Two industries have for centuries been associated with the barren heaths and sodden fens so usually found together on the sand-grounds, namely, the cultivation of buckwheat and peat-digging. The work is conducted on a regular system of fen colonization, the first operation being directed towards the drainage of the country. This is effected by means of drainage canals cut at regular intervals and connected by means of cross ditches. These draining ditches all have their issue in a main drainage canal, along which the transport of the peat and peat-litter takes place and the houses of the colonists are built. The heathlands when sufficiently drained are prepared for cultivation by being cut into sods and burnt. This system appears to have been practised already at the end of the 17th century. After eight years, however, the soil becomes exhausted, and twenty to thirty years are required for its refertilization. The cultivation of buckwheat on these grounds has decreased, and large areas which were formerly thus treated now lie waste. Potatoes, rye, oats, beans and peas are also largely cultivated. In connexion with the cultivation of potatoes, factories are established for making spirits, treacle, potato-meal, and straw-paper. Furthermore, agriculture is everywhere accompanied on the sand-grounds by the rearing of sheep and cattle, which assist in fertilizing the soil. Owing to the meagreness of their food these animals are usually thin and small, but are quickly restored when placed on richer grounds. The breeding of pigs is also widely practised on the sand-grounds, as well as forest culture. Of the fen-colonies in Drente the best known are those of Frederiksoord and Veenhuizen.