Challengers of the 11,750-home Ho‘o­pili community proposed for prime farmland in Ewa began their main assault on the plan Thursday, trying to undermine a case developer D.R. Horton has made for building the $4.6 billion project.

Eight expert witnesses primarily in fields related to agriculture testified before the state Land Use Commission, countering several key positions taken by Horton experts in earlier hearings.

The testimony presented Thursday represented the first offensive salvo from three entities — the Sierra Club’s Hawaii chapter, community group Friends of Maka­kilo and state Sen. Clayton Hee — contesting Ho‘o­pili before the commission.

One major issue attacked was whether comparable replacement land is available on Oahu that can be farmed to make up for the loss of crops — mainly vegetables — presently grown on the 1,554-acre site.

Leon Sollenberger, a farm service provider who has tilled fields all over Oahu, said the dry and sunny climate of the Ho‘o­pili site, combined with its flat topography, abundant water supply and proximity to the island’s main consumer market, makes it irreplaceable.

"We only get one time to destroy it," he said. "Once it’s gone, it’s gone."

Sollenberger countered testimony of Bruce Plasch, an agricultural economics analyst retained by Horton, who said plenty of fallow farmland exists to offset what is displaced by Ho‘o­pili.

Plasch’s conclusion that urbanizing the Ho‘o­pili site would have no adverse impact on agriculture also was criticized by Linda Cox, a community economic development specialist with the University of Hawaii’s College of Tropical Agriculture and Human Resources.

Cox, an economist who works with farmers, told commissioners she doesn’t consider Plasch an expert on agriculture because he is an expert witness for developers and doesn’t work with farmers.

Part of Plasch’s view that land isn’t a limiting factor for agriculture in Hawaii was also based on the premise that a lot of food can be grown in greenhouses and without soil using hydroponics.

Hector Valenzuela, a vegetable crop specialist who has worked with UH, said hydroponics are capital-intensitive systems that work for some crops like lettuce but not others that need lots of room or soil to grow.

Texas-based Horton, through its local Schuler Division, has attracted support for Ho‘o­pili from the Hawaii Farm Bureau Federation and the state Department of Agriculture, in part because those entities respect the city’s urban growth boundary that supports urbanizing the Ho‘o­pili site and protecting farmland elsewhere on Oahu.

But Glenn Martinez, president of Hawaii Farmers Union United, challenged the notion that Hawaii’s agricultural industry supports Ho‘o­pili.

The local farmers union has around 300 members who are mostly small farmers. He said the group opposes Ho‘o­pili based on a member poll, and said no such representative support exists at the Farm Bureau. The bureau left its decision to its board, which includes its president, Dean Oki­moto, who is promoting Ho‘o­pili as a Horton consultant.

Martinez, who owns the Wai­ma­nalo organic farm Olo­mana Gardens, is a member of the Farm Bureau.

A major reason the Agriculture Department supports Ho‘o­pili is because Horton has committed to provide 251 acres for commercial farming, community gardens and home gardens.

Martinez said Horton is committing its least viable land for the 159 acres of commercial farming.

Horton’s plan also calls for eight acres of community gardens and 84 acres for home gardens.

Home gardens sound good, according to Martinez, who sells raised-bed systems for such use. But he said his experience is that only 1 out of 100 homeowners will be interested. "There’s no backyard gardening going on," he said. "It’s a few individuals doing it."

Other experts who testified Thursday included Gary Mau­na­kea-Forth, managing director of MA‘O Organic Farms in Nanakuli. He testified about the importance of farming and efforts to grow a future crop of local farmers.

Hawaii tourism industry consultant Peter Apo testified about the negative impact on tourism as green landscapes are converted to look like mainland housing subdivisions.

Tom Giambelluca, a climatology expert and UH professor, questioned projections that there will be enough water for large housing projects like Ho‘o­pili.

Mike Lee, a Native Hawaiian cultural practitioner, expressed concern that developing the site would produce storm water runoff with contaminants that damage limu, or seaweed, along the shoreline makai of the project. Lee also claimed that Ho‘o­pili might disturb Hawaiian burials, though Horton attorney Benjamin Kudo said any burial caves in the area wouldn’t be disturbed by the relatively shallow foundations of Ho‘o­pili buildings.

The hearing is scheduled to resume today at 9 a.m. Only two witnesses are scheduled to testify because of availability. Public testimony will be accepted immediately after the experts are done, and also at 3 p.m.