Potatoes may be grown on any kind of soil; but production seldom proves remunerative except from what may be termed "potato soils." These soils are difficult to define, though easily enough recognised by practical farmers. Deep rich sandy loams, such as the red soils of East Lothian, Scotland, or the well drained alluvial "silts" and "warp" lands of Lincoln shire, England, are excellent for potato cultivation. Some of the black peaty soils, provided they are well drained and do not dry out in the summer time, also yield heavy crops of low quality tubers. The crops from the heavy clays and the light sands are small, though much can be done in improving these by using long dung. This causes better aeration and drainage of the heavy clays, and a water retaining capacity in light sands. Generally, the potato plant is very accommodating in its habits, but only develops its best crops in well aerated soils which at the same time are always moist. It is because of these facts that potato culture, the world over, is largely practised on sandy loam and the peats, as seen in Cambridge, Lincolnshire and Lancashire (England), Ulster (Ireland), eastern Germany, and America.
Early potatoes are often grown on other soils, but for these crops the site and temperature is of more importance than soil. In some cases these early soils are planted each year to potato crops, and some land in Ayrshire, Scotland, in Cornwall, England, in Spain and in Florida, U.S.A., must have produced potatoes continuously for many years. These cases are, however, excep tional, for in ordinary cases the potato crop is included in a three or four course rotation, generally following in Europe a corn crop, though in America the potato crop almost invariably follows a leguminous crop.
Where pota toes follow the cereal crop, cultivation should begin with an autumn cleaning followed by a deep winter ploughing and spring cultivation to give a moderately fine tilth. In April, the ridging plough is used to throw the soil into ridges between which lie the furrows (distance between the furrows being about 27 inches) into which the potato-sets will go; before these are put out it is customary to scatter the dung and artificial fertilisers in the furrows, so that they lie close to the potato-sets. These sets are placed by hand at intervals of ten inches to one foot in the furrows. This done, the ridging plough is driven down the ridges which are split so that the soil falls into the furrows on either side and so covers the sets. Cultivation is continued to keep down weeds. In recent years potato-planting machines have been introduced and in the use of these some modification is neces sary. The land is ploughed, cultivated and left flat, for these machines are able to cut their own drills, place the potato-set in position and refill the drill as they proceed across the field.
Dung is always beneficial to the potato crop, and within reason the greater the dressing the larger the crop. In practice not more than twenty tons per acre can be spared ; often it has to be cut down to ten or twelve tons and where this is done the deficiency should be made good by using some other substance. Seaweed for instance is an excellent substitute and is much used by the potato-growers of Scotland, Ireland and Cornwall. Inland growers, unable to obtain this, have to use artificial fertilisers and a note concerning these is necessary. Most experimenters are agreed that a compound fertiliser which contains potash, phosphates, and nitrogen is de sirable, but the exact proportion in which they should be mixed is a matter of controversy. The following mixture per acre may generally be depended upon to produce good crops : 2 cwt. sulphate of ammonia.
6 cwt. super phosphate.
cwt. sulphate or muriate of potash.
The muriate of potash is usually cheaper than the sulphate of potash, and in most seasons is as effective though there is some basis for stating that the quality of the tubers is usually higher where the sulphate is used.
In the trials conducted at the Rothamsted experimental sta tion (England), the nitrogen fertilisers proved over a period of years the most consistent in their action, giving every year, with rare exceptions, an increase of about 20 cwt. of potatoes per cwt. of sulphate of ammonia used, whatever the season and whether farmyard manure was used or not. At Rothamsted there was curiously little variation from season to season in the maximum yield of potatoes, obtainable by good manuring. Their maximum was II to 13 tons per acre, and usually 4 cwt. of sulphate of ammonia and 4 cwt. of sulphate of potash per acre was neces sary to secure this. Economy of either ammonia or potash re duced the yield. The action on other soil would probably be somewhat different.
The importance of using "good seed" cannot be over-emphasized, and the term "good seed" needs careful definition. A potato tuber of any size when planted is capable of giving a crop, but in practice it is found most economical to select for seed tubers weighing approxi mately two to three ounces in weight. These tubers are too small to be used for eating, and when planted give as good results as those from larger sets. In fact if larger tubers are used they may be cut into pieces. Eighteen hundredweight of seed of this size is sufficient to plant an acre of land. If the "seed" has been raised in red soils, silts or sands, the tubers are clean and have bright clean skins ; such seed is often spoken of in correctly as "good seed." If the seed has been raised from heavy clays or peaty soils it is dark, dull, but not necessarily inferior.