CHEMISTEY OF SOILS 115 



land soils, which contain much organic matter and little 

 lime, is produced by the oxidation of iron dissolved out 

 by acids from ferruginous minerals in the soil. By .a 

 similar process ferric hydrate sometimes accumulates in 

 the subsoil of cultivated lands. It cements the particles 

 together and, as it gradually becomes dehydrated, forms 

 an impervious indurated layer called "iron pan" or 

 " moor-band pan." Pan formation may also be due to the 

 presence of compounds of manganese, but these are of 

 rarer occurrence. 



The degree of acidity can be determined by mixing a 

 quantity of soil with alkali solution of known strength, 

 and then estimating the amount of alkali neutralised by 

 the soil. In soils of neutral reaction the capacity for 

 neutralising acids can be estimated in a similar manner. 

 The soil is mixed with acid of known strength, and 

 *the amount of acid neutralised is then determined in 

 the usual way. This value when calculated as calcium 

 carbonate is often called "available lime," i.e., lime 

 available to neutralise acids. The experiment, however, 

 should be performed on a fresh sample, because it is found 

 that a soil which gives an acid reaction while moist will, 

 in some cases, after air drying, absorb a certain quantity 

 of acid. This makes it appear as if the soil contained some 

 available lime when, as a matter of fact, it does not. In 

 any case the result is only approximate and is affected 

 by the strength of the acid employed. 



The presence of lime in soils promotes oxidation. It 

 causes the organic matter to decay faster and favours the 

 production of nitric acid, with which it unites, forming 

 calcium nitrate. In the absence of a sufficient amount of 

 lime, the activity of the nitrifying organisms is reduced, 

 putrefactive decomposition takes place instead of oxida- 

 tion, and sulphides and other substances poisonous to 

 vegetation result. The question of nitrification will be 



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