Field Experiments on Potatoes, 
415 
potato crop per acre a manure composed of 4 cvvts. of mineral 
superphosphate, 2 cwts. of potash-salts and 2 cwts. of sulphate 
of ammonia. 
This application produced the large crop of 12 tons 5 cwts. 
and 40 lbs. per acre, and gave an increase of over 5^ tons of 
potatoes over the yield of the unmanured plots. 
2. That next to the compound artificial manure used on Plot 1 
dung had the most beneficial effect upon the potato crop. 
3. That mineral superphosphate and potash-salts, without 
sulphate of ammonia, yielded much less increase than the same 
mixture with sulphate of ammonia. 
4. That the addition of nitrate of soda to superphosphate and 
potash-salts had a less beneficial effect than the addition of 
sulphate of ammonia to the same fertilising agents. 
5. That a compound artificial manure, suited to the require- 
ments of the crop intended to be raised, and to the character of 
the soil to which it is to be applied, frequently has a better effect 
than Peruvian guano. 
6. That common salt, applied to potatoes in considerable 
quantities, rather injures than benefits the crop. 
A general review of all the recorded experiments on the 
potato crop, if I am not mistaken, warrants the conclusion that 
on light land excellent crops of potatoes may be grown at a 
comparatively small expense by means of artificial manures, 
consisting of superphosphate, potash -salts, and sulphate of 
ammonia, and that on heavy land, in a good agricultural condi- 
tion, sulphate of ammonia may be omitted from a potato manure, 
either altogether or in part, and that on such land a small 
quantity of nitrate of soda, added to superphosphate, generally has 
a better effect than sulphate of ammonia. 
Laboratory, 11, Salisbury Square, Fleet Street, E.G. 
July, 1870. 
XXII. — On the Composition and Practical Value of Several Samples 
of Native Guano prepared by the ABC Sewage Process of the 
Native Guano Company. By Dr Augustus Voelcker, F.R.S. 
Of the various plans which have of late been recommended for 
the purpose of effecting the purification of town sewage, and of 
extracting from it a dry and portable manure of sufficient ferti- 
lizing value to pay the manufacturing expenses, and by the 
sale of the manure to realise an income leaving a margin 
for profit, none has attracted so much public attention as 
