SOIL ACIDITY— ITS NATURE, MEASUREMENT, 

 AND RELATION TO PLANT DISTRIBUTION. 1 



By Edgar T. Wherry, 

 United States Bureau of CJiemistry. 



[With 2 plates.] 

 INTRODUCTION. 



The studies described in the essays of which abstracts are here 

 brought together belong in a border-line class between well-defined 

 sciences; they represent the application of certain principles and 

 methods of chemistry, of physics, and of geology, to the solution of 

 problems in botany, in plant ecology, and incidentally in horticul- 

 ture. It has been the aim of the writer to sum up in relatively simple 

 language the principles of physical chemistry which bear on soil 

 acidity : to develop a method for measuring soil acidity in the field ; 

 and to apply this method to the study of the distribution and the 

 cultivation of native plants. 



SOIL ACIDITY. 2 



According to the almost universally accepted electrolytic-dissocia- 

 tion or ionization theory, many chemical compounds, under certain 

 conditions, exhibit dissociation into electrically charged portions, 

 known as ions. These may consist of single atoms or of groups of 

 atoms. Only ionization connected with the dissolving of substances 

 in water need concern us here. Compounds differ widely in the ex- 

 tent to which they are dissociated or ionized in dilute aqueous solu- 

 tion. Among inorganic compounds — acids, bases, and salts — many 

 are almost completely, others only partially, ionized. Of organic 

 compounds a few, especially acids, are markedly, a considerable num- 

 ber slightly, and many not appreciably ionized. 



Water itself dissociates into hydrogen-ion, made up of positively 

 charged hydrogen atoms, and accordingly symbolized by H + ; and 

 kydravyl-ion, made up of negatively charged hydroxyl (hydrogen 



1 The field work on which these essays are based has been curried on largely at the 

 writer's own expense, during vacation periods, but funds for certain of the trips were ob- 

 tained from the Bureau of Plant Industry, through Mr. Frederick V. Coville, botanist of 

 the Department of Agriculture. It is a pleasure to acknowledge herewith the aid received 

 In the preparation of these papers from Mr. Coville, Dr. E. Q. Adams of the Bureau of 

 Chemistry, and other colleagues. 



2 Abstracted from an essay entitled " Soil acidity and a field method for its measure- 

 ment." Ecology, 1, 160-173. 1920. 



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