = Rab + Rac, 



whence R^ = R<^ + ^a^ -JR^ + ^q) ^ 



