After months of private negotiations, the details of the county’s deal with agribusiness giant Monsanto are now public.

According to the terms, which are set to go before the Board of Supervisors for approval Tuesday, Pima County will lend its support to the company’s application to the U.S. Commerce Department for a 10-year free-trade-zone designation, which comes with substantial savings in county property taxes.

In exchange, Monsanto pledges to spend at least $90 million on its 7-acre automated corn-growing greenhouse; employ at least 25 full-time and 25 part-time workers at an average annual salary of at least $44,000; and provide health, dental, and retirement benefits, according to a memorandum from County Administrator Chuck Huckelberry released Thursday.

The greenhouse will be on agricultural land near the intersection of Twin Peaks and Sanders roads near Marana and will employ a number of water-conserving measures. The company intends to break ground before the end of 2016.

Monsanto’s plans to come to Pima County were first reported by the Arizona Daily Star in August.

A free-trade-zone designation substantially lowers property tax assessment ratios, in this case from the property’s current 15 percent ratio to 5 percent.

Assuming the company spends roughly $95 million on the project, its estimated county primary and secondary property tax bill for the first year would be nearly $190,000, according to county calculations in documents obtained by the Star through public-records requests. Without the designation, that bill would be more than $500,000. The previous owners paid just shy of $2,000 in all property taxes in 2015 on the undeveloped agricultural land.

The economic impact of the project is estimated at around $280 million between 2016 and 2025, according to an analysis conducted by Sun Corridor Inc.

“Monsanto will become the largest taxpayer in the Marana Unified School District,” Huckelberry wrote in his memo to the board. “It will have twice the taxable value of the Ritz Carlton Hotel and Resort in Marana.”

“Basically it means everyone else’s school district taxes are going down,” he said in an interview.

The agreement to be considered by the board deals only with the tax rates the county directly controls. Other taxing districts will have to work out their own arrangements. For example, Marana Unified School District’s Governing Board approved a $500,000 cash payment from Monsanto instead of a so-called payment-in-lieu-of-taxes agreement in late October, according to the memo.

Among some environmentalists and farmers, Monsanto is a controversial company. To address such concerns, Huckelberry proposed the creation of two commissions tasked with looking out for potential “adverse impacts” of Monsanto and separating “fact from fiction” when it comes to claims about the company.

The commissions, one of which would focus on agricultural science and the other on social and policy concerns, would meet at least quarterly. Monsanto would pay up to $50,000 annually for any research or technical analysis deemed necessary by the science commission.

Supervisor Richard Elías said the conditions set for Monsanto were “heading in the right direction” but that they were not enough to secure his vote next Tuesday. He pointed to health and social concerns raised by Monsanto’s critics as “serious fears that need to be confronted here.”

Supervisor Ally Miller, who has also criticized the deal, did not return a call for comment.

While acknowledging what he called “historic missteps” of the company, Supervisor Ray Carroll said those “certainly shouldn’t impact the decision on this clean and environmentally sustainable effort to feed the world’s hungry.”

“I’m definitely leaning towards supporting it,” he said.

Supervisors Sharon Bronson and Ramón Valadez did not immediately return a call for comment.

Contact: mwoodhouse@tucson.com or 573-4235. On Twitter: @murphywoodhouse