WILTING COEFFICIENT. 9 



temperature. It is important then to recognize that the moisture 

 in a soil is not nonavailable until it has been reduced to the moisture 

 content of air-dry soil. Since growth practically ceases when the 

 plants are in a permanently wilted state, any reduction of the soil 

 moisture below this point constitutes an actual deficit which must be 

 made up before the growth of any plant can be resumed. This 

 deficit may be brought about either by direct evaporation from the 

 soil to the air or by indirect evaporation through the plant when in a 

 wilted or a dying condition. 



The permanent wilting of the plant does not then mark any limiting 

 condition in the movement of water from the soil through the plant 

 to the air. It is simply a point on the moisture curve corresponding 

 to which the forces opposing the further removal of soil moisture 

 exceed the osmotic force exerted by the cell contents of the plant. 

 Under such conditions transpiration will exceed absorption — that is, 

 a part of the water transpired will be supplied from that stored in the 

 leaf tissues — and loss of turgor will result. It consequently appears 

 advisable to use a more specific term for the moisture content of the 

 soil corresponding to the wilting point of a plant, and we have 

 employed the term " wilting coefficient" in this sense in the present 

 paper. 



The wilting coefficient of a soil is then defined as the moisture con- 

 tent of the soil (expressed as a percentage of the diy weight) at the 

 time when the leaves of the plant growing in that soil first undergo 

 a permanent reduction in their moisture content as the result of a 

 deficiency in the soil-moisture supply. By a permanent reduc- 

 tion is meant a condition from which the leaves can not recover in 

 an approximately saturated atmosphere without the addition of 

 water to the soil. In the case of most plants wilting accompanies 

 this reduction of the water content of the leaves and is the criterion 

 used to determine the wilting coefficient of a soil for that plant. The 

 definition as stated is applicable also to those plants which, owing to 

 structural peculiarities, do not give visible evidence of a reduction of 

 the moisture content of the leaves. The wilting coefficient for such 

 plants is determined by means of the balancing method to be de- 

 scribed later. 



Two independent variables enter into the determination of the 

 wilting coefficient : (1) The moisture retentiveness of the soil used and 

 (2) the kind of plant used as an indicator. Any determination is 

 therefore to be considered as the wilting coefficient of the particular 

 soil employed for the particular plant used as an indicator. 



The nonavailable moisture must, then, strictly be considered as the 

 moisture content of the soil when in equilibrium with the air; while 

 the available moisture is of course represented by the difference 



230 



