Let’s start by making one thing crystal clear: soil is not the same as dirt. Dirt is dusty and dirty. Soil is clean, fragrant, nurturing, and essential to life as we know it. You’ll never catch a gardener refer to soil as dirt.
Here’s the technical definition:
Soil is the natural loose covering on land that serves as the medium for growing plants. It provides anchorage and nutrition, and stores water and oxygen. It also insulates plants’ roots from temperature extremes. Gardeners care about soil because we want to grow plants in it. This means garden soil should have good texture, contain necessary nutrients and minerals, and be able to hold both water and oxygen to be absorbed by plants roots. That is the beautiful job of soil!
Soil can be divided into three basic categories: sand, silt, and clay.
Clay soil is made up of the smallest particles and as a result, is often heavy and slow to drain. On the plus side, clay soils tend to be nutrient rich. Sandy soils are composed of the largest particles and water drains from them very quickly. They are often nutritionally poor. Silt based soils are right in the middle.
The ideal soil for plant growth is loam,
which is a combination of all three kinds of soil particles in approximately equal amounts. Unfortunately, very few of us have ideal soil. Here’s a quick test you can try to see what kind of soil you have. Pick up a handful of soil and feel it. Is it abrasive? Then it’s probably mostly sand. Is it slightly silky, like talcum powder? That’s what silt feels like. If it’s sticky and feels a little like plastic, your soil is mostly clay.
For a reference Opus Grows Potting Mix #2 (green bag) is mostly like clay soil; Opus Grows Potting Mix #3 (purple bag) is mostly like sand soil; and Opus Grows Mix #1 (blue bag) is mostly like loam - well balanced and suitable for any plants
So why does soil texture matter? Soil texture has a dramatic effect on how plants grow, based on the following properties.
Small soil particles hold onto water longer, making clay soils drain more slowly than sandy soils. Knowing how fast your soil drains is important when choosing garden plants. For example, lavender (Lavendula) is a plant that requires fast drainage. It will thrive in a sandy soil and struggle in clay. On the other hand, blue flag iris (Iris versicolor) grows best in moist soils. It grows well in clay and would not be happy in a sandy, fast draining soil.
Large soil particles have more space for oxygen in between each particle. Heavy soils with small air spaces may be poorly oxygenated. Some plants, like false indigo (Baptisia) grow well in poorly oxygenated type of soil, while plants that require good oxygenation, like yarrow (Achillea) may suffer from root rot if they don’t get enough oxygen at their roots.
Ease of digging:
Heavy clay soils are harder to dig than sand or silt. (Come on, everyone knows easy digging is happy digging.)
Fine textured soils (clay and silt) hold more nutrients than sandy soils. Some plants thrive in nutrient poor soil, like bush clover (Lespedeza thunbergii), while others require more nutrition to grow and flower well.
Soil texture is of prime importance, but it’s also important to know your soil’s pH. This number is based on a scale of 0 to 14, with 7 being neutral. Most garden plants grow best in slightly acid soils (5.5 – 7), although some, like clematis and phlox prefer alkaline soils (numbers higher than 7) and others, like azaleas and blueberries, grow best in acid soils (numbers lower than 7). For a reference the pH of all of Opus Grows Potting Mixes is between 6 and 6.5 at the time they reach your garden.
If this sounds like a lot to understand, don’t worry. Most co-op extension services will analyze a soil sample for a reasonable price. And if all the plants in your garden are growing well, there’s no need for a soil test. But if you notice certain plants are suffering, or you want to plant something that you know has specific soil requirements, start with a test so you know what you’re dealing with.
Here are a few professional tips:
It does NOT help your plants to heavily amend the soil in your planting hole. If you greatly improve the soil in the hole, magically transforming heavy clay into perfect loam, the roots of your newly planted plant will be enormously happy and grow like crazy UNTIL they reach the border with the native soil. At which point they will circle back around into the improved soil, rather than growing outward. Eventually they will girdle the plant and the plant will die. Instead of amending the soil in your planting holes, top dress your plants by spreading an inch or two of organic matter like compost and mulch above the root zone of the plant. Heavily amending the soil in a planting hole can also create drainage issues.
Most soils contain very little organic matter…about 5%. You may read that adding organic matter helps plants grow, and that’s true, up to a point. But do NOT add massive amounts of organic matter to your soil. If you increase the percentage of organic matter to 25-30%, as that matter biodegrades (and it WILL biodegrade), the particles become smaller and the soil will collapse in, leaving a sunken planting hole of compacted soil that is once again low in organic matter. Top dressing with small amounts of organic matter (as described above) is a better solution.
Do not use garden soil in your containers. Even if you have the best, most perfect loam in the world, garden soil in containers isn’t a good idea. The confines of a container slow down drainage and reduce aeration. Also, garden soil is heavy, making containers very hard to move. So what should you use instead? Potting mixes (or potting soils) are composed of lightweight ingredients like peat moss, perlite, vermiculite, bark, and coconut coir (shredded coconut hulls). These mixes drain well in containers and provide good oxygenation to plant roots.