Potting Soil for Indoor Plants and Container Plants

As a gardener, you ask a great deal out of potting soil. You want it to support and nourish your plants, often for years at a time. But the truth is, most bagged soils just aren't designed for this. Here's why and what you can do to give your indoor and container plants the best potting medium.

The Nature of Soil Mixes

Most commercial soilless potting mixes are peat-based, often made with reed or sedge peat, and ​pH adjusted with lime. They look good fresh out of the bag, and often they are enhanced with fertilizer or water-retention crystals. Unfortunately, plants rarely thrive in this kind of potting medium for too long. Instead, after one or two growing seasons, the plant no longer grows as fast or looks as vibrant. With poor-quality bagged soils, plants are lucky to survive even for a few months. 

This happens because purely peat-based potting mixes aren't designed for long-term use. They're not actually designed for plants at all, they're made for your convenience. They're cheaper to produce, and they are lightweight and easy to bag and sell.

The problem is that peat decomposes quickly. Like any organic material, all soils decompose over time, but peat is an especially rapid decomposer.

Problems With Decomposing Potting Soil

As the potting mix decomposes, several negative forces will affect your plants:

• The peat slowly compresses. In the pot, it'll look like the dirt is "settling," when in fact, it's actually breaking down. As it does, the particles pack themselves around the roots, slowly starving them of oxygen. The youngest, newest, and smallest roots are affected first. Just like plants need water and fertilizer, they also need ample air around the roots. A plant with good aeration in the root zone is a healthy plant. A plant that can't breath is a dead plant.
• Drainage is impeded. As the soil particles become smaller and smaller, it's harder for water to drain through the pot. A layer of drainage pebbles at the bottom of the pot will actually make this problem worse. All you're doing is reducing the amount of soil in the pot, so the plant will have even less room to grow.
• Salt builds up. As the drainage slows, it allows for a faster buildup of salts and solids from fertilizer. Over time, this stresses the plant and can scorch the same tender roots that are stressed by the soil impaction.

With all of this happening in a single season, is it any wonder that plants that thrive for a few months in their new pots begin to lose vibrancy within a year?

How to Improve Your Potting Soil

Take these steps to ensure your plants have the soil they need:

• Repot every year if you are using peat-based bagged potting mix.
• Improve bagged potting mix. It's not a long-term fix, but you can improve on peat-based potting mixes by adding some organic matter and perlite.
• Flush the soil thoroughly every month, at a minimum. Take the plant to the kitchen sink or outside and thoroughly flush the soil to wash out accumulated salts from fertilizer and deposits from tap water.
• Wick your pots. Insert a wick through the drainage hole in the bottom of the pot. This won't help with compaction, but it will wick away excess water in the pot and help drainage, thus reducing the chance of root rot.
• Make your own potting mix. Many gardeners mix up their own potting mixes based on composted bark, coconut coir, peat, perlite, vermiculite, pumice, and other soil additives. This is a more advanced option, but it is possible to build a soil that will last for two or more seasons if you make it yourself.

What Indoor Plants Need From Soil

Houseplants need a different type of soil than plants in the landscape. In pots, their roots grow in a very confined space yet the soil needs to be able to perform the same functions: retain moisture and provide good drainage at the same time to give the plants access to water and air. Indoor plants also require fertilizer. They need to be typically fertilized more often than plants in garden soil because the nutrients don’t remain in the soil but get washed out from frequent watering.

Just like outdoor plants, indoor plants also have specific soil pH requirements. Many houseplants thrive in a slightly acidic to neutral pH range (6.0 to 7.0) but orchids, succulents, and cacti do better at lower pH levels.

Best Potting Mix for Indoor Plants

Indoor plants should be grown in potting mix and not in potting soil. The two are not the same, potting mix is much lighter and less dense. The best potting mix for indoor plants has a fluffy, light texture to maximize aeration and allow good drainage. A variety of materials is added to the potting mix, such as perlite, vermiculite, peat moss, sand, wood fiber, and coconut fiber. Often, a slow-release fertilizer is also added. 

Best Potting Mix for Container Plants

When it comes to soil, potting mix for container plants share the two main characteristics of a good indoor potting mix—moisture retention and excellent drainage—but they also have some other requirements depending on the type of plants. For example, vegetables in containers need nutrient-rich, fertile soil that sustains the fast growth of the plants over a single season. In addition to materials that ensure good drainage, sphagnum peat moss and perlite, potting mix for containers is enriched with humus or compost and organic materials such as earthworm castings, alfalfa meal, kelp meal, or feather meal.