During this period of expansion, farmers introduced modern production practices. The widespread use of modern inputs—chemical fertilisers and insecticides, agricultural machinery, and improved seed varieties—sharply improved crop yields. Yet the modernisation process again made farm production dependent on supplies from abroad, this time on imports of petroleum and fertilisers. By 1984 domestic sources of energy covered only about 20 percent of farm needs, while in 1950 domestic sources had supplied 70 percent of them. In the aftermath of the oil price increases of the early 1970s, farmers began to return to local energy sources such as firewood. The existence of many farms that were too small to allow efficient use of tractors also limited mechanisation. Another weak point was the existence of many fields with open drainage ditches needing regular maintenance; in the mid-1980s, experts estimated that half of the cropland needed improved drainage works. At that time, about 1 million hectares had underground drainage, and agricultural authorities planned to help install such works on another million hectares. Despite these shortcomings, Finland's agriculture was efficient and productive—at least when compared with farming in other European countries.