118 J. W. SHIVE AND B. E. LIVINGSTON 



presented, do observed values of this residue fall beloiv the cal- 

 culated value, which .they do in the case of the first three main 

 averages. But, if the third average were on the generalized 

 curve, it would depart but slightly from the value given by the 

 formula of the Washington writers. This suggests that an evap- 

 oration intensity of 1.8 cc. per hour might be expected, with this 

 soil, to give observed values about equal to the calculated one. 

 In connection with the discussion of figure 4 it was pointed out 

 that the observed soil moisture residue (average (1), series IV), 

 occurring with the lowest evaporation intensity is but little 

 greater than the calculated value; this being apparently related 

 to the nature of the soil used. In the present series, which em- 

 ployed a soil with still greater water holding power than that 

 possessed by the soil of series IV, this relation is still more pro- 

 nounced; the observed values for the lower evaporation inten- 

 sities here fall below the calculated value and those for the higher 

 intensities of evaporation approach the calculated value. There 

 can be no doubt that this feature is related to the nature of the 

 soil employed. 



GENERAL CONCLUSIONS 



The experiments above presented lead first to the substantia- 

 tion of the general principle, aheady established by Caldwell, 

 that the amount of water left in any given soil at permanent 

 wilting of plants rooted therein is a function of the intensity of 

 atmospheric evaporating power for the period during which 

 permanent wilting is attained. It thus appears, as Caldwell 

 has also emphasized, that the assumption by Briggs and Shantz 

 of universality in the application of their conclusions is not in 

 accord with empirical fact. To the statement of the last named 

 authors, that atmospheric conditions are practically without 

 influence upon the soil moisture residue at permanent wilting, 

 must be added a statement to the effect that the first proposition 

 can, from its very physical nature, be supposed to express the 

 truth only within some as yet unestablished range of atmospheric 

 and plant conditions, and that it does not at all apply in cases 

 where permanent wilting occurs under evaporation intensities 



