Nov. II, 1918 Soil Acidity as Affected by Moisture Conditions 327 



Truog (7) and Meacham found that the reaction of the plant sap of a 

 number of agricultural plants was always acid. As most plants will 

 grow in slightly acid soils and in slightly acid water cultures, it does not 

 seem necessary nor even possible that in such cases calcium is first 

 transformed into bicarbonate before it is assimilated. As a result of 

 varying the moisture conditions of acid soils it is very evident from the 

 results given in Tables II and III that chemical reactions take place as 

 different conditions of equilibrium due to moisture and aeration are 

 established. These reactions are in the nature of reduction, oxidation, 

 and hydrolysis as well as interactions following the law of mass action 

 between compounds which may be made chemically active. All of these 

 chemical changes in the soil cause variations in the degree of water- 

 soluble acidity, as shown by the ethyl-acetate reaction as well as of the 

 less-soluble acidity which is shown by the soluble-salt methods. These 

 changes would no doubt also affect the toxicity of acids and other com- 

 pounds in the soil. For instance, it may be readily seen that the oxida- 

 tion of iron in the soil from the ferrous to the ferric condition would 

 reduce toxicity as well as acidity. Acid marsh soils containing much 

 iron are unproductive until some time after they have been drained. 

 These soils when properly drained become quite red from oxidized iron, 

 in which condition they are much more productive. This fact is a matter 

 of common knowledge among observant farmers in such regions. It is 

 undoubtedly true that soil processes in which carbon dioxid is evolved 

 also produce material changes in soil acidity (5). Nitrification also 

 increases water-solubility acidity. These biological reactions, of course, 

 depend materially upon soil-moisture conditions. 



FACTORS AFFECTING SOIL ACIDITY 



Primarily soil acidity is due to an excess of acid-reacting compounds, 

 or, in other words, to a deficiency of bases. This deficiency of bases is 

 caused to a large extent by the leaching of the calcium and magnesium 

 in the drainage waters. A lesser factor is the removal of mineral bases 

 by crops. Under ordinary conditions of decay the carbonaceous and 

 nitrogenous matter of plants takes on an acid character, tending to 

 neutralize bases in the soil. The acidity of peat soils is very largely 

 organic, as shown by the fact that the ash of the most acid peat is basic 

 in reaction. In mineral soils there is an enormous excess of silicic oxid. 

 This silicic oxid when free is insoluble and inactive as an acid; but it is 

 potentially acid, and under humid conditions tends to form chemically- 

 active acid-reacting silicates of iron, aluminium, and manganese. The 

 degree of acidity of aluminium silicate is in proportion to the ratio of 

 silicic oxid and aluminium oxid and also to the amount of combined 

 water the silicate contains (j). Everything else being equal, the more 

 water there is in the silicate, the more active it is chemically and the 

 more acid it is in reaction. The measurable acidity of the organic matter 



