California farmer Michael Robinson's 120 acres in the Sacramento-San Joaquin River Delta might seem like the perfect site for a 20-megawatt solar array to power thousands of homes. It's near transmission lines and lacks the endangered tortoises, long waits for federal permits and other obstacles that have tripped up larger solar projects in the Mojave Desert.

But it's not that simple. As solar developers increasingly focus on agricultural land and farmers look to cash in, California's scramble to meet its renewable energy mandate inadvertently threatens some of its richest soil, pitting individual farmers against farmland advocates. "It stirs a lot of emotion," says San Joaquin County Farm Bureau Executive Director Bruce Blodgett, who's in the tough spot of opposing Robinson, a friend.

California has lost more than a million acres of irrigated farmland since 1984, mostly to housing. "Solar sprawl is no better than any other kind of sprawl," says Chris Scheuring, an environmental attorney with the California Farm Bureau Federation. "We've got to hold the line on our best soils because they're irreplaceable. They're only renewed at geological rates."

Nearly two-thirds of California's agricultural land is currently protected under the 1965 Williamson Act, which offers farmers a property tax break in exchange for not developing their land for 10 years. These contracts apply only to the most productive "prime" agricultural lands and are between farmers and counties, renewing automatically each year so the end is always a decade out. Today, the contracts cover 16 million acres, including Robinson's 120.

But farmers are increasingly tempted to break them. Profit margins are often slim, and the contracts' tax breaks are far outweighed by the financial benefits of leasing or selling land to a solar company. And there are a few ways out. Public utilities can cancel the contracts through eminent domain. Dozens of large-scale solar projects are in the works on farm land around the state. A particular hot spot is Fresno County, where Pacific Gas and Electric plans 30 solar installations totaling 250 megawatts -- about half on farmland protected under contract. "We're sensitive to the need for preserving farmland, but these are willing sellers," says PG&E spokesman Denny Boyles.

Counties, which get more property taxes from solar projects than from farmland, can also cancel the contracts under "extraordinary circumstances," such as pressing public needs that can't otherwise be met. The Fresno County Board of Supervisors did just that for a 90-acre parcel last summer, arguing that it was the perfect place for a 40-megawatt project, given its lack of irrigation water and proximity to transmission lines.

The Farm Bureau sued over this decision in October. "This land is … very productive. Almonds and honeydew melons have been grown there," says Scheuring. "We want a precedent that the Williamson Act can't just be tossed aside to promote lucrative developments."

Last August, Robinson asked San Joaquin County to let him lease his own parcel to Northern California solar company DG Power. Robinson, who didn't respond to repeated requests for comment, has said the soil is too salty to be productive.

Blodgett, who joined Scheuring in arguing against Robinson's request, agrees that the parcel is on the unproductive side. But he thinks there's a simple fix: "Our soil conservationist said the irrigation could be managed to increase productivity."

San Joaquin's supervisors denied Robinson's request in January, but they also said the solar project was compatible with their general plan. That opens the way for it to become the first test case of a new compromise from State Sen. Lois Wolk, D, whose district includes Robinson's land. Her solar-use easement act (SB 618), which took effect this year, lets counties break Williamson Act contracts if the state Department of Conservation deems the land unproductive. An official application will have to wait, though, since the state only began setting soil criteria and other SB 618 regulations in May.

Whatever the outcome of Robinson's case, this is just the beginning of the fight over solar projects on protected farmland. "The word on the street is everyone's going to be asking for these easements," says Conservation Department spokesman Donald Drysdale. The Farm Bureau hopes Wolk's act will shield the best soil from the rush. "We do have less productive land," says Blodgett, "and we should put solar there."