Healthy soil is essential to the health and vigor of your garden. This About Your House introduces the basic soil properties that influence plant growth; outlines simple field tests to identify your soil condition and provides tips if you need to amend your soil.
Soil has eight basic properties. Knowing your soil type (that is, its texture) and its other properties is important for three reasons:
Note that, depending on your yard’s size, topography and past uses, soil properties may vary in different locations in your yard.
A soil amendment is any material added to a soil to improve its condition, for example, its fertility, pH or structure, to provide a better environment for your plants.
Soil Properties and Field Tests
Soil horizons and depth
A relatively undisturbed soil viewed in profile typically reveals, from the surface down, an organic layer, a topsoil layer (the "A" horizon) and two layers or horizons below.
Soil profiles vary considerably from one locale or region to the next, depending on geology, hydrology and climate. On sites with a history of construction or agricultural use, mixing, tilling, filling or excavating may have significantly altered some horizons.
If your soils are relatively undisturbed, be careful not to overmix the layers when working the soil. Overmixing can bury organic matter and healthy soil organisms too deeply, which can harm other soil properties.
Soil depth is the depth of the soil above bedrock. Soils more than 75 cm (30 in.) deep offer better growing conditions than shallower soils and can support a greater variety of plants. Some tree species, such as oaks and hickories, send out long taproots, so they need deep soil. However, many plants are well adapted to shallow soils. If you live in a region with shallow soils, confirm that species are suitable when selecting plants.
Organic matter and humus
Humus is produced from the decomposition of raw organic matter, such as leaves, branches and lawn clippings, which accumulates on the soil surface. Decomposition returns vital nutrients to the soil for use by living vegetation. Humus is later carried down to the "A" horizon (topsoil layer) by means that include water, insects and earthworms.
A soil’s humus content has a direct bearing on its fertility, structure, porosity, moisture and drainage. In sandy soil, added humus can improve water- and nutrient-holding capacity. In clay soils, it can improve aeration and drainage. Repeated cultivation can deplete your soil’s humus content by accelerating decomposition or burying organic matter too deeply to decompose.
Humus requirements can vary significantly from one plant species to the next. It is possible to increase humus content by adding organic matter, such as compost, manure or decayed leaves. However, it may make more sense to plant species that are adapted to infertile soils that have lower humus content, if you have those conditions.
Field test: Humus and Organic Matter
Examine the surface and the "A" horizon in the pit you dug for the soil horizon test, or dig a small pit at least 30 cm (1ft) deep. Is there a layer of organic matter and humus at the surface? How deep and how dark is the next layer (the "A" horizon)? The darker the soil, the more humus it contains and the more fertile it is. Hardened, compacted layers of soil generally indicate low humus content and low fertility. Sandy soils are naturally lighter and lower in nutrients than clay or loam.
Soil texture
>Soil texture is the relative proportion of sand, silt and clay particles that make up a given soil. These particles are graded according to their diameter, with sand particles being the coarsest and clay particles the finest. Soils are typically rated by their texture (Table 1).
A soil’s texture directly influences its nutrient content, moisture and drainage capacity. Clay soils tend to be fertile, but are often wet and poorly drained. Sandy soils drain easily but can be drought-prone and infertile. Loams retain moisture and are fertile and friable (crumbly and easy-to-work). Loam soil contains about 40 per cent sand, 40 per cent silt and 20 per cent clay, along with plenty of humus. Many plants tolerate a variety of soil textures, while some have more specific soil requirements.
Soil Structure and Porosity
Soil structure is the size and arrangement of particles in a soil. This arrangement determines the porosity of the soil, that is, the volume of air between particles.
Healthy soil with good structure may contain up to 25 per cent air. Structure and porosity are functions of several key factors, including soil texture, the burrowing activities of earthworms and insects and the presence of bacteria, fungi and other micro-organisms. Adequate porosity is essential for the gas exchange between the root zone and atmosphere, to contribute to a capability to absorb, drain, and retain water, and to enable roots to easily penetrate the soil to access nutrients.
Compaction eliminates the vital air spaces between soil particles and is the single most significant impact on soil structure and porosity. Although most plants prefer soil that is porous, some species can grow in compacted soils.
