ORGANIC ACIDS IN THE SOIL 163 



This reaction can be considered as representative not only of the 

 solvent effects following nitrification but also of the development 

 of increased acidity through the absorption or transformation of a 

 cation of an inorganic salt. By removing or changing the ammo- 

 nium ion, the sulfate radical takes on hydrogen, which gives to it as 

 great solvent powers as the nitric acid formed from the oxidation 

 of the ammonia. 



When urea is added to the soil, the reaction of the soil is first 

 made alkaline, by the transformation of the urea to ammonium 

 carbonate, then acid following the oxidation of the ammonia to 

 nitric acid: 



CO(NH2)2 + 2H2O = (NH4)2C03 



(NH4)2C03 + 4O2 = 2HNO3 + CO2 + 3H2O 



These general considerations of the solvent action of nitric 

 acid on soil minerals are equally applicable to sulfuric acid, with 

 the exception that sulfuric acid generally appears in soils in smaller 

 amounts, and consequently its effect is of less importance. 



Organic Acids in the Soil. — In addition to the inorganic acids, 

 various microbes produce in the soil organic acids, which can play 

 the same function as the inorganic acids but are weaker than nitric 

 or sulfuric acids although stronger than carbonic acid. Among the 

 organic acids produced by soil bacteria, the following may be 

 mentioned: lactic, butyric, acetic, propionic, and valerianic. 

 Fungi produce the following acids: gluconic, citric, oxalic, fumaric, 

 and succinic. Various acids may be formed from carbohydrates 

 under suitable conditions : 



C6H12O6 = C4HSO2 + 2CO2 + 2H2 (anaerobic bacteria) 



Butyric 

 acid 



CeHioOe + 30 = CsHsOt + 2H2O (fungi) 



Citric 

 acid 



CeHiaOe + 90 = 3C2H2O4 + SHoO (fungi) 



Oxalic 

 acid 



C6H12OG = 2C3HCO3 (anaerobic bacteria) 



Lactic acid 



