It also serves to show that any differences in the effectiveness
of the nitrogen of nitrate of soda and of ammonium-salts is
most likely to be due to the differences in habit of growth
of the plant induced by the two manures, since the conversion
of the ammonium-salts into nitrate is so easily and coinpletely
effected except in such soils as are short of the base necessary
for nitrification. Only in the wheat experiments is there
any indication that a wet and cold year may so check
nitrification as to make the ammonium-salts a less . valuable
source of nitrogen than usual. Again, we see how cereals, and
especially wheat, are specially dependent on artificial supplies of
nitrogen, and have earned the character of being exhausting



COMPOSITION OF DEAINAGE WATERS 237



crops. Their growth is almost completed before nitrification
has reached its greatest activity (from floAvering time onwards
the cereals take no more nitrogen from the soil), and being
harvested in August or early September, they leave the. ground
bare at a time of rapid nitrate formation, thus exposing it to
all the risks of washing away by the autumnal rains.
VI. — Other Constituents of Drainage Waters.
Complete analyses of the mineral constituents of the waters
di-aining from the various Broadbalk plots were made at various
times by the late Dr Voelcker and by Sir Edward Frankland ;
these analyses still constitute almost our only information as to
direct losses of the land by drainage.
Table LXXXVI. gives an average of the five analyses
made during the years 1866, 1867, and 1868.
