422 Clover as a Preparatory Crop for Wlieat. 
afterwards. An enormous quantity of nitrogenous organic matter, 
as we have seen, is left in the land after the removal of the clover- 
crop ; and these remains gradually decay and furnish ammonia, 
which at first and during the colder months of the year is retained 
by the well-known absorbing properties which all good wheat- 
soils possess. In spring, when warmer weather sets in, and the 
wheat begins to make a push, these ammonia compounds in 
the soil are by degrees oxidized into nitrates ; and as this change 
into food, peculiarly favourable to young cereal plants, proceeds 
slowly but steadily, we have in the soil itself, after clover, a 
source from which nitrates are continuously produced ; so that 
it does not much affect the final yield of wheat whether heavy 
rains remove some or all of the nitrate present in the soil. The 
clover-remains thus afford a more continuous source from which 
nitrates are produced, and greater certainty for a good crop of 
wheat than when recourse is had to nitrogenous top-dressings in 
the spring. 
The remarks respecting the formation of nitrates in soils upon 
which clover has been grown, it should be stated, do not emanate 
from mere speculations, but are based on actual observations. 
I have not only been able to show the existence of nitrates 
in clover-soils, but have made a number of actual determinations 
of the amount of nitric acid in different layers of soils on which 
clover had been grown ; but as this paper has grown already to 
greater dimensions than perhaps desirable, I reserve any further 
remarks on the important subject of nitrification in soils for a 
future communication. 
SUMMAKY. 
The following are some of the chief points of interest which I 
have endeavoured fully to develope in the preceding pages : — 
1. A good crop of clover removes from the soil more potash, 
phosphoric acid, lime, and other mineral matters, which enter 
into the composition of the ashes of our cultivated crops, than 
any other crop usually grown in this country. 
2. There is fully three times as much nitrogen in a crop of 
clover as in the average produce of the grain and straw of wheat 
per acre. 
3. Notwithstanding the large amount of nitrogenous matter 
and of ash constituents of plants in the produce of an acre, clover 
is an excellent preparatory crop for wheat. 
4. During the growth of clover a large amount of nitrogenous 
matter accumulates in the soil. 
5. This accumulation, which is greatest in the surface-soil, 
is due to decaying leaves dropped during the growth of clover, 
