Turnip (Brassica rapa; see BRASSICA), a biennial plant, with lyrate hispid leaves; the upper part of the root becoming, especially in cultivation, swollen and fleshy. It is a native of Europe and the temperate parts of Asia, growing in borders of fields and waste places. It is commonly regarded as a native of Britain, although in most cases where it is found apparently wild it may be doubted if it has not derived its origin from cultivated varieties. It has been long cultivated, and is to be found in every garden of the temperate and cold parts of the world as a culinary esculent; it is also extensively grown in fields for feeding cattle and sheep. It was cultivated in India long before it could have been introduced by Europeans, and is common there in gardens and about villages. The cultivated varieties are very numerous. In them the upper part of the root assumes a globose, oblong, or roundish depressed form. Some are common to the garden and the farm, and some of the largest kinds attain such a size as to weigh 20 or 25 lb. Although the turnip is of great value for feeding stock, it is not very nutritious, no less than 90 to 96 parts of its weight actually consisting of water. Garden turnips are sown from the end of March to the end of August; field turnips generally in June, it being requisite that they should not be sown so soon as to incur a risk of their throwing up flower-stems in the first year, which, when it takes place, prevents in a great measure the swelling of the root and renders it coarse and fibrous. In dry weather the plants are apt to throw up flower-stems, and so disappoint the hope of the gardener. Moist cloudy weather is most favourable. Garden turnips are sown and allowed to grow much closer than field turnips, being gradually thinned out, and the thinnings used even when of small size. The garden turnips are generally of comparatively small size, more rapid in growth, and more delicate. The Swedish Turnip, or Ruta Baga, which was introduced into cultivation in Britain, from the north of Europe, more recently than the common turnip, and has proved of very great value to the farmer, is regarded by some botanists as a variety of the same species, and by some as a variety of B. napus, but more generally as a variety of B. campestris, a species common in cornfields and sides of ditches in Britain and the north of Europe.
The cultivated turnip grows best in a rich free soil. The mode of culture varies with the soil. Where the soil is light and dry a smaller amount of ploughing, harrowing, and drilling is necessary than on stiff soils. The turnip is not well suited to clay soils, although it is often grown on them. A complete pulverisation of the soil is requisite before the sowing of the seed. On light soils a crop of turnips generally succeeds wheat or oats. Turnip-land is generally made up in raised drills by the plough, and the seed is sown by the drilling-machine on the top of the narrow ridges, which are about 27 inches wide. Moderate dressings of artificial manure, such as superphosphate of lime, crushed or dissolved bones, kainit and nitrate of soda, or other such manures, produce great crops of turnips. The more general practice, however, is to give a mixed dressing of farmyard dung and artificial manure. The young plants are thinned out by the hand-hoe to from 9 to 11 inches apart, and the ground is stirred and carefully kept clean by the plough or horse hoe. The turnip-crop is thus of great use in clearing the land of weeds. In many places part of the crop is eaten on the ground by sheep, which are confined to a small part of the field by means of movable fences. It is common to leave one of each three rows of turnips for this purpose, the other two rows being carried to the farmyard for feeding cattle or stored. Turnips are stored either in a house or in conical heaps, covered with straw and earth. They are sometimes protected from frost by being earthed up in rows by the plough. Some kinds are much more easily injured by frost than others; the Swedish turnips least of all.
The introduction of the turnip as a field-crop is one of the most important events in the history of British agriculture. It has rendered possible a rotation of crops which has been extremely advantageous, and has made the supply of butcher-meat more constant, by providing a supply of winter-food for cattle and sheep, whereas formerly all depended on the pasture. As a field-crop turnips were not raised in Britain till the end of the 17th century. Turnip-husbandry was introduced into Roxburghshire from Leicestershire about the year 1764, but was soon carried to a perfection in Scotland far beyond what it had previously reached. The climate of Scotland is well adapted for it, as is also that of Ireland; whilst the climate of North America is so unfavourable to it that it has not become an important crop there. Turnip-crops in Britain suffer very much from the two distinct diseases called anbury, or club-root, and finger-and-toe (see ANBURY). Superior culture is the best means of preventing these diseases. Plants weakened by drought are liable also to suffer from a white mould, a species of Oidium, which attacks the leaves and greatly injures the plant.
The turnip crop has to encounter many insect enemies. The most destructive in recent years has been the turnip-fly (Phyllotreta nemorum, Chevrolat), which devours the young plants before they are strong enough to be thinned. The leaves of the young turnip-plants are also attacked and often much injured by the green-fly or turnip-aphis (Aphis rapæ, Curtis); by the maggots of two kinds of Diptera—Phytomyza nigricornis, Macquard, the black-leaf miner, and Drosophila plana, Fallen, the yellow-leaf miner; by the caterpillar of the diamond-back turnip-moth (Cerostoma xylostella, Curtis); by the caterpillar of the common dart moth (Agrotis segetum, Westwood); by the grubs of the turnip sawfly (Athalia spinarum, Fabricius); and by at least two varieties of weevil, Curculio, the most destructive being the little Curculio, Centorhynchus contractus, inch long, which punctures the seed leaves with its rostrum. The diamond-back moth fortunately seems to pay its visits of destruction at long intervals. It played great havoc with the crop in England and Scotland in 1891, destroying it beyond recovery in many parts. In Ireland the attack was lighter. Earlier serious attacks occurred in 1851, 1837, and 1826. For all insect attacks the best preventive measure is a dressing of stimulating manure to force on the growth of the plants. Spraying the crop with a mixture hurtful to the insects, by means of the Strawsoniser, an air-power distributor, is also recommended.