Jefferson had an abiding interest in improving the technology of farming. One of his more important contributions to agriculture was the "mouldboard of least resistance" for a plow.
While serving as minister to France, Jefferson had the opportunity to observe European plow designs. Their deficiencies inspired him to set down in a memorandum (1788) his plans for an improved moldboard, the wooden part of the plow that lifts up and turns over the sod cut by the iron share and colter. He wished to make that lifting and turning action as efficient as possible, so that the plow could be pulled through the soil with the least expenditure of force.
He brought his love of mathematics to his design, which he declared was "mathematically demonstrated to be perfect."
By 1794, Jefferson had put his plans into action at Monticello. He had a plow fitted with a wooden moldboard of his design and reported to Sir John Sinclair (23 March 1798) that "an experience of five years has enabled me to say, it answers in practice to what it promises in theory." In addition to offering the least resistance as it was pulled through the soil, Jefferson's moldboard had a further advantage: "It may be made by the coarsest workman, by a process so exact, that its form shall never be varied by a single hair's breadth." Ease of duplication was thus another measure of the usefulness of his design.
In 1814 Jefferson began to have his moldboards cast in iron. He informed Charles Willson Peale (21 March 1815) that the plow with his iron moldboard was "so light that the two small horses or mules draw it with less labor than I have ever before seen necessary. It does beautiful work and is approved by everyone."
Just how widely Jefferson's moldboard was adopted by others is unclear. He never sought to patent it, and in fact sent numerous models to friends at home and abroad, where his design met with general approval. Jefferson's moldboard was featured in James Mease's Domestic Encyclopedia (Philadelphia, 1803), and the French Society of Agriculture awarded Jefferson its gold medal and membership as a foreign associate.
A full-scale model of a plow with Jefferson's moldboard is on display at the Monticello Visitors Center near Monticello. For further reading, see Edwin M. Betts, ed., Thomas Jefferson's Farm Book (pp. 47-64) and Betts, ed., Thomas Jefferson's Garden Book (pp. 649-654). Publishing information is available.
Jefferson is also credited with the invention of the wheel cipher, also featured in "Resources," as well as the spherical sundial.
More information about ploughing at Monticello and a compilation of Jefferson quotes about agriculture are available.