THE NITRIC ACID OF THE SOIL. 267 
* 
smaller ‘doses of lime or alkali than those employed by 
Boussingault would have been found promotive of nitri- 
fication, especially after the lapse of time sufficient to 
allow the first rapid decomposition to subside, for then 
we should expect that its presence would favor slow oxida- 
tion. This view is in accordance with the idea, universally 
received, that lime, or alkali of some sort, is an indispensa- 
ble ingredient of artificial niter-beds. The point is one 
. upon which further investigations are needed. 
d. Free oxygen, i. e., atmospheric air, and the porosity 
of soil which ensures its contact with the particles of the 
latter, are indispensable to nitrification, which is in all 
cases a process of oxidation. When sesquioxide of iron 
oxidizes organic matters, its action would cease as soon as 
its reduction to protoxide is complete, but for the atmos- 
pheric oxygen, which at once combines with the protoxide, 
constantly reproducing the sesquioxide. 
In the saltpeter plantations it is a matter of experience 
that light, porous soils yield the largest product. The 
operations of tillage, which promote access of air to the 
deeper portions of carth and counteract the tendency of 
many soils to “cake” to a comparatively impervious mass, 
must also favor the formation of nitrates. 
Many authors, especially Mulder, insist upon the physic- 
al influence of poros‘ty in determining nitrification by 
condensed oxygen. The probability that porosity may 
assist this process where compounds of nitrogen are con- 
cerned, is indeed great ; but there is no evidence that any 
porous body can determine the union of free nitrogen and 
oxygen. Knop found that of all the proximate ingredi- 
ents of the soil, clay alone can be shown to be capable of 
physically condensing gaseous ammonia (humus combines 
with it chemically, and if it previously effects physical 
condensation, the fact cannot be demonstrated). 
The observations by Reichardt and Blumtritt on the 
condensing effect of the soil for the gases of the atmos- 
