78 
Journal of Agricultural Research 
Vol. VII, No. 2 
Such like treatment has, of course, been accorded all the soils. It 
would therefore seem that the figures are illuminating if not exactly for 
the use of dried blood in general, at least for i per cent of dried blood as 
the nitrifiable material. Of course, the possibility still remains that 
with 0.05 per cent or 0.1 per cent of blood, instead of the 1 per cent 
employed in the cultures, the results in the blood series may have favored 
the arid and not the humid soils, as they did in these experiments. That, 
however, is quite unlikely, since it is unreasonable to suppose that smaller 
quantities of dried blood would have been nitrified with less vigor in the 
humid soils than the large quantity employed. Except, therefore, as 
it might have changed absolute values, the procedure in the use of dried 
blood appears to be no factor in the comparison here made between the 
powers of humid and of arid soils to nitrify dried-blood nitrogen. 
The depression of nitrification in most arid soils by 1 per cent of blood 
in nitrification experiments was briefly explained by the senior author 
(6, 7) on the following hypothesis: Ammonification of dried blood 
proceeds very rapidly. Ammonia is poisonous to the nitrifying organism. 
If a soil has a large internal surface for adsorption, as, for example, in 
the presence of large quantities of decaying organic matter, in organic 
colloids, or a similar material, the ammonia produced by the process of 
ammonification is quickly adsorbed and removed from harmful action 
on the nitrifying bacteria. If the contrary is true of the internal surface 
of soils, such as would obtain in sands poor in humus or in closely packed 
soils of finer texture, the ammonia given off in the ammonification of 
dried blood would be toxic to the nitrifying bacteria. This would take 
place by a direct toxic effect and also by the depressing effect on nitrifi¬ 
cation of large quantities of soluble organic matter, in the soil solution 
introduced through the solvent effects of ammonia on the dried blood. 
Very few arid soils have been found incapable of nitrifying 1 per cent of 
blood which did not also yield a very dark soil solution, indicating the 
active dissolution of the organic matter present. Very few such soils, 
moreover, have been noted which do not give off a strong odor of ammo¬ 
nia during the period of incubation. That both dissolved organic matter 
in the medium and free ammonia deter or inhibit the process of nitrifi¬ 
cation by pure cultures has been proved for solution cultures by Wino¬ 
gradsky’s experiments (14). That the same is true for soils in the presence 
of mixed cultures has not been known, however, until now. Since the 
hypothesis was first formulated, the writers have carried out experiments, 
not possible of description here, which clearly point to the highly toxic 
nature of relatively small quantities of ammonia to the nitrification of 
dried blood in soils which otherwise transform the nitrogen of that 
material into nitrates without difficulty. It is further gratifying to 
note that Hutchinson (4) has arrived at a similar result to that which 
the writers have obtained on the toxicity of ammonia to the process of 
nitrification. 
