Report of Experiments on the Growth of Wheat. 455 
taken as sufficiently nearly indicating tlieir comparative effects 
in a practical point of view. 
If, however, the increase on plot 5 during the 12 years is to 
be referred in any great part to previous accumulation, what an 
insignificant amount remains as the effect of the mixed mineral 
manure in restoring the productiveness of the wheat-exhausted 
soil. It will, perhaps, be said that it would have been greater 
if silica in an available form had also been supplied. Baron 
Liebig has, however, maintained that, provided there be a 
sufficiency of available alkali in the soil, there will never be a 
defic iency of available silica. Our own analytical results do not 
justify this conclusion in all its fulness. At the same time, it 
may' be stated that the mixed mineral manure employed did 
supply a great excess of available alkali ; and that when to the 
same mineral manure 400 lbs. of ammonia-salts were annually 
added there was a further annual increase of nearly 18 bushels 
of dressed corn, and nearly 20J cwts. of straw, notwithstanding 
the exclusion of silica from the manure. 
The next -selection of results affords even more direct and 
more striking evidence of the comparatively small immediate 
effects of the supposed unexhausted residue from previous nitro- 
genous manuring. 
During the first 8 years of the experiments, plots 17 and 18 
received much about the same amounts of nitrogen, potass, and 
phosphoric acid, and yielded about the same amounts of total 
produce as plot 5 ; plot 1 8, however, received rather less than 
the others. The accumulation of nitrogen and mineral con- 
stituents was, in fact, practically very nearly the same on all 
3 plots. From this time, instead of receiving mineral manure 
every year as plot 5, each of the other two plots (17 and 18) 
received ammonia-salts and mixed mineral manures alternately. 
In other words, when plot 17 was manured with ammonia- 
salts, plot 18 was manured with the mixed mineral manure, 
and vice versa; so that, each year, the one had ammonia- 
salts immediately succeeding the mixed mineral manure, and 
the other the mixed mineral manure immediately succeeding 
ammonia -salts. The detailed results of this most interest- 
ing experiment are recorded in the Appendix Tables, and 
some of them are exhibited in the coloured diagram No. II. 
(facing p. 461), to which reference will be made further on. 
But the point to which attention is now to be particularly 
directed is the amount of increase obtained when the mixed 
mineral manure each year succeeded ammonia-salts, as on plots 
17 or 18, compared with that obtained when the same mixed 
mineral manure was employed year after year on the same plot, 
as on plot 5. Table XXVI. illustrates this point : — 
