Understanding Soil

Author: Barbara Long, Master Gardeners of Ottawa-Carleton; published with permission.

We begin our study of what lies beneath our feet by looking at the five soil layers, also called horizons. From top to bottom, we have: organic (leaf litter and humus), topsoil (minerals with humus), the leaching layer (sand and silt with very little minerals), subsoil (clay and minerals), and parent/bedrock material. Understanding more about these layers can help us grow healthy food and beautiful plants. Think about a kitchen sponge; it is a flexible block of plastic with holes of various sizes that hold water. When garden soil has a structure like a sponge, it can hold air, water, and nutrients that plant roots can absorb over time as needed, and roots can move through it readily.

Aggregates

Aggregates and soil pores are terms we use when talking about soil structure and texture. A well-aggregated soil is one that has lots of clumps (aggregates) composed of sand, silt, clay, organic material, fungi, mycorrhizae (more on this later), bacteria, water, and air. Organic matter helps to form soil particles into aggregates. Aggregates have soil pores within them and between them. Large soil pores, called macropores, allow water to drain through the soil and provide air exchange; smaller ones, called micropores, hold water.

From gophers, snakes, and mice to ants, beetles, and earthworms, right down to tiny mites, springtails, and amoeba, soil is teeming with a variety of animal life. Some aerate the soil by moving through it, improving drainage, while others digest and excrete waste that fertilizes the soil.

Microorganisms in Soil

Microorganisms such as bacteria and fungi in the soil provide nutrients important to plants, such as nitrogen and phosphorous, by breaking down organic matter through decomposition. Roots, mosses, algae, and mushrooms are types of soil flora that decompose over time as well, contributing to the nutrient mix.

As mentioned earlier, many types of fungi are present in soil. When plant roots absorb nutrients from the soil, some are aided in their work by these fungi. Beneficial fungi attach to the root hairs and create an additional web of threads in the soil to collect water and nutrients for the plant. In return, the plant roots provide nourishment in the form of sugars to the fungi that don’t have chlorophyll and so cannot make their own food through photosynthesis. This symbiotic relationship between roots and fungi is referred to as a mycorrhizal association. To sum up, there is a lot going on down there!

No-Dig Gardening

Now that we understand more about the value of the intricate web of interconnectedness beneath our feet, we can make good decisions on how to maintain it when we garden. If we leave the soil undisturbed, the roots of the plants we grow will absorb more water and nutrients.

We are accustomed to practices such as tilling the soil and double-digging techniques for our gardens. Rethinking these practices to embrace a technique called ‘no-dig gardening’ is a huge topic that I will touch on briefly.

Basically, no-dig gardening means planting on top of the existing soil, lawn, or garden. One technique is to place multiple layers of newspaper over the area and then cover it with several inches of organic matter, such as wood chips, leaves, and compost.

Decomposing Layers

Over a period of time, depending on a variety of factors, the newspaper layer will decompose, and you can then plant directly into the organic matter. The roots of your plants will grow downward through the newspaper layer into the undisturbed soil, where they can benefit from the well-aggregated soil with its nutrients and mycorrhizal networks.

No-dig gardening takes more time at the front end of the process but is easier in successive years since cultivating the soil each year before planting is not needed. When more organic matter is added each year, the layers below become richer with nutrients as rain carries them down by gravity.

The wide variety of fauna (worms, etc.) in the soil travel up to the organic matter and bring it down into the undisturbed soil to consume and excrete, thus releasing nutrients. This process also loosens up soils that may be heavy with clay (which can become waterlogged) and bulks up soils that are sandy (which drain water too quickly).

Another benefit of no-dig gardening is that the seed bank remains dormant. Millions of seeds exist in the soil, some for decades. When they are exposed, germination can be triggered by moisture, air, and sunlight. Many of these seeds are weeds, unwanted in our gardens. Using the no-dig gardening method saves time weeding.

In Support of Better Soil

I encourage you to look into how you could garden using the no-dig method to grow healthier plants, reduce your workload, and keep the soil beneath our feet the way nature intended.