Soil is one of the most important concerns for gardeners, so I want to take a moment to address a few basics. It’s the medium into which we add our plants, it’s where non-woody plants retreat when dormant, it’s where water is stored, nutrients are recycled and it is the home for countless bacteria, fungi, insects and other organisms that play an important role in the garden ecosystem that we usually don’t get to see.
Defining soil
For most of us, the question “what is soil?” seems too obvious to ask. Maybe we don’t have an encyclopedia definition in our heads, but we know dirt when we see it: it’s that workable, wetable brown stuff that covers the Earth’s surface naturally, that you find wherever you dig, that must be cleansed from clothes and scraped out from under our fingernails, that tracks into the house and needs to be swept. In the garden, it’s the stuff plants use to anchor their roots.
But this is a good opportunity to draw a line and explain something that turns out to be exceedingly important to us gardeners: what soil is not. Specifically, potting mix is not soil. That stuff is better described as a “potting media” or “soil replacement.”
Native soils
Soil in any natural ecosystem on Earth is a blend of decaying organic particles and living and dead things, water, dissolved gas and small open pores of air suspended between very small particles of rock—a lot of rock.
By dry weight, native soil is almost invariably going to be between 90 and 95 percent rock, in grains that range from small (sand) to very small (silt) to microscopic (clay). The proportion of particle sizes determines much of your soil’s character. Whether your native soil is fluffy, powdery, sandy, hard as porcelain, dark and moist, gritty and dry or easily waterlogged, it’s still around 90-95 percent rock. The remaining proportion of composted organic material makes the rest of the difference in soil’s potential water and nutrient retention capabilities and texture. (I say “potential” because another important aspect to soil is its structure, which is something I’ll return to later).
Being mostly rock, soil from the ground is heavy—between 75 and 125 pounds per cubic foot. And that is one way you know the mixture of blended materials you call “potting soil” is really not soil at all. Your potted plants would be really difficult to lift and move if they were in soil. Additionally, every time you water, the container’s drainage holes would leak muddy brown water containing microscopic clay particles and dark-staining dissolved substances. When it comes to larger containers, root systems planted in native soil would mostly be concentrated in top 8-12″ of the soil volume where oxygen is most available, or wrapped around the outer edge of the container and clustered around the drainage holes were additional pockets of highly-oxygenated soil are available. In other words, dense native soil in a container would not oxygenate evenly, causing root systems to form distorted structures and waste the space in the center of the container.
Potting blends
Potting mix manufacturers make potting blends out of non-decomposed and decay-resistant shredded plant material, such as peat moss, or most commonly, shredded bark and wood pulp that comes from the byproducts of logging operations. Light, fluffy types of rock such as pumice could be added in to help reduce compaction and slow the breakdown of organic components. More commonly, perlite or vermiculite serves that function. Because the the types of organic material used are mostly nutrient-poor and there are no mineral particles to supply minerals to plants, slow-release fertilizers or compost blends will be mixed in to provide them. This sort of mix resists compaction and is mostly air, so dry potting media can weigh as little as 8 pounds per cubic foot. The abundant air space also helps the roots penetrate deeply and use the entire volume of the container. Generally, plants in containers eventually need to be fertilized, while plants in real soil generally do not.
The fact that containerized plants are not growing in soil turns out to be important when it comes to planting. That’s because a plant transplanted from a nursery container into the ground exists with its root system confined to a plug of non-soil in a soil environment. Ideally, the first “watering in” that is always recommended after planting a will help some of the native soil dissolve and infiltrate the potting mix plug, getting the roots in contact with the denser particles that tend to be better at releasing nutrients and carrying water. But that’s not always the case, and the failure of roots to integrate into the native soil bed in time for the next drought is a major reason newly-planted plants die. It’s why I advocate breaking up the rootball as much as possible when you’re planting a new plant in the ground, and, in some cases, shaking off or flushing out the potting media as much as possible to get roots in contact with native soil.
Where the nutrients are
Most gardeners are well aware that organic material releases a burst of nutrients into the soil as it decays. Additionally, small organic particles in soil are useful for storing nutrients in a way that makes them available to plants, and the bacteria and fungi that live on these organic particles provide a host of benefits to plants.
(As a side note, this understanding gives us some insight into the popularized practice of fertilizing gardens with “compost tea.” That’s the practice of running water through compost and capturing its tea-colored extrudate to water a garden as a form of organic fertilizer. It’s true that compost tea contains some dissolved nutrients and free-floating microorganisms, but it doesn’t contain the bulk organic particles that hold them in place and provide the most benefit to soil biodiversity. Why go through the extra effort to separate dissolved nutrients from the substrate that puts them to work? As clever as the idea may seem, there’s no scientific support for the idea that supplying nutrients in tea form improves plant health compared to a top-dressing of compost on soil.)
