286 HOW CROPS FEED. 
throughout the period of experiment, but only those 
amounts that remained at the time of taking the samples. 
What the vegetation took up from the planted plots, what 
was washed out of the surface soil by rains, or otherwise 
removed by chemical change, does not come into the 
reckoning. 
Those plots, the surface soil of which was most occupied 
by active roots, would naturally lose the most nitrates by 
the agency of vegetation; hence, not unlikely, the vetch 
and oat plots contained so little in June. The results up- 
on the beet, and vacant ground plots demonstrate that in 
that month a rapid furmation of nitrates took place. It 
is not, perhaps, impossible that nitrification also proceeded 
vigorously in the loose soils in July and August, but was 
not revealed by the analysis, either because the vegetation 
took it up or heavy rains washed it out from the surface 
soil. In the brief account of these experiments at hand, 
no information is furnished on these points. Since moiést- 
ure is essential to nitrification, it is possible that a period 
of dry weather coming on shortly before the soil was 
analyzed in July, August, and September, had an influence 
on the results. It is certainly remarkable that with the ex- 
ception of the vetch plot, the soil was destitute of nitrates 
on the 9th of September. This plot, at that time, was 
thickly covered with fallen leaves. 
We observe further that the nature of the crops influ- 
enced the accumulation of nitrates, whether simply be- 
cause of the different amount of absorbent rootlets pro- 
duced by them and unequally developed at the given 
period, or for other reasons, we cannot decide.* 
From the third Table may be gathered some idea of the 
total quantity of nitrogen that was present in the soil in 
* It is remarkable that the large-leaved bect plant had a great surplus of ni- 
trates, while the oat plot was comparatively deficient in them. Has this fact any 
connection with what has been stated (p. 84) regarding the unequal power of 
plants to provide themselves with nitrogenous food ? 
