Winter Soil
Contributed by Kerry Lake, Master Gardener
What happens to our garden soil during the winter? Does it indeed ‘sleep’? Or is it full of life that can sustain itself while waiting for new plant growth in the spring to bring fresh nourishment? The answer depends on what you did late last fall. Did you till and leave the ground bare? Did you mulch the soil with compost or plant a cover crop? Did you leave non-diseased, chopped plant material to breakdown over the soil?
As you know by now, our soil is a ‘vast underground kingdom of microorganisms’ as Kristin Ohlson tells us in Soil Will Save Us. The microbes need living or dead plants for their foods (sugars, carbohydrates, and proteins) even in the winter. Cutting or mowing the plant material, but leaving it in the garden bed to protect the soil surface will provide nutrition for the microbes both above and below ground. We want to keep these organisms alive during the winter so they will be ready to go to work as soon as the soil warms up and you plant your first crop in the spring.
Leaving the garden bed this way will look messy with (dead plant debris and fallen leaves, however this is what a forest floor looks like). Keep reminding yourself that all of this provides food for worms and the other soil creatures that aerate and enrich the soil, helping to make it more porous and absorbent, creating great erosion control from the rains and melting snows but also howling winds. This residue provides another benefit of protecting the soil from the sun so that it doesn’t dry out as much. Moist soil provides a great environment for organisms as they eat the garden residue, creating this looser, absorbent and fertile soil.
It has been said that a single teaspoon of healthy soil can contain 1 to 7 billion organisms. In that teaspoon could be 75,000 species of bacteria, 25,000 species of fungi, 1,000 species of protozoa, and 100 species of tiny worms (nematodes). Add to this soil ecosystem the different species of earthworms, millipedes, and some insect larvae. This tiny ecosystem is necessary for our plants, and in turn, good for us.
Most of the microbial diversity and soil life occurs in the rhizosphere, the zone of soil extending roughly two millimeters out from plant roots. Plants secrete sugars and amino acids from the cells at the tips of new roots. These secretions promote the growth of bacteria and fungi, concentrating microbial decomposition and important nutrient release at the plant’s root tips where it can easily be absorbed. Some fungi form symbiotic associations with plant roots. These are called mycorrhizae and this association allows for the exchange of water and nutrients between the fungal and plant cells. The mycorrhizae fungi can send out networks of very fine strands for many meters from the plant root into the surrounding soil. This increases significantly the absorptive surface of the plant root system. This association of the fungi and plant enhances plant growth and can help plant roots be more resilient to drought stress.
Removing the plant roots in the fall (tilling the soil for example), destroys this mycorrhizae network until it can be built up again the next growing season.
Soil organisms provide other indispensable services for plants. They can convert atmospheric nitrogen into mineral form for plants to use (nitrogen fixation); break down organic compounds for their nutrients (organic matter); aeration of the soil through burrowing to provide better water absorption and storage; and even suppression of some soil-borne diseases. All of this continues in the winter soil, providing we did our part to keep these soil systems in place and fed.
At one of WMMGA’s community service projects, the Hospice at Fisher Home, we have started a new demonstration vegetable garden. We are counting on this soil activity described above to create our garden bed this winter. We employed a ‘layered garden’ approach by layering right over the lawn: first a layer of newspaper and corrugated cardboard, then some composted material (humus) from previous garden weeding material, and a final layer of dried leaves. All of this was tied down with mesh, garden rope, and landscape pins to keep the layers in place during windy winter storms. This system allows the roots of the grass to remain intact in the soil while simultaneously killing the grass by eliminating its access to sunlight. The lawn will die off, but the underground network will remain intact. In the spring, the soil microorganisms and worms will have eaten through the dead grass blades, newspaper and cardboard, and most of the dried leaves. Throughout the winter, the garden “prepares” itself! It will be ready to plant (and seed) as soon as the soil temperature allows. At the end of next summer’s growing season, we will cut back the last of the garden growth, mulch with that growth and other composted material to keep the soil fed throughout the next winter.
For more on soil, please go to our previously featured articles on this topic.
Sources:
Healthy Soils for Sustainable Gardens from the Brooklyn Botanic Garden All-Region Guides, copyright 2009.
The Soil Will Save Us by Kristin Ohlson, published in 2014 by Rodale Inc.