Soil supplements help organic crops ward off disease

Organic growers have the challenge and opportunity of building a living community in the soil that supplies their crops with far more than nitrogen, potassium and phosphorous.

Some of the living soil supplements help ward off the effects of plant disease, while others help the crop endure under extreme drought conditions.

"Drought tolerance is a huge benefit of mycorrhizae," said Larry Simpson, a biorational specialist with Valent USA. "Whatever moisture is in an area of the soil seven or eight times as large as the roots is available to the plant."

He made his remarks as growers and researchers discussed biological amendments or supplements to produce better crops during the California Association of Pest Control Advisers seminar on Sustainable and Organic Production in Northern California, held in Chico.

As evidence of the powers of mycorrhizae, Simpson showed a slide of a small but healthy looking oak tree growing out of rocks in a desert region of Sonora, Mexico, that gets just 4 inches of rain.

"How does it do that?" Simpson asked of the magical oak tree. "It largely has to do with mycorrhizal fungi, which grow on the roots of plants and do amazing things to help tolerate drought. They send these microfilaments out into the soil that are extensions of the roots. In exchange, the plants feed the fungi."

He recounted the story of a cotton grower in Arizona who was able to grow his crop with an acre-foot less water because it was colonized by mycorrhizae, and said University of California, Davis, researchers are doing a field trial of almonds with and without mycorrhizae.

Researchers at Guelph University in Ontario already completed a study on the benefits of mycorrhizae on 13 different plant species.

"They planted them in spring, watered them, then left them alone until the fall," Simpson said. "There was 41 percent survival among the non-treated plants, and 90 percent among those treated with mycorrhizae."

The fungus helps crops by sending out microscopic filaments that access water and nutrients from far more of the soil than the roots can reach, he said.

"These filaments are very tiny, but they'll grow 18 to 24 inches out into the soil," Simpson said. "You can get up to a mile of these filaments in a spoonful of soil. These hyphae or filaments are the preferred habitat for many of the beneficial organisms in the soil. Not only do you get the benefits of the hyphae, but you expand many of the other beneficials in the soil."

While mycorrhizae extend the roots, many growers say they find that seaweed extracts bring a whole new level of health to their crops.

"I'm a seaweed addict," said Scott Park, who grows more than 1,700 acres of 30 different organic crops in the Sacramento Valley. "I put it on every crop; it's just part of the whole package. I put a lot of emphasis on quality, and I think it helps. I could be a seaweed salesman."

For Park, the benefits of seaweed are measured, not in yield, but in the health and appearance of his crops.

"We don't want gigantic plants, we want healthy plants," he said. "The quality is there because they get all the micronutrients from healthy soil, and they can withstand water stress. For diseases and insects, we almost never have problems in all these crops. We see the diseases we hear about, but over time they dissipate."

Farmers have been applying seaweed, or seaweed extracts, to their crops or soil for centuries.

"Even the Pilgrims used it, because the fish was too valuable as food," said Tom Quick, sales manager for the San Diego biostimulant company GrowMore. "If I were a salesman, I would tell you that this will give you 5 to 20 percent more roots and yield. It sounds like snake oil, but it's true."

It takes a couple applications of a seaweed product to achieve optimal benefits, according to Quick.

"Seaweed extract is applied two times in a four-week period; that's how we use it," he advised. "You apply it to either the roots or the foliage. Seaweed extract is one of the largest biostimulant categories."

While seaweed extract is best applied multiple times to each crop, mycorrhizae is a fungus that, once established, can live in the soil as long as it has host roots.

"These spores can last up to two years in the soil, or even longer," Simpson said. "They will just sit there until a root comes along."

While these fungi are naturally occurring, they usually have to be applied to the soil.

"We often till our farmland and leave it fallow, and the mycorrhizae have to have living roots, or they die," Simpson said. "If you're working with ground that has been fumigated, you can pretty much guess the mycorrhizae are gone."

The common practice of removing non-crop plants from areas close to fields, in order to reduce weed and disease pressure, results in an environment devoid of sources of mycorrhizae to migrate to the farm.

It should only take once, however, to introduce these beneficial fungi, because they can live off the crop host roots.

"One application should be good for the life of the plant," Simpson said. "If it's a tomato plant it would be a year, but if it's a tree crop it should last 20 years or more."

(Bob Johnson is a reporter in Davis. He may be contacted at bjohn11135@aol.com.)