To organic farmers, Kenneth Noel Nelson Jr. was the man with the golden manure: It was rich with Mother Nature’s finest waste, robust for the soil and cheap in price.

But to federal prosecutors in California, Nelson’s organic fertilizer empire had developed a stench.

On Thursday a federal grand jury indicted Nelson on 28 counts of mail fraud in connection with an alleged years-long scheme to dupe farmers and agriculture product distributors. The indictment accused Nelson, 57, of selling premium-priced liquid fertilizer touted as made from all-natural products such as fish meal and bird guano that instead was spiked with far cheaper synthetic chemicals.

The Los Angeles Times reports that the scheme, according to the federal indictment, enabled Nelson to become the largest purveyor of organic fertilizer to farmers in the western half of the U.S. and pull in at least $9 million in sales from 2003 to 2009.

This is the second indictment of an organic fertilizer producer in California in the last five months. It also has fueled fears among some farmers about possible contamination of their pristine fields and has raised questions about whether consumers bought produce that was billed as organic but may not have met federal organic requirements.

The indictment is part of a growing effort by the U.S. Department of Agriculture’s Office of the Inspector General to crack down on fraud and corruption in the organic industry — a segment of the food sector that has grown to more than $24 billion in the U.S. and has emerged as a lucrative business in the Golden State.

The agency has seven open investigations involving the federal National Organic Program, officials said.