For decades, U.S. farmland has been a refuge for investors. The country’s highly productive agricultural sector has supplied domestic consumers while exporting food and commodities around the world. And people, after all, will always need to eat. Even Bill Gates, Microsoft’s co-founder, has built one of the largest privately held portfolios of U.S. farmland, reportedly totaling about 275,000 acres — presumably as a counterweight to a technology-heavy fortune. But a portfolio hedge can become ...