Studies in Soil Physics. 1 1 7 



in a given soil. It ought to be obvious that this quantity will 

 depend primarily, not on the plant nor on the soil, but on the 

 evaporating power of the air. But strange to say this apparently 

 obvious fact has been either overlooked or disregarded by nine- 

 tenths of the workers who lia\e made wilting point determina- 

 tions. '■■' The temperature and humidity are almost never stated 

 and in most cases thev were apparently neither measured nor 

 controlled. Another matter almost alwa\s neglected in deter- 

 minations of the wilting point is the rate of movement of water 

 through the soil. The plant may wilt, not because of a lack of 

 water, but because the water is unable to move through the 

 soil and to tlie plant as rapidlv as it is being removed by trans- 

 piration. Nor will this possible error be removed by determining 

 (as is usually done) whether or not the wilting be permanent. 

 For permanent wilting and death will result from any continued 

 excess of transpiration over absorption, whether the deficiency 

 of the latter be due to total lack of water or to a too slow supph-. 



To have any meaning either for plant physiologist or soil 

 physicist the wilting point must be determined with reference 

 to the evaporating power of the air. Ivven so determined, local 

 and occasional experiments are of little value and geneial ex- 

 periments, though potentially very useful, would be exceedingly 

 tedious and even more costly. If experiments are to be made 

 in this field they had better l)e u])on the rate of transjjiration 

 which is fatal with full water supply, upon the minimum rates 

 of absorption and transjiiration which are jjossible, or, best of 

 all, upon the amount of water which will be transpired (and 

 hence extracted from the soil) l^y a given ])lant in air of a given 

 evaporating i)ower. 



Available and Non- Available Water. The concej:)t of non- 

 availaljle water is closely related to that of the wilting point. 

 Non-availal)le water is usually defmecl as that percent of water 

 which still remains in the soil after the ])lant lias wilted. As 

 so determined land I am aware of no determinations made other- 

 wise) it is subject to all the errors which affect the determination 

 of the wilting jjoint and wliicli lia\e just been discussed. Hut 

 further than this there is no real assurance tliat there is any sucli 

 thing as non-a\ailable water. Nine times ottt of ten a plant dies 



♦There are, of course, a few physiologists who have given full alteiilimi t<i ihc imporumce 

 of evaporation in tliis connection. The papers of B. U. Livingston are notable examples 



