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.
In carrying out all these more or less elaborate manurial trials the fundamental error which naturally leads to utterly wrong conclusions, is the assumption that the soil is the only thing to be considered in potato culture. The soil is talked about and written about, and enormous sums of money are lavished upon it, as if it, and it only, contained all the material out of which the crop is to be made. Not a word is said about the air and the light, and their absolute necessity to the crop. Perhaps it is because they cost nothing they receive such scant courtesy. And yet the great bulk of the crop - the great weight, after water has been deducted - comes from the carbonic acid gas which is floating about in small quantities with the oxygen and nitrogen of the atmosphere. It is from this gas that all the starch in the potato is obtained, and the starch can only be secured by the healthy action of the leaves when well exposed to sunlight. If growers of crops would only realize this most important fact they would get far finer, cleaner, and healthier crops than they do at present, and at much less cost. It is a most unbusinesslike proceeding to spend from £8 to £10 per acre in manures and fungicides that are not really wanted, to get a crop of 5 or 6 tons of potatoes, when from 10 to 25 tons can be obtained at far less cost by well-known cultural methods. How these good results are to be obtained will be shown below when dealing with the distance that should be given between the rows and sets. In the meantime the following advice may be given in regard to the soil.
1. Dig it deeply, if possible to a depth of 2 ft., and bring the bottom spit to the top at least every third year. In this way the subsoil will become as fertile as the top spit by exposure to the weather, the action of the roots, and the decomposition of well-rotted manure. When the plough is used, the soil should be always subsoiled to a depth of 18 in. if possible. This may seem a dangerous and drastic doctrine to teach, but it will be less costly to carry out than allowing the crops to languish and die for want of moisture at the root in dry summers, or to become water-logged, sodden, and diseased in wet ones.
2. In wet, heavy, clay soils, deep cultivation is far more necessary than in good loamy ones. It is essential to get rid of the superfluous moisture, and thus, by letting in fresh air, not only does the soil become warmer and better drained, but the soil bacteria become more active. Of course the cost of cultivating a bad heavy soil is great, but it must be borne at first if good results are to be secured. After a few years the cost will be comparatively small, but the crops will continue to improve.
3. Light or gravelly soils are almost as bad as wet, heavy ones. They eat up larger quantities of stable manure than a heavier soil, and also require more potash and phosphates. Twenty tons of stable manure per acre would not be too much in a light soil, but a similar quantity might prove injurious in a heavy or loamy soil. In light soils it would also be beneficial to give a dressing of kainit - 4 to 6 cwt. per acre - a week or two before planting potatoes; or about 1 cwt. of muriate of potash might be applied in the same way. When the tops are well through the ground, sulphate of potash - 2 to 3 cwt. per acre - may be strewn over the drills, afterwards covering when earthing up.
4. Lime or chalk, at the rate of 80 to 100 bus. per acre, should be given to soils that have been cultivated and heavily manured for years, to rectify aoy acidity that may have arisen through excessive moisture and putrid organic matter. Where lime or chalk is difficult to obtain, such soils will benefit by a dressing of basic slag, 5 to 10 cwt. to the acre. This should be strewn in the drills at time of planting.