Organic soil amendments, including animal and green manures and wastes from processed animal products like blood, bone, and fish meal, have been used as soil supplements for centuries.

However, one reason these organic materials have never been seriously considered as plant protectants is that there is no good way to evaluate their effectiveness, according to George Lazarovits, a plant pathologist with Agriculture and Agri-Food Canada. But, with the loss of methyl bromide looming in the near future, researchers are taking a closer look at these soil amendments.

"Growers have come to rely heavily on chemical fumigants, especially methyl bromide, because it controls such a wide range of pests and dissipates quickly to allow planting soon after treatment," he explains. "In lab and field tests, we found soil amendments good candidates to replace methyl bromide for eliminating certain soilborne pests."

"While an assessment of a chemical pesticide can be completed in a matter of hours, screening organic products or biological control agents requires weeks, even months," explains Lazarovits. "Not only are biological interactions complex and difficult to evaluate, their effects are more subtle than those of chemicals. And organic products must be tested in soil, which involves soil microbes, thereby further complicating the situation."

He and colleagues at the Pest Management Research Center in Ontario are working with feather meal and soymeal; meat, blood, and bone meal; hydrolyzed pig hair; fish byproducts; chitin; and manures from various animal sources and composts.

"We mixed various quantities of these products with soils from potato fields that had a history of scab and wilt. The products are nontoxic and most are edible," Lazarovits says. "Their effectiveness varies from soil to soil, likely because soil microbiology plays a crucial role in how each product degrades."

He found that in test plots planted with potatoes and tomatoes, soil amendments increased yields, reduced scab and wilt, and virtually eradicated plant pathogenic nematodes. Plants were greener, more vigorous, and survived the entire season, whereas plants in untreated soils died soon after emergence. Treated soils sprouted fewer broadleaf and grassy weeds.

"All treatments were somewhat phytotoxic for about 2 to 4 weeks, which only delayed plant growth in some cases, but was lethal for tomatoes. But in the second and third seasons after application, there was dramatic improvement in growth of tomatoes and potatoes," Lazarovits reports. "We found that potatoes tolerate treatment reasonably well in the first season if the supplements are applied the previous fall. We're investigating ways to control this residual phytotoxicity."

Effectiveness of an amendment to kill microsclerotia (fungal spores) produced by Verticillium dahliae depends on its nitrogen content and carbon-to-nitrogen ratio, and how it changes this balance in the soil. The amendments are ineffective in heat-sterilized soil, indicating that the active ingredients are the breakdown products, he says. To prevent production of active ingredients, both amendments and soil must be sterilized. There are enough microbes in untreated amendments added to sterilized soil to release active components, but it would take some time to be effective.

Amendments will usually kill microsclerotia in the soil within 7 to 10 days, when the soil is 75 °F, with 50 percent water-holding capacity, according to Lazarovits. He says that temperature is not critical; in fact, they got excellent results at 45 °F. However, soil moisture is important to efficacy; the amendments worked faster in drier soil.

How do these soil amendments kill harmful pathogens in the soil?

"Several mechanisms are at work, here," Lazarovits says. "The most successful products are those that raise soil pH temporarily to above 8.5 for a few weeks, after which it drops back to just slightly below what it was before treatment. Early on, volatile toxic gases, likely ammonia, from the compounds, quickly kill the fungal spores. Later effects are slower and probably involve biological control."

Unlike fumigation, which reduces overall numbers of soil microbes, soil amendments increase the total number of fungi and bacteria by up to 10,000 fold, but decrease the number of pathogens.

Lazarovits and colleagues are conducting further research to determine optimum concentrations of product required, timing and depth of application, cultural requirements, and longevity of protection.

Since large amounts of these products would be needed and amendments have variable efficacy depending on soil microbiology at different locations, one concern is the economics.

"We're looking at a thorough cost-benefit analysis. Granted, this system would be expensive and moving large quantities of these materials to the field would be difficult. However, we have developed a soil test that can be done in the lab prior to field application to see if a specific formulation will do the job required," Lazarovits explains. "A better understanding of how these amendments work will also help reduce the amount required in specific types of soil."