Experiments on the Growth of Wheat. 
217 
other experiments, and on Plot 4 they were top-dressed in the 
spring. Looking to the columns of total produce (Table II.), we 
see that, taking the four years together, there is a difference of 
less than 2 bushels between the produce of the two plots, it being 
however rather in favour of the autumn-sown manure. The 
autumn-sown manure also gives on the average (see Table III.) 
a rather better weight per bushel. The produce of straw, taking- 
together the three last years (it not being weighed in the first), is 
nearly identical in the two cases, there being a difference only of 
8 lbs. in favour of the spring-sown manure. Upon the whole, 
then, the results are in favour of sowing these soluble manures in 
the autumn even in so light a soil. Comparing the produce of 
the different years by ammoniacal salts alone, we find that there 
is here again a fall in the produce of 18 bushels in the one case, 
and of 14| in the other, from the first year to the average of the 
three last years ; and that there is afterwards something like a 
gradual reduction from year to year. It is obvious, therefore, 
that the amount of nitrogen supplied in this large dose of am- 
moniacal salts is in excess over the annually available minerals of 
the soil, which it would appear are becoming gradually reduced. 
That these, however, are nevertheless considerably in excess over 
those required by the natural supplies of nitrogen, is obvious from 
the fact, that whilst by mineral manures alone we got no increase 
of corn whatever, and only a total increase of straw in the last 
three years taken together of about half a ton, the ammoniacal 
salts alone have given in the four years a total increase of 31 to 32 
bushels of corn, and in the three last years of 2830 lbs. of straw. 
By the comparison, then, of the results of the mineral manures 
alone by the side of those of the ammoniacal salts alone, we have 
beautifully illustrated not only the nature of the characteristic 
exhaustion induced by the growth of the corn, but we are also 
able to form a pretty clear idea of the actual degree or extent of 
that exhaustion, much more so at any rate than we should be by 
any analysis of the soil. 
In Experiment 5, we have in the manure both the minerals of 
Plot 2, which gave no increase of corn and but little increase of 
straw, and the ammoniacal salts of the Plots 3 or 4, which gave 
a considerable, though annually decreasing, amount of increase. 
The result of this mixture of both minerals and ammoniacal salts 
is to give, taking the 4 years together, from 53 to 54 bushels of 
corn and a large quantity of straw more than is yielded by the 
minerals alone. This, then, is an annual average of 13 to 14 
bushels of corn and an equivalent of straw due to the ammoniacal 
salts. And since there is in the 4 years about 20 bushels more 
increase by the mixture of both minerals and ammonia salts than 
by the ammonia salts alone, it is obvious that the minerals of 
