Manures and the principles of manuring

by Charles Morton Aikman

The Rothamsted experimenters have very fully investigated the conditions of the manurial requirements of potatoes. In these experiments potatoes were grown year after year in the same field. It was found that the effect of mineral manures alone was greater than the effect of nitrogenous manures alone, and that of mineral manures phosphates, as a rule, had a better effect than potash; that under the action of the growth of potatoes a greater exhaustion of phosphates than of potash takes place in the soil; and lastly, that it is essential to have an abundant supply of the different fertilising ingredients in order to grow successful crops. In the Rothamsted experiments, the slow action of farmyard manure in supplying fertilising ingredients to the potatoes is strikingly demonstrated. Thus, although farmyard manure was applied at such a rate that more than 200 lb. of nitrogen were added to the soil, the result was inferior to that obtained from the application of 86 lb. of nitrogen applied in the form of readily available artificial manure.

Effect of Farmyard Manure on Potatoes.

It may be said, in this respect, that the potato is less able to utilise the fertilising ingredients of farmyard manure than any other of the farm crops. Yet, despite this fact, farmyard manure has been found to be one of the best manures to apply. The reconciliation of these seemingly contradictory statements depends on the influence exerted by the farmyard manure on the mechanical condition of the soil, rendering it more porous and easily permeable to the surface-roots, upon the development of which the success of the crop so much depends. The beneficial effect of farmyard manure is also doubtless due to the increased temperature which large applications of it produce in a soil.

Sir J. Henry Gilbert, in his well-known Cirencester Lecture on the Growth of Potatoes, cites several examples of the manurial treatment of potatoes in different parts of the country. In Forfarshire, farmyard manure or stable manure is largely employed (at the rate of 12 to 14 tons, and in some cases even 20 tons per acre), and it is also largely supplemented by artificial manures. These latter are applied to the extent of about 10 cwt., and consist of superphosphate, dissolved bones, and potash salts. Six tons of potatoes are considered a fair crop. In East Lothian the manuring is similar, with the exception that farmyard manure is applied in even larger quantities—30 to 40 tons being often used. Sometimes potatoes are grown with artificial manures alone. It would seem that the usual crop of potatoes ranges from 4 to 8 tons per acre.

Manuring of Potatoes in Jersey.

The manuring of the potato crop, so largely grown in Jersey in the Channel Islands, is of interest. Potatoes are there grown two or three years, then corn, then grass for a few years, then potatoes again, no special rotation of crops being followed. Either farmyard manure or sea-weed is applied at the rate of 25 to 30 tons per acre, supplemented by 8 to 12 cwt. of artificial manures.

These statements show how prevalent the practice of heavily manuring the potato crop is.

The Influence of Manure on the Composition of the Potato.

The influence of manure on the composition of the potato crop is of much interest. Potatoes grown without manure, just as in the case of roots, are found to have a larger percentage of nitrogen than potatoes grown with manure. The effect of manuring, therefore, is to increase the proportion of starch, which is the most important constituent of the potato. Mineral manures have a greater effect in increasing the percentage of starch than purely nitrogenous manures; but when used together, a still greater increase is obtained than when used singly. The effect of nitrogenous manures on the composition of roots and potatoes is thus seen to be similar. In the case of both crops the effect is to increase the proportion of the characteristic carbohydrate constituent, which in roots is sugar, and in potatoes starch. Potatoes, like roots, are also much influenced by the season. The effect of season and manuring on the potato disease is worthy of notice. Wet seasons are favourable to the development of the disease. It has been found that in a highly nitrogenous manured crop the proportion of diseased tubers is greater than in a non-manured crop.

LEGUMINOUS CROPS.