Finland's climate and soils make growing crops a particular challenge. The country lies between 60° and 70° north latitude - as far north as Alaska - and has severe winters and relatively short growing seasons that are sometimes interrupted by frosts. However, because the Gulf Stream and the North Atlantic Drift Current moderate the climate, and because of the relatively low elevation of the land area, Finland contains half of the world's arable land north of 60° north latitude. In response to the climate, farmers have relied on quick-ripening and frost-resistant varieties of crops. Most farmland had originally been either forest or swamp, and the soil had usually required treatment with lime and years of cultivation to neutralise excess acid and to develop fertility. Irrigation was generally not necessary, but drainage systems were often needed to remove excess water.