But a lot of us take for granted the fact that the rock particles themselves contribute to nutrition for plants. Rocks naturally release trace amounts of nutrients, or bind excess nutrient molecules that can be re-released later. That’s a reason why volcanic regions, where soils are full of fine mineral ash have some of the most fertile soils on Earth.
Understanding soil types
As a gardener, it’s good to understand the soil type you’re working with. Above I mentioned the three main particle sizes—sand, silt and clay—which lend to different soil qualities. Most soil will be a mix of all three particle sizes in various concentrations.
Sandy soil, with large grains and pore spaces, drains and dries out quickly. It is beneficial in that it is less likely to become waterlogged, but is also more likely to dry out. Since larger particles have a smaller surface area, they don’t exchange nutrients easily, and can become nutritionally poor.
Clay soil is made of microscopic grains that fight tightly together. It holds large amounts of water and resists drying. Oxygen has a harder time penetrating clay soils deeply, so trees growing in clay will have shallower root systems that stretch out farther. Clay is particularly vulnerable to compaction and in urban areas it can be very degraded. However, clay, with its high surface area (more than 1,000 times that of an equal volume of sand) is excellent at storing and exchanging nutrients with plants.
Silty soils, as might be intuitive, combine the qualities of sand and clay.
Loam is an optimum mix of sand, silt and clay. Many farmers and gardeners consider it the best: it carries the benefits of sand and clay but avoids the drawbacks. It holds both oxygen and water, and stores and releases nutrients. Additionally, loam can be divided into sandy loam or clay loam based on the dominant particle size. A lot of gardeners seek to create a loamy soil for their garden, but…
You can’t change your soil type
The mass of soil in the ground is huge. Since each cubic foot of soil weighs close to 100 pounds, a small back yard of 40 by 40 feet has 160,000 pounds of soil in the top 12 inches alone. And, while most root system activity occurs in the top 8 to 12 inches of the ground, deep anchor roots and the roots of drought-tolerant plants and grasses can extend several feet!
So imagine you’re trying to change your soil type because your current soil is heavy clay, and you envision creating an ideal, loamy soil for your vegetable patch by adding sand. To tip the soil past the threshold where it really behaves differently, you’d have to add enough sand that that there aren’t enough clay particles to completely fill the gaps between grains of sand, providing empty space That means your soil has to be more than 50 percent sand, and to amend a 10-by-10 foot area you’d have to truck in 5,000 pounds of sand and till it thoroughly to a depth of 1 foot. That’s a lot of work, and over time, living organisms and water are going to spread that sand deeper into the soil and out into the surrounding areas so you’re going to have to add even more sand.
I think it makes a lot more sense to just work with your existing soil. In truth, even the heaviest of clays can become excellent soil with proper management. Plants can be selected to work with the existing pH. Generally, plants can weave their roots around rocks and adapt to different soil depths.
Summary
- Potting mix is a lightweight replacement for natural soil that is made of shredded plant material and added fillers and nutrients. It weighs about a tenth as much as natural soil, lets oxygen in more easily, and is ideal for plants in containers.
- Natural soil is 90-95 percent small to microscopic rock particles, with the rest being composed of organic material. It weighs 70 to 100 pounds per cubic foot.
- Soil is composed of sand (large particles), silt (medium particles) and clay (small particles). Most soil has a mix of all three, but one or two types may dominate. The balance between particle sizes affects the soil’s characteristics—for example, sandy soil dries fast and encourages deeper roots, whereas clay soil stores more water and nutrients, but is more prone to compaction. A well-balanced soil is called loam.
- Organic material helps water and oxygen move through soil, provides food for beneficial microorganisms, and stores nutrients. But microscopic rock particles in soil also supply nutrients to plants, and gives soil weight to help stabilize trees and shrubs. Ideal soil has a balance of about 90 percent mineral particles to 10 percent organic material, which is close to the natural ratio.
- Adding compost to soil will temporarily increase the soil’s organic component, but it will gradually break down until the soil reaches equilibrium.
- Soil has so much mass that it is very difficult to add enough material to change the composition of a garden. For example, a 10 by 10 foot garden would require 5,000 pounds of sand to change the soil type from clay to loam. It’s easier for gardeners to add mulch to optimize soil and choose plants wisely to work with the existing soil type.