76 THE SOU, SOLUTION 



ure, and making the further very probable assumption that the 

 capillary water in the soil (&) is never less than the fly-off or 

 the water that evaporates during rain (/), it follows from the 

 equations given that the capillary water is at least 35 per cent, 

 of the rainfall. If we assume the lower value for the drainage, 

 then the capillary water is at' least 40 per cent, of the rainfall, 

 and if we assume the extreme case that the fly-off is practically 

 negligible the capillary water becomes 80 per cent, of the rain- 

 fall. It appears, therefore, that in all probability the proportion 

 of the cut-off water which returns to the surface as film water 

 or capillary water is always greater, and generally much greater, 

 than the portion which seeps through the soil to join the run- 

 off. 



From the available data, it appears that the average concern- 

 tration of the run-off waters of the United States is about 1.8 

 parts per million of potassium (K) and about 0.6 part per mil- 

 lion of phosphoric acid (PC^), 1 while the concentration of the 

 capillary groundwater is some ten or twelve times greater. But 

 even if these concentrations were the same, it is altogether prob- 

 able that very much the greater part of the mineral plant nu- 

 trients dissolved by meteoric waters is continually, if slowly, 

 moving towards the surface of the soil. 



The average rainfall of the United States may be taken as ap- 

 proximately 30 inches. 2 If it be assumed that the discharge 

 into the sea is 25 per qent., then the capillary cut-off water is 

 at least 37.5, and probably nearer 70 per cent, of the rainfall. 

 King's experimental work 3 indicates that the higher figure is 

 much nearer the truth. Computing from the concentrations 

 just cited, with the equations given above, it is found that ap- 

 proximately 3,500,000 tons of potassium (K) and 1,200,000 tons 



1 Estimated from data in Bull. No. 330, U. S. Geological Survey, The 

 data of geochemistry, by Frank Wigglesworth Clarke, 1908, pp. 53-90. 



* The latest authoritative statement is that the average annual rain- 

 fall of the United States is 29.4 inches ; see : Water Resources, by W. J. 

 McGee, vol. i, pp. 39-49, and Distribution of rainfall, by Henry Gannett, 

 vol. 2, pp. 10-12, Report of the National conservation commission, Senate 

 doc. No. 676, 6oth Congress, 2d session, 1909. 



* King : loc. cit, p. 85. 