Soil Moisture
Soil moisture depends on climate, topography and other soil characteristics, and is typically graded as wet, moist or dry. Some plant species are highly adaptable and can tolerate a range of moisture conditions. Others have very specific moisture requirements. While it is possible to irrigate plants that need more moisture than your soil can provide, it is better to select plants suited to your soil's moisture.
Soil Fertility
Soil fertility is the soil’s capability of storing and releasing plant nutrients. The main nutrients are nitrogen, phosphorus and potassium (N, P, K), along with a range of trace elements. Fertility depends in part on soil texture. Clay soils are generally more fertile than sandy soils, partly because of the tendency of some nutrients to bond to clay particles. Fertility is also directly proportional to the amount of humus and organic matter in the soil.
Fertility requirements usually vary from one plant species to another. Some, such as certain woodland wildflowers, require deep, rich soils. Others, such as prairie or meadow species, prefer infertile soils and will develop spindly stems and foliage in deep, humus-rich soils at the expense of flowers and fruit.
Soil pH
pH is the measure of a soil’s acidity or alkalinity. Most plants do well in soil with a neutral pH of 6.6 to 7.4, or in slightly acid soil with a pH of 6.0 to 6.5. Some plants have very specific needs for soil that is acidic (for example, azaleas, rhododendrons) or alkaline (for example, chrysanthemums, columbine).
Always check soil pH before selecting plants. Although many gardening books give plant pH requirements, you may need some additional research for specific species. While it is possible to change a soil’s pH, it generally makes more sense to select plants that are adapted to your soil’s pH. The exception is soil that is so alkaline or acidic it cannot support any plants. Concrete surfaces, salt spray, intensive irrigation and other factors can increase soil pH and create conditions that are hostile to many plants.
Life in the Soil
A great many life forms — from microscopic fungi, bacteria, algae, protozoa and nematodes, to larger creatures such as springtails, ants, earthworms and moles — live in healthy soils. They contribute directly to the health and vigour of plants. Bacteria, for instance, decompose organic matter and release the nutrients essential for plant growth. Earthworms, ants and other insects, through burrowing, aerate the soil and carry humus from the surface to lower layers. Mycorrhizal fungi form symbiotic (mutually beneficial) relationships with the roots of certain plants, helping them get water and nutrients.
Improper use of pesticides can seriously affect your soil’s microbial community, as can repeated cultivation. A soil depleted of its microscopic flora and fauna loses its ability to decompose organic matter and becomes less fertile. It will have poorer structure and porosity, and be less hospitable to plants than a soil rich with life. A diversity of beneficial organisms can also help control organisms that can harm certain plants.
Chemical Tests
While the field tests described in this About Your House can help you gather basic information about your soil, a chemical analysis may be necessary for more accurate information about texture, fertility, and pH, among other properties. Contact a local garden centre, soil testing laboratory or provincial agriculture ministry or department for more information about testing your soil. Home testing kits are also available. For reliability, use a higher cost kit. Laboratory results are generally the most accurate.
Amending Your Soil
When field tests are completed, you will have a basic understanding of your soil’s properties. The next step is to select plants that are suited to your soil and site conditions, such as shade and anticipated rainfall. Local nurseries, conservation agencies, plant catalogues, books and Web sites can help you create a suitable plant list. Refer also to the plant list in CMHC's Landscape Guide for Canadian Homes.
Although selecting plants suited to your soil minimizes the need for amendments, there are situations where amendments may be unavoidable. These situations include areas in which soil health has been seriously compromised; food gardens or other specialized plantings that may not be as easily tailored to your soils; and, areas in which even well-suited plants are not thriving. Amendments can also be helpful when establishing new plantings.
Table 2 sets out some common soil problems and non-synthetic solutions. As a renewable and readily available resource, compost is a particularly desirable amendment. The preferred approach is to plant suitable species, but you can work in amendments, or use a combination of the two approaches, depending on site conditions and your goals.
Amend your soil selectively, only where needed, rather than amending soil over the whole yard whether it needs help or not. Work the amendment material into the soil, rather than leaving it on the surface (Figure 3). This helps spread it around, prevents drying, particularly of organic matter and avoids harmful concentrations. It also supports vigorous plants by encouraging roots to spread rather than concentrate only in the amended area.
References and Resources
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