This is a fascinating question, that is answered at some length in Andrea Wulf’s Founding Gardeners.

The short version is that, while Jefferson was probably the most influential, he was certainly not the only American luminary to have a keen interest in agricultural improvement. Most of the American founding fathers were also agriculturalists who invested a great deal of time and effort into research, both to enrich their own estates and out of a desire to improve American agriculture generally by importing new crops and developing agricultural methods to compete with Europe. Washington at Mount Vernon, Madison at Montpelier, and Jefferson at Monticello all spent many years studying agricultural methods and exotic crops, with the ultimate goal of making America “food independent,” to transpose the modern terminology.

A lot of these experiments were flops. A few of them were remarkable successes. George Washington, for example, set out to create a breed to giant donkey to improve the supply of animal labor. The blend of Spanish, French, and other bloodlines that he ultimately selected became the American Mammoth Jackstock (of Jackson as it is sometimes known), which produces some of the largest and strongest mules in the world when crossbred with horses. In some respects, the Mammoth Jackstock was a victim of its own success - because a given donkey stud will usually refuse to breed horses if he has known donkeys in the past, it became a commercial necessity to use nearly every male Jackstock for producing mules - which are of course sterile. When the demand for mules collapsed, the Jackstock nearly went extinct. Today, the efforts of a dedicated breed organization have brought it back from the brink.

This is just one example of how the founding fathers (who were, with a handful of exceptions, wealthy planters who fancied themselves enlightenment polymaths) effected the American agricultural world with there experiments. By far the most influential - either from Monticello or from the group as a whole - was Jefferson’s lifetime work to acquire and popularize upland rice.

Before tobacco became the leading cash crop, rice was the driving agricultural product of the Chesapeake region. This was paddy rice, which had to be regularly flooded with standing water in order to yield a crop. This in term led to catastrophic malaria outbreaks, which both killed many people outright and left others with the enfeebling symptoms of chronic malaria for life.

On a visit to Europe, Jefferson observed that rice culture did not have to be done this way. Italian farmers used a variety of upland rice that did not have to be flooded. The Italian authorities guarded the cultivar jealously, and levied harsh punishments on anyone who tried to leave the country with viable seeds. Jefferson did just that, and returned to Virginia with a pocket of Italian rice ready to sprout.

Over the subsequent years, Jefferson and his rice went through a saga of experiment, failure, piracy, marketing, mutiny on the high seas, industrial espionage, and propaganda to spread the use of upland rice around the American south, which you can read much more about here.

Perhaps the most interesting question left is why Jefferson expended so much effort on introducing upland rice. The planter gentry at large saw agricultural science as a respectable and honorable pursuit, and exploits like Washington’s giant donkeys were also a way to boost national pride and personal prestige. But Jefferson’s rice was different. His stated reason for expending so much effort and risk was humanitarian - that he was concerned for the life and health of the many people (mostly slaves) who suffered from the malarial culture of Chesapeake rice. Of course, being an early adopter and sole supplier of a hot new commodity could also be a lucrative commercial opportunity.

In many ways, Jefferson’s experiments with rice encapsulate the contradiction at the heart of his person. An abolitionist who attempted to write slavery out of the constitution, he also owned and raped slaves. He risked imprisonment to smuggle upland rice out of Italy - was it for the good of the enslaved people who worked his fields, or of their captors who dined at his table?