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 
( Versuchs St., TH, 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 directly 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 Rendus, XLIX, 289) that a 
nitrogenous substance obtained by him from rotten dung 
and called fumie 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 humate of ammonia. 
