404 Clover as a Preparatory Crop for Wheat. 
potash and superphosphate of lime were emph)yed, 17 to 18 tons 
(equal to from about 4^ to nearly 5 tons of hay) were obtained. 
When salts of ammonia were added to the mineral manures, the 
produce of clover-hay was, upon the whole, less than where the 
mineral manures were used alone. The wheat sjrown after 
the clover on the unmanured plot gave, however, 29i bushels of 
corn, whilst in the adjoining field, where wheat was grown after 
wheat without manure, only 15^ bushels of corn per acre were 
obtained. Messrs. Lawes and Gilbert notice especially that in 
the clover-crop of the preceding year very much larger quantities 
both of mineral matters and of nitrogen Avere taken from the land 
than were removed in the unmanured wheat-crop in the same 
year, in the adjoining field. Notwithstanding this, the soil from 
which the clover had been taken was in a condition to yield 
14 bushels more wheat per acre than that upon which wheat had 
been previously grown ; the yield of wheat after clover, in 
these experiments, being fully equal to that in another field, where 
verv large quantities of manure were used. 
Taking all these circumstances into account, is there not pre- 
sumptive evidence that notwithstanding the removal of a large 
amount of nitrogen in the clover-hay, an abundant store of avail- 
able nitrogen is left in the soil, and also that in its relations 
towards nitrogen in the soil clover differs essentially from 
wheat? Tlie results of our experience in the growth of the two 
crops appear to indicate that whereas the growth of the wheat 
rapidly exhausts the land of irs available nitrogen, that of clover, 
on the contrary, tends somehow or other to accumulate nitrogen 
within the soil itself. If this can be shown to be the case, an 
intelligible explanation of the fact that clover is so useful as a 
preparatory crop for wh'jat will be found in the circumstance 
that during the growth of clover, nitrogenous food, for which 
wheat is particularly grateful, is either stored up or rendered 
available in the soil. 
An explanation, however plausible, can hardly be accepted as 
correct if based mainly on data which, although highly probable, 
are not proved to be based on fact. In chemical inquiries espe- 
cially, nothing must betaken for granted that has not been proved 
i'V direct experiment. The following questions naturally suggest 
themselves in reference to this subject : What is the amount 
of nitrogen in soils of different characters ? What is the 
amount, more particularly after a good and after an indifferent 
crop of clover? Why is the amount of nitrogen in soils larger 
after clover than alter wheat and other crops ? Is the nitrogen 
])rescnt in a condition in which it is available and useful to 
wheat? and lastly. Are there any other circumstances, apart 
I'rom the supply of nitrogenous matter in the soil, which help to 
