Indoor Plants & Selecting The Perfect Soil, by Jim Hole

The quality of potting mix can mean the difference between life and death for a houseplant, so invest in a high-quality mix—one that offers the correct balance of water and oxygen. This balance is important because the soil must be able to retain moisture long enough to sustain a plant between waterings yet also allow for proper drainage.

Don’t reuse soil from the pots of ‘plants past.’ If the plant died because of pests or disease, the soil could be contaminated. Even if the plant died because you dried it out, chances are the soil has too few pore spaces (pockets of open spaces that can be filled with water) to sustain a new plant. As soil decomposes, it starts to lose pore space and becomes too dense for air to infiltrate and for roots to grow properly. Pots can, however, be reused—just be sure to scrub them clean and then soak them in a solution of 10% bleach and water.

If I wear one snobbish indulgence proudly, it has to be my penchant for correcting people when they refer to soil as dirt. Soil is not dirt. Soil is defined as a dynamic natural body composed of inert and organic solids, gases, liquids and living organisms, which can serve as a med­ium for plant growth. Dirt is just the stuff under your fingernails, thank you very much.

Hydroponics is a method of growing plants in the absence of soil. Instead, a plant’s roots grow in nutrient-rich water. Hydroponics might seem like something new and high-tech, but the practice goes all the way back to the Hanging Gardens of Babylon.


Potting Mix vs. Soil Mix

Soil is the term most people use to describe the black medium in which we pot plants. But the truth is that most of the ‘soil’ to which we refer is actually soil-less—completely free of what we traditionally think of as garden soil. It looks
like rich field or garden soil and even smells like it but is completely different.

Most potting mixes contain at least one of the following materials: peat moss, vermiculite, perlite, sand and lime (to neutralize the peat moss), bark, pumice or compost. Soil mixes, on the other hand, contain a blend of soil. So when you’re looking for ‘soil,’ be sure to read the bags carefully and choose a high-quality soilless potting mix.

Specialty Potting Mixes

African violets, cacti and orchids require special potting mixes. Because of the popularity of these plants, distributors have come up with special commercial blends of each.

Orchid mixes: To the uninitiated, this planting medium might look unable to sustain anything other than a beaver. Many contain two or three types of bark, coarse sphagnum peat, fine-grade pumice and sponge rock. It’s an odd combination, but one that serves an important purpose.

Some species of orchids grow on trees in their natural habitat. These orchids are referred to as epiphytic plants—those having roots exposed to the air. One of the reasons orchid mixes contain bark and moss is to allow air to move freely through the medium. This air movement allows an orchid’s roots to absorb moisture and nutrients from the humid air.

Cacti mixes: Even someone who doesn’t know much about cacti knows that these plants prefer dry soil. It should come as no surprise to find that the standard potting medium for cacti is composed of coarse sand, potting mix, peat and perlite. Although the formula varies from one commercial mix to another, all cacti mixes are designed to provide rapid drainage.

African violet mixes: African violets like a light, loose, porous soil, so most commercial African violet mixes consist of three parts peat moss, two parts coarse vermiculite and one part perlite. Lime is also often added to bring the pH level to the 5.8–6.0 range. Because African violets hate their roots sitting in water, the loose, porous soil is important for the health of these plants.