To be honest it has little to do with reform in America and is more of a political philosophy that had to do with the developing nature of the country and the divide between America's first political parties. The first party the Federalists had a power base in New England which had always relied heavily on merchant shipping and trade over straight agriculture. As a result this was usually the party of many merchants and traders. Hamilton, John Adams, and Washington were Federalists. The Federalist party grew into a party that wanted to expand the power of the national government. Jefferson and James Madison grew disillusioned with the government's expanding power and decided to create an alternative party that became known as the Jeffersonian-Republicans.
Jefferson based his party around his own philosophical ideas about agrarianism that became known as Jeffersonian Democracy. This philosophy relied heavily on Jefferson's own distrust of all things urban and heavily borrowed ideas from the Roman Republic i.e the citizen farmer soldier. To Jefferson only farmers who owned their own land and relied on their own selves to survive and prosper could sustain a true democracy as they would not be living at the beneficence of others like the majority of people's living in the urban areas did. In many ways it was an anti-political machine platform that become so pervasive in the 19 century. Jefferson was very elitist in that he believed that only men who owned agrarian property were capable to be educated in the way that democracy would survive and thrive and thus were the only ones capable of voting. In other words Jefferson supporters favored small government, a strong legislature, a very limited interpretation of the Constitution, and a weakened court system.
This agrarian philosophy carried into said court system as the Federalists and Jeffersonians struggled to gain control of the Supreme Court. One of the great ironies of American history was the Supreme Court led by John Marshall, a Federalist, deciding numerous times to interpret the Constitution in a way that James Madison disagreed with vehemently, and this was the guy that penned the Constitution.
The Jeffersonian party eventually died out, but not before the Federalist Party was completely extinguished, and was succeed by Andrew Jackson's political party. Jackson took many of Jefferson's agrarian ideas but put them in a more populist slant, opening up voting to all white men regardless of if they had property or not. Jackson gets knocked around a lot on this board a lot but people have to remember that he was one of the most popular contemporary presidents of all time.
In other words the word agrarianism in the United States probably has a different meaning than in other countries like Germany because of how much it tied into the early American political parties and really became an power struggle between those who supported a stronger versus a weaker central government. Culminating to Jackson who sort of merged the two together. I really need to learn more about 17th and 18th century Europe because I only know about it from an American and British Perspective.