Soil: The Underrated Secret Sauce
CONTAINED
BRILLIANCE
Soil is the secret sauce to beautiful container plantings that's often not considered enough.
Choosing the right soil is the foundation to any successful container planting. The quality of your potting soil is the key to ensuring that you have happy, healthy plants which will receive the nutrients they need to grow. If you invest in a healthy plant but plant it in poor soil, that plant is likely to fail.
What is the right kind of soil for your containers? It’s a soil that is both smooth in texture and lightweight—a soil with the capability to retain enough water and air to supply the plant with all it will need in an artificial environment. Yes, growing plants in a container is not like growing plants in your garden—you’re actually creating an artificial garden environment for your plants. Many beginning gardeners are surprised to learn that using a garden soil is NOT the right answer. My peers and I have experimented with numerous soil mixes over the years, and generally have found bagged “soil-less” (no loam, sand, or clay) potting soils to be the best for container gardening.
What are the ingredients for a "good" potting soil?
The best soil is one that provides air space between particles and holds moisture long enough for the plants to take it in but not so long as to inhibit oxygen (air) needed for root hair survival. One of the biggest challenges with container plantings is that the soil dries out quickly. Mixes with peat and coir have moisture holding capacity and are lightweight. Those with perlite and vermiculite hold air and can absorb and hold nutrients for the plants resulting in a medium that remains lightweight and loose even through daily watering. While there are some environmental concerns being debated about sphagnum peat moss, the jury is still out on its impact and I have yet to find a good potting soil which doesn’t contain it.
"A thing is good because it's good, not because it's natural. A thing is bad because it's bad, not because it's artificial" ~ Tom Robbins, "Even Cowgirls Get the Blues"
Organic doesn't always mean better when it comes to potting soil. Although there are some good organic mixes available, “organic” labeling is not a guarantee it works in containers. Tina Dixon of Plants a la Cart, my mentor and peer, performed planting trials on a number of mixes listed as “organic” and found many of the mixes contained too much organic matter, which caused them to hold moisture too long. In addition to harming the plants, there were weight concerns on terrace plantings. On the other hand, organic mixes high in bark dried out too fast and wouldn’t rehydrate. In both cases, the plants didn’t have the foundation needed to thrive. Also, the frequent watering that container planting requires often washes out any extra benefit of having organic compost matter in the soil.
Another issue she found was that soil high in organic matter settled significantly as the organic matter broke down. If you’re regularly replanting and changing out your containers often, this may not matter to you. However, if you have a large focal center plant and choose not to add soil and raise it every season, any additional annuals or perennials planted will be too low in the container to view and enjoy.
Along with choosing the proper soil, there are a few other soil-related things to keep in mind which will help you in successful planting this upcoming season.
- Hydrate first. It’s best to water new soil as you’re filling the pot and allow it to settle before you plant. If you plant directly into dry soil it will settle considerably and your plants will be sitting far too low in the pot. At Garden Revelry, our process is to “premix” the soil in a gardening trug before adding it to the container. We’re careful to moisten—but not waterlog!— the soil, as it can take too long to dry out if over-moistened. If you have time and can plan ahead, it helps to do this a day or two ahead of planting so the soil has time to properly settle.
- Fill it up. It’s tempting to add extra material to the bottom of your container to save money, lessen or increase weight. But most plants want room to run their roots deep and can live longer in your container with more soil mass. Additionally, you create “interfacing” when you combine two layers of dissimilar textures—such as soil and gravel, rocks, plastic water bottles, Styrofoam or whatever you might choose to put in the bottom of the pot—to reduce the amount of soil needed to fill up the container. When you create an interface, the water flow backs up causing a zone of supersaturation. This water-soaked layer fills pore space, suffocates root hairs, and results in root rot, causing plants to suffer and potentially die. This is particularly prevalent in the Pacific Northwest given our propensity for wet winters. So, make sure to fill the container only with soil. When you choose not to completely fill your pot with soil, whatever money or labor you thought you saved on soil just gets spent on plant replacements.
- Remember drainage. Anytime we’re discussing soil, it’s important to make sure that your containers have drainage. The same dangers of waterlogged soil detailed above, such as root rot, also can occur when a pot has insufficient or non-existent drainage holes. Three 5/8” holes per square foot of pot bottom is a good guide.
Make the Investment
Good soil can be expensive, but it’s worth the upfront investment. Read the ingredients. Choose wisely. When you choose a good container gardening soil, you don’t have to change it out for years. The soil will have the proper balance of moisture retention and pore space needed for the plants to thrive. You are giving the plants a solid environment which will allow them to continue to draw in the air, water and nutrients they will need to flourish.
Below are several brands of potting soil that many of our clients and peers have had successful results with:
Sunshine, E.B.Stone, Fox Farm, Miracle-Gro, Espoma, Happy Frog, Gardner & Bloome and locally, Molbak’s House Blend.
Our favorite is Sunshine, but there are other reputable brands and creating your own mix is always a fun and rewarding option as well. I encourage you to do a little research and check out what’s available in your local area.
"The true method of knowledge is experiment ~ William Blake
Resources:
Tina Dixon, Plants a la Cart
Don Marshall, The Plantsman, Emeritus Horticulture Instructor, Lake Washington Institute of Technology
