254 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



ing the water used for mixing with the soil. If calcium bicarbonate 

 is present in this water, the soil acidity will be diminished, while if 

 neutral salts, such as sodium chloride, and especially calcium sul- 

 phate, are present in any considerable amount, the acidity will be 

 appreciably increased. The former effect is a direct neutralization; 

 but the latter is due to the fact that the clay and the humus in the 

 soil adsorb the basic elements from neutral salts and set the acid 

 free. In the laboratory distilled water can be used, and to attain 

 the greatest precision air freed from carbon dioxide can be blown 

 through it until it reacts quite neutral; when one is traveling, dis- 

 tilled water can usually be purchased from a drug store and will 

 give satisfactory results without special purification. In the wilds 

 the best that can be done is to obtain spring or well water rising 

 through rocks as free as possible from soluble constituents — such 

 rocks as sandstone, shale, or schist. In calcareous regions it may 

 be necessary to test waters from one source after another until a 

 sample is found which reacts neutral — is colored green by a drop 

 of bromthymol blue indicator — and to arrange the trip so that the 

 water supply can from time to time be replenished from this source. 

 With these points in mind, the following approximate directions 

 have been drawn up : 



A sample of soil a gram or two in, weight is shaken from living 

 roots into an empty vial, and 5 cubic centimeters of the most nearly 

 neutral and salt- free water available is added, the vial being shaken 

 well to insure complete mixing. After the soil and water are 

 thoroughly mixed, the solid matter may be compacted with a glass 

 rod or a stick, and the vial then supported at an angle of 45° and 

 allowed to stand until the bulk of the suspended matter has settled. 

 The more or less clear liquid is then decanted or pipetted off into 

 another vial, 9 a drop or two of bromthymol blue or one of the other 

 indicators, the color changes of which occur near the neutral portion 

 of the table, are added, and the color assumed is noted. If either 

 of the extreme colors is shown, the process is repeated with the indi- 

 cator 10 whose color changes come next in the corresponding direc- 



9 Instead of a vial a " porcelain plate with, cavities, for color reactions," sold by dealers 

 in chemical apparatus or in artists' supplies, may be used. The color changes can be seen 

 very clearly in such a plate, but great care must be taken that too much indicator solution 

 is not added. It is well to place a tiny drop of the indicator in one of the cavities, and 

 then to add successive portions of the soil extract until the color can be barely seen. 



10 In some soils, because of the colloid-matter acting differently on different indicators, 

 the reactions indicated may not agree ; for instance, bromcresol purple may show a color 

 corresponding to specific acidity 10, and methyl red specific acidity 100 ; in this case the 

 mean of the two values, 30 + , is used. It should also be noted that the color changes are 

 much more gradual than it was practicable to show on the chart, so that intermediate hues 

 between those shown are often obtained, leading to reaction values between those tabu- 

 lated. 



