294 
Journal of Agricultural Research 
Vol. IX, No. 9 
HISTORICAL REVIEW 
INFLUENCE OF MOISTURE 
Long before the process of nitrification was known to be due to micro¬ 
organisms, the underlying principles governing the speed of the reaction 
had been investigated nationally by France, Germany, and Sweden. 
Among other things, they had learned that there must be a certain 
proportion of water, and, in order that the maximum yield of nitrates 
be obtained, that this must be diminished as the soil becomes richer in 
nitrates. As early as 1887 Deh£rain (9) found that the most active 
nitrification took place when the soil was allowed to become partially 
dry between the applications of water, and later (12) he found that 
there was a relationship between the speed of nitrification and the moist¬ 
ure content of fallow soil, the nitrification increasing with the water. 
Boussingault (63) taught that, when soils contain as much as 60 per 
cent of water, they lose in a few weeks the greater part of their nitrates. 
This teaching gave rise to the general belief that dentrification may take 
place to a great extent in soils, but recent work has amply demonstrated 
that it is only under extremely abnormal conditions that this becomes 
an important factor. For this reason literature bearing on this phase 
of the subject is not considered here. 
Deh6rain and Demoussy (14) found that the bacterial action of a soil 
was at its maximum when a rich soil contained 17 per cent of water, but 
that it decreased if the proportion of water fell to 10 per cent or rose to 
25 per cent. With soils less rich in humus a somewhat higher propor¬ 
tion of water was necessary to retard oxidation to any marked degree. 
The optimum moisture content for nitrification, according to Deh£rain 
(13), is 25 per cent. An insufficient supply of moisture checked both 
nitrification and nitrogen fixation. This occurred when the water had been 
reduced to 16.5 per cent. This, however, would vary with the soil, for 
Schloesing (50) found bacterial activity less in fine-grained soils than in 
lighter, coarse-grained soils. In order that nitrification be equally active 
in both light and heavy soils, the latter must have a higher percentage 
of water than the former, a difference in moisture content of soil of 1 per 
cent, according to Dafert and Bollinger (8), being sufficient to produce* 
a marked change in the oxidation going on in the soil. 
Fraps (16) found that the number of nitrifying organisms in a soil 
varies with the moisture and that their activity was periodic, rapid nitri¬ 
fication being preceded and followed by periods of less activity. Later 
he (17) found nitrification to be at its height in soil containing 55.6 per 
cent of its water capacity. Excessive quantities of water practically 
stopped nitrification and was much more injurious than too small a 
quantity. The water requirements, however, varied considerably with 
the soil. 
