Building the Mycorrhizal Connection

Summer 2017
Toki Oshima illustration

By C.J. Walke

As spring rolls into summer, we should see young, month-old fruitlets on our trees, slowly swelling with growth in the sunlight of our longest days of the year. Nutrition for that growth is centered in the soil, where we look to build a biologically active ecosystem for soil microbes and plant root hairs to exchange water and nutrients, supporting each other. Essential to this healthy exchange is the vast hyphal network of mycorrhizal fungi that colonize root systems, connecting plant to plant beneath the soil and creating a community like none other on Earth. As organic farmers and gardeners, we would do well to support this microbial magnificence and share in the benefits of a healthy soil.

Mycorrhizal Planet

I’ve been reading Michael Phillips’ recent book, “Mycorrhizal Planet: How Symbiotic Fungi Work with Roots to Support Plant Health and Build Soil Fertility,” which gives a thorough account of current research and knowledge on how healthy fungal soil communities support the health of us all. We’ve known for decades that mycorrhizal fungi are beneficial in the soil and have the ability to expand plant root systems by 10 to 100 times in the search for water and nutrients, but what I’m finding most interesting in Phillips’ book is the role mycorrhizae play in supporting complete photosynthesis and efficient protein synthesis.

We’ll start with the simple sugar glucose, which is formed through the transformation of carbon dioxide and water using light energy from the sun that is absorbed by chlorophyll. Photosynthesis. Glucose is the basis for creating more complex sugars (polysaccharides) and other carbohydrates. Complex carbohydrates, such as starches, are stored in plant cells as stable forms of energy that can be broken back down into simple sugars when the plant needs energy.

Sugars and nitrogen are the base materials needed to synthesize amino acids, and amino acids are the building blocks of proteins. Many insect pests lack the specific enzymes needed to digest complete proteins, so they feed on sources of incomplete proteins and soluble amino acids in plant sap. Fungal pathogens source their nutrients from simple sugars in plant cells as well as from soluble amino acids in plant sap. So insects and diseases are searching for nutrients from simple forms of organic molecules but cannot easily access nutrients from complex sugars and complete proteins.

Enter mycorrhizae. When mycorrhizal networks are strong and functioning efficiently in the soil, plant root systems have ready access to the macronutrients and trace minerals needed for complete photosynthesis and protein formation. In turn, plants send more carbon sugars to their roots, and that carbohydrate energy is needed by soil microbes to flourish. As this symbiotic relationship of mutualism gains momentum, we see what Sir Albert Howard spoke of nearly a century ago – healthy soils support healthy plants, and healthy plants are less susceptible to pests and pathogens.

Beneath the Trees

To support these mycorrhizal networks in our soils, we need to minimize soil disturbance and promote methods of non-disturbance, in both orchard and garden. Deep tillage tears apart fungal connections and pulverizes soil structure, and although it may be a necessary evil once in a while, it should not be a regular method of choice. Beneath trees, tillage would clearly damage tree roots and lead to death or decay, so non-disturbance is key, and we need to build soil from the top down.
Mulches are an excellent choice for minimizing soil disturbance, since they suppress weed growth while also retaining moisture in the soil for tree roots and microbes. Hay or straw in a thick layer will get the job done, but be mindful of potential weed seed in mulch hay, which is not much of an issue on the orchard floor but could bring a host of issues into your garden. Ramial wood chips, made from limbs 2 1/2 inches in diameter or less, recycle tree nutrients as they decompose and feed soil fungi. The decomposition layer where mulch meets soil is where the action happens and where evidence of biological activity can be seen.

Cover cropping, where grasses and/or legumes grow for weeks or months, minimizes soil disturbance until the cover crops flower and approach seed set, when mowing and, sometimes, incorporation are necessary. Yes, the dreaded tillage! But in dwarf apple tree cultivation, cover crops suppress weed growth and keep the soil root zone more open and accessible for tree roots, as well as for mycorrhizal networks. Legumes help build nitrogen levels in the soil (remember those amino acids and proteins), while grasses produce ample organic matter. Both can be mowed and used as mulch, rather than being tilled under.

I have a row of dwarf apples growing on each of the long sides of my rectangular half-acre garden plot. Those trees are mulched heavily with wood chips. I do run the BCS tiller around the garden perimeter to keep pesky quackgrass at bay, but after that tillage I sow different cover crops or mixes to compete with the grasses that try to creep back in. In spring I may use oats and field peas along the edge, which I eventually mow and rake toward the trees as additional mulch. Buckwheat will follow to shade out weeds and provide food for beneficial insects, adding more mulch when mowed. Then I sow another round of oats, which grow into the fall, winterkill and protect the soil until spring returns.

When I want to add nutrients to the soil beneath the dwarf apple trees, I incorporate amendments during one of these mowing and mulching cycles.  Before mowing one of these cover crops, I walk the row of dwarf trees and sprinkle amendments along the drip line of the trees, adding quantities recommended from soil analysis. I make two passes through the cover crop strip with the cutter bar on the BCS machine, which lays the crop down nicely. Then I walk back along the row with a fork or rake, and mulch the cover crop material along the drip line of the trees, covering the newly spread soil amendments. The decay cycle of the fresh mulch provides soil microbes with a boost of food sources and activity while incorporating the rock powders into the soil.

Healthy soils grow healthy crops. By mulching, cover cropping and incorporating rock powders, we can build healthy soil under our fruit trees, making our fruit crops less susceptible to insect pests and disease pathogens. It’s certainly not a cure-all, but I can’t wait to see the look on that sawfly’s face when it stings one of my young fruitlets, spits in disgust at the taste of complex carbohydrates and complete proteins, and decides to move on.

C.J. Walke is MOFGA’s organic orchardist. You can contact him at [email protected].

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