402 STATE BOARD OP AGRICULTURE 



each reading to flush out engaged soil particles and allow contact with 

 the soil solution with a fresh portion of the potassium chloride. 



Calibration of the potentiometer consisted in setting the indicators 

 to read the same voltage as given for the standard cell and adjusting 

 resistance R until no deflection Tvas observed in the galvanometer with 

 the cell circuit momentarily closed. Rough adjustment was first made 

 with a normal calomel cell, and final calibration with the Weston 

 Standard Cell. The potentiometer then read in volts and was adjusted 

 before each titration. After starting the apparatus for each determina- 

 tion, potential readings were taken at five minute intervals until the 

 change in potential over the last reading was constant within one 

 millivolt. Fifteen to fortj^-five minutes time was usually required to 

 complete a determination. Voltage readings were converted to pH 

 factors by means of the Schmidt and Hoagland conversion tables (6). 

 The author is indebted chiefly to Clark (1) and Hildebrand (3) for the 

 theory and applications of the hydrogen electrode. 



SOILS AND REAGENTS 



The neutralizing values of four acid soils — a medium sand, sandy 

 loam, silt loam and clay loam — were studied using the hydrogen elec- 

 trode, in the manner described, as an end point indicator. These soils 

 were air dried and passed through a two millimeter mesh sieve. Re- 

 sults of this experimentation are shown in the curves, Figs. 2 to 9 in- 

 clusive. As indicated under the heading. Method of Operation, the 

 titrations were set up in a series of units in order that pH readings 

 might be taken at regular intervals of time. The reagents used were 

 Ca(0H)2, ALClg and HCl, and these reagents were added in O.IN 

 equivalents so that all the curves might be plotted on the same scale. 

 Curves are shown in two groups containing four figures in each group, 

 Figs. 2, 3, 4 and 5 comprise the first group and Figs. G, 7, 8 and 9 the 

 second. All alkali titrations were made with Ca(0H)2 and the second 

 group of alkali titration curves were duplications of the first group. 

 The acid titrations shown in the first group of four figures were made 

 with Alo Cle, and in the second group with HCl. 



