Expert: Soil biodiversity is key to healthy garden

SHIRLEY– “Having a biodiverse soil is the best way to support plant growth,” said Michele Cannon, as she began her introduction to soil science for an audience of Gardeners Exchange members at the Hazen Memorial Library on Sept. 25.

“There is more biodiversity below the ground than above the ground, and the biodiversity underground supports everything above the ground,” she explained.

The presenter said that she began growing her own vegetables organically 34 years ago, and gradually progressed to flowers, shrubs and fruit trees. A resident of Townsend, she is the former owner of MotherEarth Landscaping, where she designed, installed, and maintained many private gardens.

She has worked with the conservation commissioner of Townsend, and last year was named an “Unsung Heroine of 2009” by the Massachusetts Commission on the Status of Women.

What microorganisms do for plants

As Cannon showed slides of soil organisms growing around plant roots and on humus, and fungi growing in strands and connecting smaller soil aggregates, she explained that fungi and bacteria build soil structure, suppress disease, help retain nutrients, and decompose toxins. They also create a shield around roots that help to protect plants from harmful organisms, she said.

Soil nutrients generally occur in two forms: Inorganic compounds attached to minerals dissolved in water, and organic compounds that are part of living organisms and dead organic matter. Bacteria, fungi, nematodes, protozoa, and arthropods are always transforming nutrients between these two forms, Cannon said.

Some bacteria and fungi initiate many of the biochemical reactions that occur in soil. The decomposition of leaves, for example, requires the enzyme cellulase, which is produced by soil organisms.

The role of plants in the soil, said Cannon, is that they produce simple sugars, proteins, and carbohydrates that are exuded through the roots and feed the sets of organisms that are most beneficial to the plants.

In one handful of healthy soil, she said, there are generally 100 billion cells of 100,000 different types of bacteria, 50 grams of fungal threads (or hyphae) from more than 500 types of fungi, 100,000 cells of protozoa from hundreds of species, and 10,000 nematodes from thousands of species.

The reason that such a large abundance of organisms can be found in one handful of soil is due to the pore space found within soil, which is where the organisms live. While it may appear to be solid, soil normally contains a large amount of pore space. In some soils, the pore space can make up 5 percent of the total volume, and the pore space itself can generally be divided between air- and water-filled space, with the exception of times of water logging.

In this underground biological system of microorganisms, said Cannon, it is a “bug-eat-bug world.” Protozoa eat bacteria, nematodes eat plant roots and protozoa, arthropods eat nematodes, and birds and other animals eat arthropods.

“In a bug-eat-bug world there are pests that are bad, and some are pretty destructive. But maintaining beneficial organisms is like having a neighborhood watch group: The bad guys are watched and can’t get away with stuff, so they leave. If they are in balance with the good guys, then they stay in balance,” she said.

As for which plants grow best with which decomposers, Cannon said that weeds strongly favor bacteria, annuals thrive where there is somewhat less bacteria, and vegetables and grasses favor both fungi and bacteria. Shrubs, vines, and perennials slightly favor fungi, deciduous trees favor fungi more so, and conifers strongly favor fungi.

She went on to explain that most microorganisms are beneficial to plants in some way. In a healthy garden, she said, “healthy plants are rarely attacked by disease or critters.”

Plant stressors

“A little stress in the garden is good,” Cannon said. “It boots up the carotenoid system. Too much stress is bad, as stressed plants take in less light.”

She said that the primary stressors on plants are extreme weather, compaction, weeds, invasive plants, and human actions such as the application of pesticides, herbicides, feet, tillage, improper mulching and watering, and imbalanced soil biology.

Fixing stressors include “walking lightly on the earth,” not over-mulching, soil aeration, and improving the texture of the soil.

To tell whether your garden is stressed, she suggested using your eyes. “You can use weeds as stress indicators, or your ‘cultural practices report card.'”

For weed control she suggested using a scuffle hoe, asparagus knife, soil knife, dandelion popper, and, as a last resort, a flamer, which could be something as simple as vinegar soaked into the cracks within walkways.

Another technique is to shade or crowd out weeds by smothering them with an old piece of carpet or mulch, but, she said, there is nothing like good old-fashioned hand pulling.

Harmful practices

Cannon cautioned against using pesticides and herbicides, which have not been adequately tested despite having an EPA registration.

Insecticides, fungicides, herbicides, and weed-and-feed type products are all registered as pesticides, she said. Many are endocrine disruptors, and the breakdown products and inert undisclosed ingredients may be even more toxic than the original active ingredient. “Pesticides are never safe,” she said.

All commercial applications of pesticides must be sign posted with a yellow warning flag, placed in a visible spot. Cannon said to report any suspected misuses of pesticides with the pesticide bureau: 617-626-1781.

She also warned against the old-fashioned practice of tilling in the garden, which damages soil structure, destroys the homes of microbes, and brings up weed seed to where they can germinate.

Composting

One of the best ways to improve conditions for beneficial soil microorganisms is to use compost, Cannon said. The four methods of composting she discussed are thermal, worm, static pile (“let it rot”) and sheet composting.

Sheet composting is not turned, but needs to be layered more carefully than static pile composting. “It has to be at least 3-1/2 feet in each direction, or you won’t get good compost for a year to three years.”

To get good a carbon to nitrogen ratio in compost, which favors composting microorganisms, “mix brown and green,” Cannon said.

She said that fall is a good time to do “lasagna” sheet composting. This style of composting requires no digging or tilling.

It consists of alternating layers of “browns” such as fall leaves, shredded newspaper, peat, and pine needles, with layers of “greens” such as vegetable scraps, garden trimmings and grass clippings.

In general, the brown layers should be about twice as deep as the green layers. “If you take care of the soil, the soil will take care of you and the planet,” Cannon said.

For more information on the Gardeners Exchange, visit www.gardeners-exchange.org, or contact President Janet Tice at jetice9@aol.com or 978-425-9067.