THE WILTING COEFFICIENT FOR PLANTS IN ALKALI SOILS. 1 



By Thomas H. Kearney, 2 Physiologist in Charge of Alkali and Drought Resistant 



Plant Investigations. 



INTRODUCTION. 



In a previous publication of this bureau 3 the wilting coefficient was 

 defined as " the moisture content of the soil (expressed as a percentage 

 of the dry weight) at the tune 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 reduction is meant a condition from which the leaves 

 can not recover in an approximately saturated atmosphere without 

 the addition of water to the soil." It follows that when the moisture 

 content of a given soil falls below the percentage representing the 

 wilting coefficient, the plants are unable to continue their growth. 

 In the present publication, therefore, the soil water in excess of the 

 wilting coefficient will be referred to as "moisture available for 

 growth." 



The term "alkali" is here used in the popular sense to designate 

 those alkali and alkali-earth metals (Na, K, Ca, Mg) which are most 

 frequently present in excess in the soils of arid regions, together with 

 the acid radicles (CI, S0 4 , C0 3 , HC0 3 ) with which these bases are 

 generally associated hi alkali soils. 



The experiment described in the following pages was designed to 

 ascertain whether an alkali content of the soil not sufficiently high to 

 cause evident injury to the plants would have the effect of raising 

 the wilting coefficient; in other words, whether the plants under 

 such conditions would reach a permanently wilted condition before 

 they had exhausted the soil moisture to a point corresponding to the 

 wilting coefficient as calculated from the moisture equivalent for the 

 type of soil used. It was assumed that if this proved to be the case 

 it would be possible to determine the minimum concentration at 

 which alkali of a given composition becomes the limiting factor for 

 plant growth from the inability of the plants to reduce the soil 

 moisture to the calculated wilting coefficient. 



For this purpose it was decided to grow seedling plants of wheat 

 in a series of soil mixtures having a gradually increasing salt con- 

 tent. Each soil was to contain at the outset the same amount of 

 water available for growth — i. e., the same percentage of moisture 

 .above the calculated wilting coefficient — and no additional water 



1 Issued Jan. 4, 1913. 



2 Dr. L. J. Briggs, Physicist in Charge of Biophysical Investigations, Bureau of Plant Industry, cooper- 

 ated by having the moisture equivalents determined and the soil extracts prepared in his laboratory. The 

 chemical analyses of the extracts were made by Mr. J. F. Breazeale, of the Bureau of Chemistry. 



3 Briggs, L. J., and Shantz, H. L. "The wilting coefficient for different plants and its indirect determi- 

 nation," U. S. Department of Agriculture, Bureau of Plant Industry, Bulletin 230, 1912. 



71357— Cir. 109—13 2 17 



