After more than two decades of debate, abandoned plans and false starts, Simsbury is on the verge of deciding whether 288 acres of former tobacco fields will be set aside as open space and athletic fields. If voters approve on May 4, Simsbury will put about $2.2 million toward a $6 million purchase of the land, part of a farm where Martin Luther King Jr. worked in his youth. Griffin Industrial Realty, the owner, had fought for years for permission to create a massive subdivision with an affordable housing component, but never went ahead with the project. Instead, Griffin changed course and last year negotiated to sell to the Trust for Public Land in a complex $6 million deal that includes a patchwork of state agriculture, historic and open space preservation grants along with the town money. It’s unclear how Griffin would proceed if the deal falls through. The company went to court more than a decade ago to get zoning approval for 296 homes on the site, including 88 intended for affordable housing. But the land required extensive environmental remediation because of fertilizer chemicals, and Griffin never moved forward with the housing project after it was approved.