for Tioenty Years in succession on the same Land. 297 
The evidence taken as a whole, therefore, gives no indi- 
cation of any deterioration in either the quantity or the 
quality of the produce as the result of the continuous growth of 
the crop, provided the necessary constituents are supplied by 
manure. 
It is seen that whilst the average annual produce over the 
twenty years is, with ammonia-salts and superphosphate of lime 
47^ bushels of dressed corn and 21^ cwts. of straw, with the same 
quantity of ammonia-salts and a mixture of sulphates of potass, 
soda, and magnesia, it is only 35^^ bushels of corn, and only 20f 
cwts. of straw. Even with the ammonia-salts and both the super- 
phosphate and the " mixed alkali-salts," it is only 46 1 bushels 
of corn, and 28^ cwts. of straw ; or rather less corn, though rather 
more straw, and total produce, than with the ammonia-salts and 
superphosphate without the salts of potass, soda, and magnesia. 
It is further remarkable that the yield of corn has increased more 
over the later period where the superphosphate was used without, 
than where in conjunction with the mixed alkali-salts. The details 
show, however, that the produce, at any rate of straw, where the 
mixed alkali-salts and the superphosphate are used together, has 
been of late years somewhat gaining upon that where the super- 
phosphate is used alone. 
It may be mentioned, though not shown in the Table, that the 
increase over the unmanured, or over the corresponding mineral 
manured produce, is much greater over the second period com- 
pared with the first, than is the augmentation of the actual pro- 
duce itself. This is explained by the fact that the produce without 
manure, or by the mineral manures alone, was much the less over 
the later period, and hence, though there was much the same 
actual amount of produce over the two periods when ammonia 
was also used, still the increase over that without ammonia is 
much the greater. 
Over the whole period of twenty years the average annual 
increase of produce due to the combined action of mineral and 
nitrogenous manures is, with the ammonia-salts and superphos- 
phate, 26|- bushels of corn and 15^ cwts. of straw ; with the same 
and the mixed alkali-salts in addition, 25|^ bushels of corn and 
16| cwts. of straw ; with the same and sulphate of potass (without 
soda and magnesia) 23|^ bushels of corn, and 15|^ cwts. of straw; 
but with the ammonia-salts and salts of potass, soda and magnesia 
(without superphosphate) only 14^ bushels of corn and 8| cwts. 
of straw. Or, if the increase be reckoned over the produce by the 
corresponding mineral manure without ammonia, in which case 
it is the increase due to the ammonia itself that is more nearly 
represented, it is, when used with superphosphate of lime 21^ 
bushels of corn, and 141 cwts. of straw; when with superphos- 
