220 The Electrical Method of Soil Moisture Determination 



The observation at 1|" is doubtless aSected by the nearer a^jproach to 

 a three dimensional condition and the fact that the electrodes were not 

 small in comparison with the mdth of the field thus restricted. The 

 correction for evaporation was made by interpolation from the original 

 and a final observation at 1\". We see then that no grave error is likely 

 to have been introduced by the assumptions made at the beginning of 

 this section. 



This being so we are probably not far wrouj^ in inter])reting our soil 

 phenomena in terms of the calculation made for the three dimensional 

 case, more especially as it explains all the phenomena observed. Each 

 electrode may obviously still be considered to possess a definite re- 

 sistance value of its own at any stated distance, and calculations based 

 on these values will still yield approximately correct results but slight 

 differences will doubtless appear according to the direction in which the 

 second electrode lies. Since with increasing distance apart local differ- 

 ences in specific resistance will tend to cancel out in their effects it is 

 clear why a better agreement was obtained in the hexagon than in the 

 square. The result of the pot experiment is explainable on either theory, 

 but it is noteworthy that the theory agrees with the halving of the re- 

 sistance at 4" depth. Calculation actually shows an external residue of 

 current of about 8 per cent, but since the surface is plane it follows that 

 a considerable portion of this would, in the circumstances of the experi- 

 ment, be included. The steady but slow increase of resistance as the 

 electrodes are moved further apart beyond 18" is explained by the 

 cutting off by the air layer of regions of increasing current density. 

 Combining this with the fact of the existence of a moisture gradient in 

 the soil we obtain an explanation of the minima observed in the plot 

 trials (Table II) since, starting with the electrodes in close proximity 

 and gradually moving them apart the fir.st effect is to cause a fall in 

 resistance by causing the current to dip down into lower moister layers. 

 After a time however, with increa.sing distance, the non-conducting air 

 laver above begins to cut into regions of greater and greater current 

 density till the resistance introduced by this more than compen.sates 

 for the other effect. In the second series of experiments in Table V there 

 is no minimum. These were made at the end of the long drought of 1921 

 and it appears probable that the second effect overbalanced the first 

 from the beginning, owing to the moisture gradient in the soil being 

 less steep. An explanation is also afforded of the increase of the distance 

 apart at minimum resistance with increasing depth since it will in this 

 case be necessar}' to move the electrodes further apart before the air 



