ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 95 
ment, with no other than the atmospheric supply of 
carbon. 
Hydrogen is adequately supplied to crops by water, 
which equally belongs to the Atmosphere and the Soil, 
although it enters the plant chiefly from the latter. 
Nitrogen exists in immense quantities in the atmosphere, 
and we may regard the latter as the primal source of this 
element to the organic world. In the atmosphere, how- 
ever, nitrogen exists for the most part in the free state, and 
is, as such, so we must believe from existing evidence, un- 
assimilable by crops. Its assimilable compounds, wmmo- 
nia and nitric acid, occur in the atmosphere, but in pro- 
portions so minute, as to have no influence on vegetable 
growth directly appreciable by the methods of investiga- 
tion hitherto employed, unless they are collected and con- 
centrated by rain and dew. 
The subjoined Table gives a summary of the amount 
of nitrogen annually brought down in rain, snow, etc., 
upon an acre of surface, according to the determinations 
hitherto made in England and Prussia. 
AMOUNT OF ASSIMILABLE NITROGEN ANNUALLY BROUGHT DOWN BY 
THE ATMOSPHERIC WATERS. 
om : rn | Netrogen Water 
Locality. Year. | ner Acre. per Acre. 
Rothamstead, Southern England...... ........ 1855* | 6.63 1bs./6,633,220 Ibs. 
ee a Seige ose emedaten rae Bax 1856* | 8.31 ‘ |6,160.510 “ 
Kuschen, Province Posen, Prussia....| 1864-5t| 1.86 ‘°* [2.680.086 ‘‘ 
* 4 ss +b) tee | 1865-64) 52:50. °°" 14 008-491) ~** 
Insterburg, near Konigsberg, > asde | LOO4=OT | ADs o> ||Greee Abbe * 
4 te re Se), 2 Shs -6 hh Gs8L. <2) 5.383 478s" 
Regenwalde, near Stettin, SO 2. O64 5 Fila. 09k 252 |Sthe abe 
es s of ‘eg. | 1865-61/10.38 “* [4,358,053 °° 
Ida-Marienhiitte, near Breslau, Silesia, ‘‘ ....| 1865* {11.83 ‘* |4,877.545 ‘ 
Proskan, Silesia. oe) 1864-55/20. 98. 8: 4208L-7See. 5 
Dahme, Province Brandenburg, es oq. estos: 36.60)" (3-868:64m) 
NADIE ea ee a each Sle shee Se SOAs kee ee 8.76 lbs./4.867.0%5 lbs. 
Direct Atmospheric Supply of Nitrogen Insufficient 
for Crops.—To estimate the adequacy of these atmos- 
pheric supplies of assimilable nitrogen, we may compare 
their amount with the quantity of nitrogen required in the 
