258 HOW CROPS FEED. 



ficient time this oxidation extends so far as to leave the 

 board loose upon the nail, as may often be seen on old, 

 unpainted wooden buildings. Direct experiments by Knop 

 ( Yersuchs St., Ill, 228) strongly indicate that ammonia is 

 oxidized by the agency of iron in the soil. 



b. The organic matters of the soil, either of vegetable 

 or animal origin, which contain nitrogen, suffer oxidation 

 by directly combining with ordinary oxygen. 



As we shall presently see, nitrates cannot be formed in 

 the rapid or putrefactive stages of decay, but only later, 

 when the process proceeds so slowly that oxygen is in large 

 excess. When the organic matters are so largely dilut- 

 ed or divided by the earthy parts of the soil that oxygen 

 greatly preponderates, it is probable that the nitrogen of 

 the organic bodies is directlj^ oxidized to nitric acid. 

 Otherwise ammonia is first formed, which is converted in- 

 to nitrates at a subsequent slower stage of decay. 



Nitrogenous organic matters may perhaps likewise yield 

 nitric acid when oxidized by the intervention of hydrated 

 sesquioxide of iron, or other reducible mineral compounds. 

 Thenard mentions {Comptes Mendus, XLIX, 289) that a 

 nitrogenous substance obtained by him from rotten dung 

 and called fumic acid* when mixed with carbonate of 

 lime, sesquioxide of iron and water, and kept hot for 15 

 days in a closed vessel, was oxidized with formation of 

 carbonic acid and noticeable quantities of nitric acid, the 

 sesquioxide being at the same time reduced to protoxide. 

 The various sulphates that occur in soils, especially sul- 

 phate of lime (gypsum, plaster), and sulphate of iron 

 (copperas), may not unlikely act in the same manner to 

 convey oxygen to oxidable substances. These sulphates, 

 in exclusion of air, become reduced by organic matters to 

 sulphides. This often happens in deep fissures in the 

 earth, and causes many natural waters to come to the sur- 



According to Mulder, impure liumate of ammoaia. 



