GILLESPIE: HYDROGEN-ION CONCENTRATION IN SOILS 7 



dip; B, the direction angle, is read off directly on the graduated 

 circle; OC (= tan C) is the apparent dip; thus the line of inter- 

 section of a stratum, dipping at an angle of 35° (OA), with a ver- 

 tical section which includes an angle 44° 30' (B) with the line of 

 strike of the bed, has a slope angle (OC) of 26.1°. In figure 16 

 the angles A and C are greater than 45° and the numbers in 

 parentheses on the scales are used; thus on a vertical section 

 including an angle of 44° 30' (B) with the line of strike of a bed 

 dipping at an angle of 72° (OA) the apparent dip is 65.1°. In 

 figure lc the angle A is greater than 45° while C is less than 45°; 

 in this case the numbers in parentheses on the scale of the rotat- 

 ing arm are used, while for the angle C the unbracketed num- 

 bers along OC are read; thus the trace of a bed, dipping at 60°, 

 on a vertical section, which includes an angle of 24° with the 

 line of strike of the bed, has a slope angle of 35.2°. 



CHEMISTRY. — The reaction of soil and measurements of hydro- 

 gen-ion concentration. L. J. Gillespie, Bureau of Plant 

 Industry. (Communicated by Oswald Schreiner.) 



The reaction of soil is conceded to be of great influence upon 

 soil fertility. Certain plants seem to require a certain degree of 

 acidity, and may flourish at an acidity sufficient to be very 

 harmful to others. That excessive acidity is a common cause 

 of infertility n) long-cultivated soils is generally recognized. On 

 the other hand, sodium carbonate frequently occurs in the alkali 

 soils of arid lands and imparts an excessive alkalinity. The study 

 of the reaction of soil is therefore not only of considerable scientific 

 but also of great practical importance. 



Let us consider briefly the case presented by a sour soil. Two 

 questions are of especial interest: (1) What is the quantity of 

 acid substance? and (2) what is the intensity of the acidity? 

 The amount of liming or other ameliorative treatment required 

 to neutralize the acidity will depend upon the quantity of acid 

 substance. It seems probable, on the other hand, that charac- 

 teristic effects of acidity upon soil fertility will be more clearly 

 correlated with the intensity of acidity than with the quantity 



