242 LIME 



They may occur in the soil by natural changes which take 

 place particularly when the soil is cultivated and crops 

 grown. Or they may be the result of the application of 

 fertilizers, in which case soil acidity may be said to be 

 caused by artificial means. 



(a) Natural Acidity. — When organic matter in the soil 

 undergoes decomposition through bacterial action, organic 

 acids are some of the intermediate products. Decomposition 

 in the presence of air is an oxidation process, and with 

 thorough aeration results ultimately in the formation of 

 carbon dioxide, water, and inorganic salts resulting from the 

 combination of mineral elements with the organic matter. 

 Nitric acid of course is formed from nitrogenous compounds 

 and possibly mineral acids, but before complete oxidation 

 occurs, and especially in soils not thoroughly aerated, 

 organic acids are formed. These acids are sometimes fairly 

 simple, like acetic, or butyric, or oxalic, but more frequently 

 they are very complex, and many of them unknown. In 

 the absence of air, as in water-logged soils, acids are even 

 more commonly produced as a result of what may be called 

 intermolecular oxidation, where one compound is oxidized 

 at the expense of another. 



Another natural cause of acids is the removal of bases 

 from salts by plants and by certain hydrated compounds 

 in the soil. Plant root hairs exercise a so-called "selective 

 absorption" in taking up plant foods (Section 17). Sulphates 

 and chlorides, for example, are split up, the base element 

 entering the plant and the acid radicle being left in the soil 

 as an acid. Certain colloidal compounds like hydrated 

 silicates and humus produce this same result, the base 

 being adsorbed or perhaps chemically combined with the 

 colloid. It is possible that hydrolysis takes place previous 

 to the absortive phenomenon. This means the formation 

 of a hydroxide of the base, and an acid, thus : 



KCl + HOH = KOH + HC1. 



(6) Artificial Acidity. — The application of ammonium 

 sulphate results in the production of nitric acid from the 

 ammonia by the nitrifying bacteria, and of sulphuric acid 



