What Is Potting Soil?

It's more than just dirt. Lets take a closer look at what lies beneath the surface of your plant

When it comes to soil, I believe houseplant enthusiasts fall into one of two categories: those who don't consider it enough and those who think about it excessively. The first group is content to place their green companions in any dirt they have available or that happens to be on sale at the garden center. On the other end are what I call the "dirt collectors." If they have ten different kinds of plants, they have ten different kinds of commercial potting soil to match. I can't blame the dirt collectors, really. After all, a garden center is likely to carry dozens of different types of soil and soil amendments, and it can be confusing to decide which ones are appropriate for a given plant. Perhaps we can sort this out by taking a closer look at what is actually in these soil mixtures. Maybe we can even learn to make our own.

Commercial soils can contain a wide range of ingredients, but most of those components can fit into three categories: bases, moisture regulators, and fertilizers. Let's examine each category in more detail.

Soil Bases

These bases form the majority of soil mixtures. They serve various purposes and possess different properties. Besides offering a medium for plant roots to grow, they have specific qualities related to aeration and water retention. Some can also supply nutrients to your plants as the organic matter within them continues to decompose over time. Here’s what you need to know about the most common ones:

• Peat moss is a nutrient-rich organic soil harvested from the top layers of peat bogs. Commercial potting soils are primarily composed of peat moss, making up between fifty to eighty percent of the total content. Recently, sustainability concerns have prompted environmentally conscious gardeners and houseplant enthusiasts to seek more eco-friendly alternatives, but for most people, this remains the primary soil base. Peat moss offers excellent aeration and drainage, maintaining the ideal moisture balance for most plants.

• Coir, also known as coco fiber, is a more sustainable alternative to peat moss. It consists of the fibrous "hairs" covering the exterior of coconuts and is a byproduct of the coconut industry. Besides serving as a soil base, it has many other uses.

• Topsoil is the upper layer of earth found under lawns and gardens. While it is commonly used in outdoor gardening, it is not recommended for indoor potting soils because its poor drainage properties can lead to compaction and hinder healthy root growth. Additionally, topsoil may contain unwanted organic matter, such as root segments of grasses and weeds, which could pose problems indoors.

• Sand, specifically desert sand, is often added to soil for succulents and cacti to facilitate quick drainage of excess water. It should be used solely for this purpose.

Moisture Control

Since indoor plants are kept in pots, the ability of potting soil to drain excess water is crucial to prevent root rot. These substances also prevent the soil from becoming too compacted and help keep it aerated (a task performed in nature by worms and insects).

• Perlite is a natural mineral with excellent water-regulating properties. It is the crispy, small white beads found in commercially available potting mixes (and no, that's not Styrofoam). The irregular surface of these beads has tiny nooks and crannies that trap small droplets of water while allowing excess moisture to drain away. Because it is an inorganic compound, perlite will not break down over time like some other drainage materials.

• Bark, usually finely shredded or small chunks of pine bark, is added to potting mixes as an additional drainage and aeration material or as an alternative to perlite. Being organic, it will break down over time. While this can provide extra nutrition to your plants, it means that soils using pine bark essentially have a shelf life. This isn't a problem as long as you are repotting your plants every couple of years. It is often sold as a mixture called orchid or phalaenopsis bark, used for growing (you guessed it) orchids.

• Vermiculite can be included in soil mixtures where a bit more moisture retention is desired. Similar in appearance to perlite, it can absorb moisture and keep the soil wetter. It should be used sparingly and only for plants that prefer a little extra dampness (I'm looking at you, ivy). Use it with caution, as too much vermiculite can lead to the dreaded root rot.

Fertilizers

Since potted plants are separated from nature's cycles of decay and replenishment, some form of nutrient-rich matter must be added to the soil to nourish the plants' root systems. As the plant consumes these nutrients and grows, the soil's nutrients will become depleted, necessitating additional occasional feeding with a fertilizer solution. Here are some of the most common fertilizers found in soil.

• Compost is organic matter that has decomposed into a soil-like consistency rich in vitamins, minerals, and other organic compounds. Many varieties are available at local greenhouses and garden centers. They all seem to work pretty well. My personal favorite is Coast of Maine Land and Sea compost. You can also make your own by allowing a mixture of old coffee grounds, banana peels, and eggshells to ferment and break down for a few months. Your plants will appreciate it.

• Humus can be thought of as partially decomposed compost. While compost is completely broken down, humus is somewhat halfway there and has a chunkier texture. This means that in addition to nutrients, it will assist with aeration as well. It is often made from the layer of organic debris (such as leaves, twigs, and such) collected from forest floors and often has some peat added to it before packaging.

• Manure is the dried and sifted organic waste of farm animals. I'm lucky enough to live in a semi-rural area known for its traditional farming practices, so here in Lancaster County, PA, the phrase "shit happens" can be taken quite literally. In addition to the generic variety, some specific and even exotic types can be found in specialty stores. To give your plants a real treat, try mixing a little bat guano into your soil.

• Horticultural charcoal is another ingredient often found in orchid bark. It is almost pure carbon. Just like us, plants are carbon-based life forms and need carbon to grow and thrive.

• Chemical fertilizers, often in the form of slow-release pellets, are found in many potting soil mixes from companies like Miracle-Gro. They are mostly a mixture of nitrogen, phosphorus, and potassium. Different ratios of these macronutrients have different uses, but most all-purpose potting soils will contain a balanced mixture with roughly equal parts of each. This also distinguishes the many types of special-use soils that may use ratios suited to specific types of plants (such as tropicals or African violets).

Other Ingredients

Soil manufacturers use proprietary blends of ingredients that include a variety of substances too numerous to list here. These may include elements to balance soil pH, preservatives to protect the soil during shipping and storage, and micronutrients. A few of the more interesting ones are discussed below.

• Other mineral compounds and micronutrients such as vitamins and manganese are added to soil blends to provide additional nutrition to your plants, promoting vigor and growth. If you're making your own potting mix, these compounds can be purchased individually or in blends from a range of manufacturers, but they may not be needed if you are using natural fertilizers to enrich your soil.

• Micro rhizomes are the dehydrated "roots" of beneficial fungi that would be found in nature and have become popular in recent years. They can be purchased from specialty retailers or online if you are interested in trying them out. I've noticed recently that even big brands like Miracle-Gro have begun to include them in the mix.

• Rooting compounds are isolated hormones that promote new root development in plants and are sometimes included in soils labeled "nursery blends." Powdered versions are widely available.🪴