ABSORPTION BY SOILS 65 



of the salts in question, show conclusively that the absorption 

 phenomena taking place in the soil are in harmony with the direct 

 solubility effects in tending to produce and maintain a solution 

 of a normal concentration as regards those constituents which 

 it happens are also derived from the soil minerals. 1 It is an 

 interesting coincidence that nitric acid (in combination with 

 various bases of course) is very little absorbed by most soils, and 

 does vary in concentration, not only in different soils but in the 

 same soil, between wide limits, and within short intervals of 

 time. 2 The nitrates of the soil are not derived from minerals, 

 and should more properly be considered with the organic con- 

 stituents of the soil solution. 



An important application of these views concerning absorp- 

 tion arises in connection with certain widespread notions con- 

 cerning soil acidity. There is a popular belief that most soils 

 are acid, that the soil solution contains some free acid, mineral 

 or organic, other than dissolved carbon dioxide, and that a 

 neutral or alkaline solution is necessary to the successful pro- 

 duction of most of our crops. This belief is, however, unwar- 

 ranted, for the vast majority of soils yield an aqueous extract 

 which is alkaline when boiled to expel carbon dioxide, and some 

 of our crops, for instance wheat, seem to thrive better in a 

 slightly acid medium. This popular fallacy seems to have its 



*An extreme case is worth citing in this connection. Mr. W. H. 

 Heileman in studying the influence of various kinds of alkali upon plant 

 growth, added from 3-4 per cent, of sodium carbonate to soils known to 

 be otherwise free from alkali. Wheat seedlings grown in the soils so 

 treated showed no ill effects from the added salt. When distilled water 

 was percolated slowly through the soils, or shaken up w'th them, the 

 resulting solution contained the merest traces of the alkali. 



The ordinary method of determining the lime requirement of a soil 

 by adding lime water until the solution shows an alkaline reaction, is 

 another obvious absorption phenomenon, and is not dependent, as popu- 

 larly supposed, upon the presence of acids in the soil. Soils which by 

 no possibility could contain any free acid, frequently absorb very large 

 amounts of lime in this manner. 



2 Usually, in the growing season, the soil solution has a much higher 

 concentration with respect to nitrates in the morning than it has in the 

 evening. 



