The ash of potatoes consists of about 50 per cent of potash, and this fact has induced many writers to recommend ashes as a manure for this crop. It will be seen, however, that in this instance, at least, they have very little effect, 400 lbs. giving an increase of only five bushels per acre. One hundred pounds of plaster per acre gave an increase of six bushels. Plaster and ashes combined, an increase per acre of 15 bushels.
One fact is clearly brought out by these experiments: that this soil, which has been under cultivation without manure for many years, is not, relatively to other constituents of crops, deficient in potash. Had such been the case, the sulphate of ammonia and superphosphate of lime—manures which contain no potash—would not have give a an increase of 84 bushels of potatoes per acre. There was sufficient potash in the soil, in an available condition, for 179 bushels of potatoes per acre; and the reason why the soil without manure produced only 95 bushels per acre, was owing to a deficiency of ammonia and phosphates.
Since these experiments were made, Dr. Vœlcker and others have made similar ones in England. The results on the whole all point in one direction. They show that the manures most valuable for potatoes are those rich in nitrogen and phosphoric acid, and that occasionally potash is also a useful addition.
“There is one thing I should like to know,” said the Doctor. “Admitting that nitrogen and phosphoric acid and potash are the most important elements of plant-food, how many bushels of potatoes should we be likely to get from a judicious application of these manures?”
“There is no way,” said I, “of getting at this with any degree of certainty. The numerous experiments that have been made in England seem to show that a given quantity of manure will produce a larger increase on poor land than on land in better condition.”
In England potatoes are rarely if ever planted without manure, and the land selected for this crop, even without manure, would usually be in better condition than the average potato land of this section, and consequently a given amount of manure, applied to potatoes here, would be likely to do more good, up to a certain point, than the same amount would in England.
Let us look at some of the experiments that have been made in England:—
In the Transactions of the Highland and Agricultural Society of Scotland for 1873 is a prize essay on “Experiments upon Potatoes, with Potash Salts, on Light Land,” by Charles D. Hunter, F.C.S., made on the farm of William Lawson, in Cumberland. Mr. Hunter “was charged with the manuring of the farm and the purchasing of chemical manures to the annual value of £2,000,” or say $10,000.
“Potatoes,” says Mr. Hunter, “were largely grown on the farm, and in the absence of a sufficiency of farm-yard manure, potash naturally suggested itself as a necessary constituent of a chemical potato-manure. The soil was light and gravelly, with an open subsoil, and the rainfall from 29 to 38 inches a year.”
The first series of experiments was made in 1867. The following are some of the results:—