402 
Clove)' as a Preparatory Crop for Wheat. 
The total quantity of ash-constltuents carried off the land in an 
average crop of wheat thus amounts to only 175 lbs. per acre, 
whilst a good crop of clover removes as much as 672 lbs. 
Nearly two-thirds of the total amount of mineral in the grain 
and straw of one acre of wheat consists of silica, of which there 
is an ample supply in almost every soil. The restoration of 
silica, therefore, need not trouble us in any way, especially as 
there is not a single instance on record proving that silica, even 
in a soluble condition, has ever been applied to land with the 
slightest advantage to corn or grass crops, which are rich in silica, 
and which, for this reason, may be assumed to be particularly 
grateful for a supply of it in a soluble state. Silica, indeed, if 
at all capable of producing a beneficial effect, ought to be useful 
to these crops, either by strengthening the straw or stems of grami- 
naceous plants, or otherwise benefiting them ; but after deducting 
the amount of silica from the total amount of mineral matters in 
the wheat produce from one acre, only a trifling quantity of other 
and more valuable fertilising ash-constituent of plants will be 
left. On comparing the relative amounts of phosphoric acid and 
potash in an average crop of wheat and a good crop of clover- 
hay, it will be seen that 1 acre of clover-hay contains as much 
phosphoric acid as 2^ acres of wheat, and as much potash as the 
produce from 5 acres of the same ciop. Clover thus unques- 
tionably removes from the land very much more mineral matter 
than is done by wheat; wheat carries off the land at least three 
times as much of the more valuable mineral constituents as that 
abstracted by the clover. Wheat notwithstanding succeeds re- 
markably well after clover. 
Four tons of clover-hay, or the produce of an acre, contains, as 
already stated, 224 lbs. of nitrogen, or, calculated as ammonia, 
272 lbs. 
Y^ssuming the grain of wheat to furnish 1"78 percent, of nitro- 
gen, and wheat-straw "64 per cent., and assuming also that 1500 lbs 
of corn and 3000 lbs. of straw represent the average produce 
per acre, there will be in the grain of wheat per acre 26"7 lbs. 
of nitrogen, and in the straw 1\)'2 lbs., or in both together 46 lbs. 
of nitrogen ; in round numbers, equal to about 55 lbs. of ammonia, 
which is only about one-fifth tlie quantity of nitrogen in the pro- 
duce of an acre of clover. Wheat, it is well known, is specially 
benefited by the application of nitrogenous manures, and as clover 
carries off so large a quantity of nitrogen, it is natural to expect 
the yield of v/heat after clover to fall short of what the land might 
be presumed to produce without manure before a crop of clover 
was taken from it. Experience, however, has proved the fallacy 
of this presumption, for the result is exactly the opposite, inas- 
much as a better and heavier crop of wheat is produced than 
