When Drewsen was married in 1898, his father gave him the agricultural estate Smidstrup some ten kilometres north of Strandmøllen. By studying German literature on farming, and consulting the most successful farmers in the area, the pastors in
Gentofte and
Kongens Lyngby, and the brothers Jean and
Frédéric de Coninck at Frederikslund, he acquired a great knowledge about farming. He introduced new technology (such as the Scottish plough) as well as new practices (such as
crop rotation) and crops (large-scale cultivation of potatoes). He became a member of the
Royal Danish Agricultural Society ('') in 1812 where he became the editor and writer of several publications on farming. He received the society's silver medal for a work on
crop rotation in 1813 and its gold medal for a work on cultivation of clover. He also published a nine-volume translation of
Albrecht Thaer's works on agronomy (
I-IV, 1816–19). From 1819 to 1930, he served as one of the society's three presidents. Together with Jonas Collin, he was the founder of the Agricultural Society for Copenhagen County ('') in 1818 (closed 1824). In 1819, he went on a study trip to Germany. Together with Frédéric de Coninck and pastor Rønne, he was the publisher of the journal (I–VII, 1815–1817), which he later continued alone (VIII–IX, 1818–1819; I-IV, 1819–1825). His most important work on agronomy, , was published in 1834 (second edition 1842). In 1841 and 1842 he published some minor works on the breeding of silk worms and cultivation of
mulberry trees. He was involved in a controversy with professor Oluf Christian Olufsen. ==The paper mill==