Bacteria, fungi more numerous in organic soil systems

Organic systems have more bacteria and fungi and more different types of soil organisms than conventional systems, according to a recent study.

As part of her PhD program, Alison Nelson found that diverse soil communities “cycle nutrients, impart resistance and resilience to stress, and carry out other ecosystem functions.”

In the study, which she conducted at the University of Alberta research farm, Nelson compared five wheat varieties grown under organic and conventional management over three years.

She used phospho-lipid fatty acid analysis, a sort of microbial fingerprinting that uses chemicals in cell membranes to tell microbes apart, to look at soil organisms in samples taken from her plots.

Most soil organisms obtain their energy by consuming organic material in the soil. Greater diversity allows soil organisms to use more types of organic material as food and thus are more able to mobilize nitrogen.

Mycorrhizal fungi in the soil community can be important in helping plants take up nutrients that are difficult for plants to access, such as phosphorus, copper and zinc.

It’s hard to know why there are more soil organisms in the organic plots because many management factors differ between the organic and conventional plots.

Nelson suggests that the greater diversity of plants in the organic plots contributed to the greater microbial diversity.

Organic plots were weedier, and a greater diversity of plants, including the weeds, would have resulted in more types of roots and more types of materials secreted by the roots. The microbial food supply would be more diverse.

However, that doesn’t necessarily mean that weeds were helpful in the organic plots.

Organic plots yielded less than conventional plots, and the study’s authors attribute this to competition with weeds.

As well, Nelson found that some types of microbes were more common in the weediest of the organic plots. The increase of those microbes did not result in improved wheat yields or quality.

In the organic system, the presence of mycorrhizal fungi was associated with increased yield and improved bread making quality.

Mycorrhizal fungi were less abundant in the weediest plots.

Most of the weeds in this study, such as redroot pigweed, lamb’s-quarters and smartweed, are nonmycorrhizal, which means they do not form associations with mycorrhizal fungi.

This suggests another reason why mycorrhizal fungi may have been fewer in the conventional system – the inclusion of canola in the crop rotation. Canola is also non-mycorrhizal.

Weediness was not the only difference between the organic and conventional systems.

Chemical fertilizers were applied in the conventional system, which have been shown to decrease mycorrhizal fungi.

Crop-fungi associations cost both partners. The crop pays in carbohydrates while the fungus pays in mineral nutrients. When the crop has easy access to nutrients, the association is not beneficial to it.

In Nelson’s study, nutrient levels were similar in organic and conventional plots, but the organic system received compost rather than chemical fertilizer.

Compost increases soil organic matter and thus the food supply for soil organisms.

In the conventional system, AC Superb plots had the most mycorrhizal fungi and the most diverse soil community of the five cultivars.

Because AC Superb is the newest cultivar in the test, it suggests that generations of breeding efforts have not reduced the mycorrhizal nature of wheat.

In the organic system, there were no noticeable differences in the soil organisms among the different varieties.

However, other differences were noticed. Marquis, the oldest and tallest variety, yielded the least and had the lowest protein. AC Superb, the newest variety, yielded the most. Glenlea had the fewest weeds and the highest protein.

All varieties, in both organic and conventional systems, produced wheat with acceptable bread making qualities.

The world of the soil is still a largely unknown environment. Scientists are just beginning to understand the roles of specific organisms.

“Until we know the functions carried out by specific organisms, maintaining diversity is one way of ensuring ecosystem functionality,” Nelson said.

Organic methods may help maintain that diversity.

COMING EVENT

The biennial Organic Connections Conference is scheduled Nov. 21-23 in Saskatoon. Speakers include:

Maria Rodale of the pioneering Rodale Institute in Pennsylvania

Graham Strong, agricultural diversity proponent and mixed farmer of 4,500 acres in New

South Wales, Australia For more information, visit www.organicconnections.ca.

Brenda Frick, Ph. D., P. Ag. is an extension agrologist and researcher in organic agriculture. She welcomes your comments at 306-260-0663 or via e-mail at organic@usask.ca.

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