Photo The Soil Food Web retrieved from: Soil Biology Primer; http://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/health/biology/ [June 30, 2021]; authors: Elaine R. Ingham, Andrew R. Moldenke, Oregon State University, Clive A. Edwards, The Ohio State University; Illustrations: Nancy K. Marshall, Marshall Designs
By Darlene Belton, Master Gardener
Good gardeners see themselves as committed caregivers to their plants, but how many of us are aware of how much plants manage their own growth and health through partnerships and cooperative systems operating unseen to us, both below and above ground?
A new bioscience is gaining traction, with discoveries reported daily, about how plants obtain the nutrients essential to health and ward off disease and pests. The new understandings include: that natural systems are more cooperative than competitive; that biology controls soil chemistry; that plants direct the process; and, that conventional human interventions are more destructive than helpful, no matter how well – meant.
Fossil records reveal that approximately 450 million years ago the first plants crept onto the totally arid land. Research published in May, 2021, has demonstrated that it happened as a result of a partnership between plants and fungi. Though able to capture the sun’s energy to make sugars through photosynthesis, the first plants could not access mineral nutrients from rock. Fungi could produce enzymes to ingest minerals, but could not create sugars through photosynthesis. In cooperation, however, plants and fungi went on to colonize the earth, a partnership that continues today between mycorrhizal and other fungi species and most plants (true weeds and brassicas being two exceptions).
Simply put, it works as follows: plants produce sugars from photosynthesis in excess to what is needed to feed their own growth. The excess leaks out as ‘exudates’ through plant roots into the surrounding soil. Why? To attract soil bacteria and fungi that ‘exchange’ the minerals they have absorbed for the sugars excreted by the roots. Plants take up the minerals as nutrients to promote their own healthy functioning. Plants have even evolved to be able, through changing the composition of the exudates, to attract specific microorganisms that are able to search out over large distances mineral nutrients in short supply to the plant. The system also works for plants to obtain water from distant sites through the microbial highways established in rich soils. For this reason, plants are said to drive the ultimate biology and chemistry of the soil. When the plants die and are decomposed, the nutrients that were in their living tissues – some of which may not have originally been present in appropriate amounts – pass into the soil, providing a fuller range of immediately accessible nutrients for the new, growing plants at that site.
Bacteria also play crucial plant-helping roles. Some, partnering with legumes, ‘fix’ atmospheric nitrogen into a usable form; others solubilize the relatively insoluble phosphate, a macronutrient that promotes plant growth, or produce natural fungicides or antibiotics to help plants fight disease.
In a healthy soil rich with food, microorganisms multiply rapidly and would overrun the soil if it weren’t for what has been called the ‘soil food web’ a vast, living community of trophic levels, including ascending sizes of predators and decomposers that impose checks and balances on the system by eating the levels below and being eaten by the levels above. We are only able to see the largest of these soil-based organisms, such as earthworms, but if earthworms are in abundant supply, we can be assured the rest of the soil food web is functioning well.
Dr. Elaine Ingham, a leader in this emerging arena of soil bioscience, attests that there is no soil on earth that doesn’t contain enough of the right mineral elements for plant life and health – the entire periodic table in fact; scientists are yet to discover the function of many of these elements. She says that when the soils of a particular site appear deficient it is because soil biodiversity isn’t present enough to create the networks and systems that transport nutrients as needed by the plants. Ingham says that ‘amending’ soil consists only of promoting the diversity of the soil food web, through such mundane and inexpensive methods as adding compost and manures something that would happen naturally in a non-human dominated environment.
So, in our newly assigned, and diminished role of ‘concerned bystander’, how do we gardeners promote the health of our ornamental plants and the nutrient density of the food that we grow?
Much of the answer consists of NOT doing a lot of what humans have done for millennia, such as tilling soil. Quebec market gardener, Jean-Martin Fortier, asks “Why do farmers and gardeners till the soil?” He answers, because tilling creates a ‘clean slate’, loose easy-to-work soil, like “writing in a new notebook, rather than one that is already full”. Besides pulverizing soil aggregates (that leads to soil compaction), tilling initially infuses the turned soil with oxygen, like blowing on a fire, creating a ‘fertility inferno’ that causes new plants to burst forth, but which rapidly consumes the fragile organic matter (OM) reserves. Soon, the OM sinks below the minimum 3% required to grow plants and so, today’s farmers and gardeners add chemical fertilizers to support subsequent crops. This continues a depletion cycle; chemical fertilizers are ‘easy food’ for plants – that then cease to produce exudates for the slower process of building the soil food web and obtaining a full range of nutrients from microbial partnerships. Without the fungal pathways – chopped up and killed as they were by tilling – inter-plant communication fails, and plants fall prey to pests and disease without the ability to produce defenses. So, the farmer/gardener adds pesticides to the deadly arsenal, killing both beneficial insects as well as pests, and upsetting the balance that kept the ‘pest’ insects an integral part of a healthy ecosystem.
