Healthy soils for Garden Plants

Written November 30, 2012, published December 2012

Hand in hand with good plant choices, and planting at the right time (fall) to fit the local climate, is promoting soil health.  There are two paths to take; one is for native perennials, shrubs and trees, and the other is for vegetables. I’ll discuss vegetable soils later. Today, my focus is on ornamental gardens, especially native plants––perennials, shrubs and trees–-and the soil these plants need to grow well. 

On a hillside under pines, Evergreen Huckleberry, Vaccinium ovatum, and Soft-footed Sedge, Carex leptopoda, grow in thick layers of wood chip mulch. Photo by Kathleen Sayce

Our climate is a curious one, with wet winters and dry summers. Wet winters mean that it’s difficult for soils to hold onto nutrients, many of which are water-soluble. Long months of cold rains mean that nutrients wave at plant roots as they wash past and out of reach. Worse, those long wet months bring the ground water table up to the surface in low areas. Roots of most upland plants do not grow in water due to low oxygen levels. The result is these root systems are relatively shallow, and nutrients wash past even more quickly. Also, during severe windstorms, plants with shallow roots are more likely to blow out of the ground. Due to a long wet season, local soils are also acidic; native species tolerate and even prefer this acidity. 

Streambank orchid, Epipactis gigantea, is growing in a low wet swale, amended with compost and aged wood chips. Photo by Kathleen Sayce

In nature, soils store carbon in several forms:  living and dead wood, including logs, branches and twigs, or thatch, and living and dead roots. Many species of fungi and bacteria live on these materials. Wood, roots, branches and twigs are composed of cellulose, the most common biopolymer on the planet, which is made by living plants from sugars formed during photosynthesis to shape cell walls. Those sugars and celluloses are the plants’ building blocks and trade goods. They trade sugars with bacteria and fungi for water and nutrients. 

In thick layers of woody mulch, Stropharia fungi produce mushrooms, the fruits of soil mycelia. Photo by Kathleen Sayce

Different species of fungi live on heartwood, greenwood, cambium, bark, roots, and dead wood. Specific fungi live on living roots, dead roots, and on duff materials––twigs, needles and branches that fall to the forest floor.  Specific fungi associate with specific shrubs and trees, connecting via their mycorrhizae (fungal filaments in the soil, which are often whitish and look like thin fragile roots) with plant roots, to share water and nutrients. 

Mushrooms are abundant in garden soils with ample carbon, such as aged wood chips.  Photo by Kathleen Sayce

The fungi get simple sugars from the plants, and the plants get minerals in return. There may be bacteria in association with both that fix nitrogen, and also share with fungi and plants for sugars.  Animals that live in the soil eat roots, fungi and bacteria, and are eaten by other animals. Their bodies are food for other bacteria and fungi. Soil ecosystems are largely hidden from us by virtue of size and location, as most soil organisms are microscopic and all are out of sight underground. 

To promote healthy soils for native plants, then, it is not sufficient to provide water and fertilizer. In fact, nitrogen fertilizer by itself, without the supporting structure of soil carbon and soil organisms, throws soil out of balance, causing soil carbon to be eaten and further depleted in the soil, year after year. 

Balance is restored to the soil by adding several forms of carbon:  compost, biochar (biologically activated charcoal), tree litter and wood chip mulch. As I mentioned at the beginning, this is not a soil designed for vegetable gardens, but for native trees, shrubs and perennials, species that have lived here for thousands to millions of years. 

Blue-flowered tall camas, Camassia lechitlinii, is growing in a plant bed that was widened; the thick wood chip and compost layer is now ready to plant. Photo by Kathleen Sayce

There are efficient ways to feed these forms carbon to the soil. One is to mix in compost and biochar around the root zone in the planting hole when you put in plants. The second is to layer all of these materials on the surface, year after year. Carbon promotes the growth of soil organisms, which in turn collectively improve soil health, help it retain nutrients and water. 

Wood chips can go on the surface of the soil in a mulch layer. These aren’t fresh from the chipper, but aged chips, piled and kept damp until well-colonized by soil fungi. The piles are aged for a year or more, until fungal mycorrhizae (visible as small white threads) have thoroughly spread throughout the pile. Once a soil is on its way to improved health, in alternate years spread compost or wood chips. 

How do you know there’s enough carbon on top of and in the soil? You will see fruiting fungi (mushrooms) during the wet season. When mushrooms appear, they tell you that the soil has enough carbon to be reasonably healthy. The gain is in the garden: plants need less summer water, grow well without added fertilizers, flower abundantly, set seed, and resist drought and disease.