This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Of late years much attention has been given to the best methods of cultivation, and experiments almost innumerable on manuring, spraying, and sprouting have been carried out by various agricultural schools and colleges in England, and by the Department of Agriculture in Ireland. The results of these experiments have been tabulated, but beyond giving a great impetus to the sales of artificial fertilizers and poisonous washes there seems to be little improvement to record so far as the yield of tubers per acre is concerned. Indeed the cultural operations in the great majority of cases seem to have been ignored or not considered. As a rule, nothing is said as to the preparation of the soil, whether it is dug deeply or otherwise, or whether it is ploughed; nor is reference made, as a rule, to the distance between the rows or the number of sets to the acre and their weight. These important points sink into oblivion in comparison with the effects produced by this or that special fertilizer or wash. The results obtained are useful in a way, but they are vitiated to a great extent owing to inferior cultural methods. As a rule, the soil is not cultivated sufficiently deep, and the sets and rows are much too close together. Most potato growers are apparently unaware of the fact that the great bulk of the dry weight of the crop comes from the carbonic acid gas in the atmosphere, under the influence of sunlight, and they are under the impression that the more tubers they put into the ground the greater and better the yield. Nothing could be further from the facts. Indeed, most of our Potato troubles are readily traceable to overcrowding and bad cultivation. It is considered a "waste of ground" to put sets in a yard apart, and too little is spent in labour in preparing the soil in advance.
The ideal soil for Potatoes is a deep and gritty loam on a limestone bottom, if possible, or with a fair percentage of lime in it. Such a soil, however, should be deeply worked, and it will pay for the expense, not only because of the large and essential quantities of natural potash (a most important Potato food) that will be liberated, but also because such terrible pests as wireworms, if present, will be brought up and exposed to the keen eyes of the various birds always on the search for food. Ground that is already in a good state of cultivation would cost from 40s. to 60s. per acre to dig one spit deep. Heavy ground covered with coarse weeds would cost twice or three times as much, and would be cheaper but not better broken up by the plough in the first instance. Digging is always better than ploughing, as a greater depth is obtained, and the soil is broken up into a much finer condition. Consequently it is better ventilated, and its particles are more easily acted upon and rendered fertile by the weather. The analysis of the ash of Potatoes (see Vol. I, p. 109) will give the cultivator some idea as to the foods that are taken out of the soil. To bring the necessary supplies of potash, phosphoric acid, lime, magnesia, etc, into proper, i.e. a soluble, condition gradually, there is only one way, and that is by constant cultivation.
Perhaps there has been no crop so experimented upon with manures - artificial and natural - as the Potato. Hundreds of experiments have been carried out in all parts of the United Kingdom, on the Continent, and in America, and the results have been most bewildering. Manures, applied in accordance with certain formulae, that give apparently good results in one place, are practically useless in another. The cost of these special fertilizers varies from £3 to £7 and £8 per acre - often much more than the entire crop would realize without even considering the question of labour, rent, etc.
The following figures, taken from Leaflet No. 38 of the Department of Agriculture and Technical Instruction for Ireland, will show the highest and lowest results of field experiments with Potatoes in the year 1909: -