Fortier has what he calls a ‘toolbox’ to support soil health. Some of the tools he outlines for food gardens are:
• No roto-tilling. Light tilling of virgin turf one-time only.
• Create wide, raised rows one time only (30 inches) with permanent pathways (18 inches).
• Use a broad fork to open the soil without inverting the soil layers, to promote an increase in depth of topsoil as compost and topsoil fall into the holes created by the broad fork.
• Utilize green manures on rows, and wood chips in the pathways before turning them onto the rows some months later when somewhat composted.
• Cover bare soil with straw and grass clippings, and plant directly into this mulch.
Tips for ornamental gardens include: create new beds using the ‘lasagna’ method, rather than tearing off the existing layer of perennial grass (this captures the OM and soil food web, such as it exists, to inoculate the lasagna layers and deepen the topsoil); use organic mulch that provides food for the microorganisms in the soil; avoid excessive moving of plants as you then tear up their fungal networks that serve to help the plants communicate with their neighbours to promote mutual health.
Of course, the above is only a bare introduction to the miracle of soil life. Without a biological science background, I struggle to understand the concepts and complexity, and avidly seek out as many sources as I can for new information about these amazing ecological systems. The field is relatively new and rapidly evolving. I try to apply what I can to my gardens, both ornamental and food-producing. One area I need to learn more about in a practical way is cover-cropping/green manure. I am reluctant to try it until I can find an experienced gardener mentor, though the practice is recommended widely.
My husband uses a broad fork to loosen the substrate without mixing the soil layers. The surface compost and topsoil fall into the holes created, encouraging deepening of the topsoil layer.
Preparing the Ground for Healthy Soil (Webinar 1), Compost Council of Canada. “The Soil Food Web – Why it Matters for Gardening Success, the Science and Principles of Soil Health (Getting to Know Your Invisible Workforce), Glen Munroe, Compost Council of Canada, March 30, 2021
Preparing the Ground for Healthy Soil (Webinar 2), Compost Council of Canada. “Soil Structure: The Key to Healthy Soils for Gardeners”, Glen Munroe, Compost Council of Canada. April 14, 2021.
Soil Regen Summit, March 15-17, 2021. Sponsored by the Soil Food Web.: specific sessions:
– “How Soil Biology Works”. Dr. Elaine Ingham, Oregon State University, March 15, 2021.
– “Soil or Dirt – How Do You Know?” Dr. Elaine Ingham. March 16, 2021.
– “Succession: Above Ground and Below”. Dr. Elaine Ingham, March 17, 2021.
– “Working with Living Soils in the Market Garden”. Jean-Martin Fortier. March 16, 2021.
– “Tapping into the Parlance of Plant-Fungal Networks”. Dr. Suzanne Simard, University of British Columbia. March 17, 2021.
Teaming with Microbes: The Organic Gardener’s Guide to the Soil Food Web. Revised edition. Jeff Lowenfels and Wayne Lewis. Timber Press, Inc. 2011.
State of Knowledge of Soil Biodiversity: Status, Challenges and Potentialities, Report 2020. Food and Agriculture Organization of the United Nations. Rome, 2020. 619 pages. Digital version made available in downloadable form by Google with permission of the rightsholders.
Soil Carbon Restoration: Can Biology do the Job? Part Two. Jack Kittredge, Policy Director: North East Organic Farming.
Soil Carbon Restoration: Can Biology do the Job? Part Three. Jack Kittredge, Policy Director: North East Organic Farming.
“A plant-fungi partnership at the origin of terrestrial vegetation”. Numerous international contributors: University of Cologne and the Centre for Integrative Biological Signalling Studies: University of Freiburg, Germany; University of Zurich, Switzerland and the University of Tohoku, Japan. May 20, 2021 CNRS. Science: Vol 372, Issue 6542, 21 May 2021. As reported by the Garden Professors, June, 2021.
A Better Way to Look at Trees: What pioneering new research has revealed about the forest. Rebecca Giggs. The Atlantic. Published online, June 17, 2021.
Published: June 2021