Evaluating Methods to Enhance Microbial Degradation of Residual Soil Contaminants

Final Report for FW95-075

Project Type: Farmer/Rancher
Funds awarded in 1995: $5,000.00
Projected End Date: 12/31/2004
Matching Non-Federal Funds: $6,737.00
Region: Western
State: Oregon
Principal Investigator:
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Project Information


The study seeks methods of breaking down the residual organochlorine insecticides found in many Oregon soils or keeping them from going into plants like zucchini, carrots or melons.

Chlordane, Dieldrin and DDT, once widely used organochlorine (OC) insecticides, were designed to break down slowly. As a result, many persist in agricultural soils throughout Oregon and elsewhere, preventing organic and conventional farmers from planting crops like carrots and members of the cucurbit family (for example, zucchini and melons), which tend to draw the contaminants into their flesh at levels above EPA and FDA tolerances.

The study, conducted at Heron's Nest, a 50-acre organic farm on the banks of the Willamette River, looked at breaking down the residual insecticides or keeping them from entering the plant using four treatments: 1) activated carbon, which is know to absorb soil contaminants; 2) green manure in a combination of vetch, rye and buckwheat to stimulate soil microbial activity; 3) compost, also to stimulate microbial activity as well as buffer plant uptake; and 4) radical biological loading, which uses soybean oil to aid the soil's transition to an anaerobic environment - microorganisms that thrive in transition environments are known to degrade OCs.

Soil samples were taken from each plot and sent to labs for analysis of microbial populations and residue levels of Chlordane and Dieldrin. In each of the first two years of the study, the treatments were applied to their respective plots. In the third year, the treatments were again applied and planted to zucchini.

Unfortunately, separate grant proposals seeking funds to conduct the final soil residue and microbial population analyses failed to materialize. So the project used the remaining SARE funds to conduct plant tissue tests to determine the level of OC uptake, which indicates whether the treatments succeeded in buffering soil contaminants.

The research showed that all four treatments were effective at ameliorating or buffering plant uptake compared with control plots. The green manure treatment was the most effective, followed by activated carbon, compost and radical biological loading. In addition, the research showed that more mature fruit is likely to have a greater rate of uptake.

In summary, project coordinator J.J. Haapala says, "This is clearly work that is not easily accomplished within even a decade of research. The testing is very expensive and not particularly conclusive. However, I find it instructive that the treatments that provide the most organic material and the most microbial activity are the most promising for buffering plant uptake of soil contaminants."

Ultimately, the benefits of studies like these and others conducted in the future are to provide farmers and gardeners with techniques they can employ to make their soils safe for growing a wider variety of crops on previously contaminated soils.

The project participants have yet to cite examples of growers who have adopted any methods to break down the residual traces of these once-popular insecticides or keep them from entering plant tissue.

Future research might look at the treatments in succession, for example, using a green manure to stimulate soil microbiology and following that with the radical biological loading treatment.

The project held two field tours in August 1999. On Aug. 16, 50 gardeners and farmers visited the project on the Haapala farm. The Oregon Tilth newsletter, In Good Tilth, wrote about the field day. During the last weekend in August, a group of farmers, instructors and interns from Approvecho (a sustainable ag research institute in Cottage Grove, Ore.) toured the experiment. A final report of the project results was printed in In Good Tilth.

J.J. Haapala, a producer from Junction City, Ore., provided the farmland on which the studies were conducted. In addition, a number of farmers and farm advisors visited the project.


Participation Summary

Research Outcomes

No research outcomes
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.