A farming experiment at the University of California, Davis, has found that organically grown tomatoes are richer in certain kinds of flavonoids than conventionally grown tomatoes. And one researcher is curious to determine why this may be.

Stephen Kaffka cut his teeth in organic gardening when he was a student at the University of California, Santa Cruz, in the late 1960s. He was involved in running the now-renowned student garden there, a task that was both intellectually satisfying and physically challenging. “It was a great time,” he recalls.

Kaffka went on to get a Ph.D. in agronomy. Now, one of his latest interests has taken him back to his organic roots — a desire to figure out if the way we grow things can actually make food more nutritious.

Antioxidant Amplification

In Northern California, on some fields west of Davis, Kaffka and his colleagues have been comparing organic and conventional tomatoes grown in neighboring plots. It’s part of a UC Davis study dubbed the “Long-Term Research on Farming Systems Project,” which was begun in 1991 and is slated to last 100 years.

So far, the researchers have found that the organic tomatoes have almost double the concentration of two types of flavonoids — quercetin and kaempferol — which are considered to be healthful plant compounds with potent antioxidant activity. The 10-year mean levels of quercetin were 79 percent higher than those in conventional tomatoes, and levels of kaempferol were 97 percent higher.

The Answer in the Dirt

The increased flavonoid levels, Kaffka suspects, could stem from the difference in how organic and conventional tomatoes are fertilized.

On Kaffka’s plot, the conventionally grown tomatoes get commercial fertilizer made with soluble inorganic nitrogen, a form of nitrogen the plants can take up very quickly. The organic tomatoes get nitrogen from manure and composted cover crops. These organic materials have to be broken down by the microbes in the soil before the nitrogen is released to the plants.

“It takes time,” Kaffka says, and the nitrogen is “not instantaneously available.”

With limited nitrogen, the organic plants may grow slower, says Alyson Mitchell, a food chemist at UC Davis. When this happens, she says, the plant “has more time to allocate its resources toward making secondary plant metabolites” such as flavonoids.

Unanswered Questions

Though such findings are intriguing to researchers, Kaffka cautions that there are many factors that could confound the results: The soil types could be different, moisture or irrigation might have varied from plot to plot, and the variety of tomatoes might have played a role.

He points out that the few rigorous studies that have compared organic systems with conventional systems have returned mixed results. There is no consistent evidence to suggest that organic methods lead to more healthful foods. So Kaffka remains skeptical.

Because growing systems are so complicated, Kaffka says he needs additional research to pinpoint a cause and effect between fertilization and flavonoids. If future research confirms that limiting the supply of nitrogen to tomatoes consistently leads to more flavonoids, then perhaps conventional farmers could use this approach, as well.