for Twenty Years in succession on the same Land. 321 
1 bushel of wheat may be reckoned to weigh 61 lbs., and its 
average proportion of straw 105 lbs. Thus, whilst from 2 to 2\ 
lbs. of ammonia in manure will yield 62 lbs. barley-grain, and 
63 lbs. straw = 115 lbs. total produce, it required 5 lbs. to yield 
61 lbs. of wheat-grain, and 105 lbs. straw = 166 lbs. total 
produce. 
It is clear that it required much more nitrogen in manure to 
yield a given amount of increase of produce when applied in the 
autumn for wheat, than when in the spring for barley. 
The questions remain — what proportion of the supplied 
nitrogen is recovered in the immediate increase of crop ? — what 
becomes of the unrecovered amount, if any ? — does it, wholly or 
in part, remain in the soil ? — if so, what will be its effect on 
succeeding crops ? — or, lastly, is there any material loss, by 
drainage, or otherwise? These points will next be considered. 
Section IV. — On the Effects of the unexhausted Residue 
FROM previous MANURING UPON SUCCEEDING CrOPS, LOSS 
OF CONSTITUENTS BY DRAINAGE, AND SOME ALLIED PoiNTS. 
In the foregoing pages incidental reference has frequently been 
made to the effects of the residue from previous manuring upon 
succeeding crops ; but the subject is, in various aspects, of such 
great importance, that it has been reserved for separate consi- 
deration in this place. 
For example, it is of very great practical interest to have some 
exact data, showing — what proportion of the nitrogen, supplied in 
manure, will probably be recovered in the increase of the crop 
for which it is applied ; whether, or in what degree, the at first 
unrecovered amount will, on the one hand be retained by the 
soil, or on the other, be drained away and lost ? whether, if 
retained, it will remain, wholly, or in part, in such a state of 
combination, and distribution, within the soil, as to be available 
for succeeding crops ? and so on. 
Very similar questions obviously arise in regard to the mineral 
constituents of manures and crops ; and so far at least as some 
of those constituents are concerned, it is very important to be 
able to refer to direct experimental evidence, bearing on the 
subject. 
But, independently of facts and conclusions of great general 
interest and importance, when the same manure is applied, and 
the same crop grown, year after year on the same land, it is 
essential to a proper interpretation of the average results obtained 
over a series of years, not only to consider the characters of the 
seasons, but also whether any particular description of manure, so 
applied, induces exhaustion of certain constituents, resulting in 
