66 THE SOIL SOLUTION 



origin in the fact that most soils when moistened and pressed 

 against blue litmus paper, redden it. This reddening may some- 

 times be due to the actual presence of some acid, or to dissolved 

 carbon dioxide, but is undoubtedly due in the majority of cases 

 to selective absorption. Litmus is a red dye of an acid-like 

 character, which forms a soluble blue salt with the ordinary 

 bases. But it has been shown that most soils are better absorb- 

 ents of bases than is paper, whereas paper is a better absorbent 

 of dye, speaking generally, than is a soil. Consequently when 

 moist soil is brought into contact with wetted blue litmus paper 

 the base is absorbed more readily by the soil, and the dye by the 

 paper, the latter therefore becoming reddened. 



The reddening of blue or "neutral" litmus paper can be accom- 

 plished with various absorbents. By pressing the litmus paper 

 between moistened wads of absorbent cotton the reddening can 

 be readily accomplishe,d usually in the course of ten minutes 

 to a half hour. That the phenomenon is not due to any adhering 

 acid on the cotton can be shown in the following way: A litmus 

 solution is carefully prepared so that there is a very small excess 

 of base present over that required to give the blue color. A 

 wad of absorbent cotton is carefully washed by repeatedly sous- 

 ing it in distilled water from which carbon dioxide has been 

 expelled by boiling. When the cotton has been thoroughly 

 washed, it is stirred thoroughly in a portion of distilled water, 

 free from carbon dioxide, then withdrawn by some appropriate 

 instrument and allowed to drain for a few minutes. The litmus 

 is added in fairly large quantity to the drainings, which should 

 then have a blue color. Again stir the cotton in the water, and 

 more or less quickly, depending on the amount and purity of 

 the litmus preparation as well as the quantity of cotton used, 

 the solution will become red. The only criterion for deter- 

 mining surely that a soil is acid, is to make an aqueous extract, 

 expel the dissolved carbon dioxide by boiling, or by passing 

 through the solution an inactive gas, such as nitrogen, and then 

 to test the reaction of the solution. Acid soils undoubtedly do 

 exist, but they are by no means common or wide-spread, and 

 are to be regarded as exceptional and abnormal. 



