for Twenty Years in succession on the same Land. 365 
deficiency of nitrogen in the soil ; for the falling off was con- 
siderably more marked with mineral manure alone, than with 
ammonia-salts alone. 
It will be obvious that an average of 50 bushels of barley-grain, 
and 30 cwt. of straw, would not be maintained without great 
fluctuations from year to year, according to season. Indeed, in 
no two years of the twenty did one and the same manure yield 
precisely the same result both as to the quantity and the quality 
of its produce ; nor were the seasons which were more or less 
favourable than the average for one description of manure 
equally favourable for other descriptions. Thus, comparing the 
least and the most productive seasons of the twenty, there were 
obtained (reckoning the total corn at 52 lbs. per bushel) — without 
manure Ib^ and 37| bushels, or a difference of 22 bushels ; with 
farmyard manure, 32 bushels and 60 bushels, or a difference 
of 28 bushels: lastly, with the two most productive artificial 
manures, there were obtained 30f and 36J bushels in the worst 
season, and 66 and 68 bushels in the best season, or a difference 
in favour of the good season of 35|- and 31f bushels of grain. 
That is to say, with one and the same expenditure for manure, 
there was a difference in the quantity of the produce obtained in 
the two seasons, of from nearly 32 to over 35 bushels of corn, 
besides, in one case, nearly a ton of straw. 
Not only, then, has the average produce over twenty years, 
by artificial, nitrogenous and mineral, manures, considerably ex- 
ceeded the average barley crop of the country with rotation, 
but the difference between the produce by one and the same 
manure in the least and the ""most favourable seasons of the 
twenty was, itself, not much less than would represent the 
average barley crop of many localities. 
As we have in substance frequently said, it is but a truism to 
assert that the growing plant must have within its reach a 
sufficiency of the mineral constituents of which it is to be built 
up. But the results obtained with barley, as well as those with 
wheat, have shown that, whilst it is essential that there be a 
liberal provision of mineral constituents within the soil, the 
amount of produce is more dependent on the supply by manure 
of available nitrogen than of any other constituent. 
The practical question obviously arises — How much ammonia, 
or its equivalent of nitrogen in some other form, will, on the 
average, be required to yield a given amount of increase of 
wheat or barley grain, and its proportion of straw ? 
In our Report on the growth of wheat for twenty years in 
succession on the same land, it was shown that the quantity of 
increase obtained for a given amount of ammonia, or its equi- 
valent of nitrogen, in manure, varied exceedingly according to 
the amount applied, to the provision of mineral co7istituents within 
