wherry: determining soil acidity 221 



In accordance with the above considerations, a simplification 

 of the procedure previously recommended has been adopted; 

 modifications may still be desirable in special cases. But before 

 giving the directions, a word should be added concerning the 

 watei 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 

 sulfate, 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 neu- 

 tral 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, distilled water can usually 

 be purchased from a drug store, and will give satisfactory re- 

 sults 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 sand- 

 stone, 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 brom- 

 thymol 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 direc- 

 tions have been drawn up : 



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

 roots into an empty vial, and 5 cc. 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 soHd matter may be compacted with a 

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



' Gillespie and Wise, op. cit. 



10 This is, of course, the reason that the so-called lime-requirement methods in 

 which a neutral salt solution is mixed with a soil yield so much higher results than 

 can be obtained by direct titration of water extracts of the soils. 



