I went to the local center garden center where I used to run the greenhouses yesterday to breathe some oxygen-rich air. While I was looking around and visiting with friends, I got asked some question. This always seems to happen; I think I have an eager “talk to me” face on when I’m around plants. A customer asked me to recommend a soil for container plants. I recommended a couple of standards from the vast array of choices. She asked me what the difference was between them and what follows are the ideas we talked about when choosing, or making, a container soil.
Container soil, like garden soil, serves four basic purposes: Plant support, root aeration, nutrient retention, and moisture retention. Each component of a soil mix addresses at least one of these purposes, and when assembled correctly plants thrive.
Most container mixes start with something organic. There are lots to choose from; pick one based on availability and cost. Some of the choices include peat, coir (coconut husk), compost, pine bark, hardwood bark, loamy garden soil, and rice hulls. The organic part you choose serves all the main purposes of a container soil in some way, but usually can’t do all the work itself.
Most plants thrive in well-aerated container soil with good drainage. To this end, most container soils have something in them to promote drainage and good air penetration. Perlite, sand, and polystyrene are all common choices. Perlite and polystyrene are good for lightening the weight of heavy mixes, while sand is a cheap. Sometimes the size of the organic pieces allows for good drainage, as with crushed bark, but these mixes usually require something to help hold in the moisture.
If your soil mix is really porous, if you have small containers, if you grow big plants in little pots, or if you live in a hot or windy place, your plants may do better if you add something for moisture retention. Some common additions are vermiculite, polymers, or long fiber sphagnum moss. All of these help hold the moisture while maintaining good aeration and drainage.
Now that you know what all those little pieces in your soil mix are doing, which one do you choose? The easiest thing to do may be just buying something labeled for containers. This is a great because the work is done for you. But if you’re like me and can’t leave well enough alone, or you like to experiment, there are lots of mixes you can create. I love experimenting, and I’ve found that messing around with container soil mixes can produce very different results, one of which may be the best thing you’ve ever had. Here are a few basic ratios to start, and experiment, with.
- 1 soil: 1 peat moss: 1 sand
- 1 vermiculite: 2 peat moss: 1 perlite
- 3 peat moss: 1 perlite
- 2 vermiculite: 2 pine bark: 1 perlite
- 1 coir: 2 peat moss
- 1 peat moss: 3 hardwood bark: 1 sand
Some people want to include garden soil in their container mixes. While garden soil by itself is usually too heavy for containers, it can be a part of a container mix. If you want to add garden soil to your containers, mix together equal parts good garden soil, peat, perlite, and compost. There can be some benefit to adding garden soil. It may contain beneficial microorganism and nutrients that will help your plants.
Once you’ve got a good soil mix, remember that with a finite amount of soil comes a finite amount of nutrients. To grow the best plants, they need a constant supply of nutrients. You can do this a couple of ways. The most common approach tends to be the use of water-soluble nutrients. These are the crystals or liquids that you dilute into your irrigation water. These are good, but you have to add them regularly to achieve the benefit. Another approach, one I really like, is to add slowly-soluble fertilizers to the soil mix before filling the containers. This way, there is always food and I can add a little water-soluble food if I want a boost. Here is a recipe I got from Rodale’s Encyclopedia of Organic Gardening: to each cubic foot of soil (about 7.5 gallons) add 4oz dolomitic limestone, 1lb rock phosphate, 4oz greensand, 1lb granite dust, and 2oz blood meal. This is a good recipe…I’ve experimented with this, too. If you do want to adjust the ratios, do it a little at a time. There are also pre-mixed formulations that work great, too.
If you’re going to make your own soil mix or mix nutrients into your container blend, be careful. Most of the components are a powder or create dust. Work in a well-ventilated area, wear a dust mask, and don’t be afraid to dampen the ingredients ahead of time to cut down on dust.
Here are a couple more tricks that I learned by trial and error (more error) that will make your life a little easier.
- Containers can be heavy when full of soil. Move them into place, and then fill them.
- Don’t cover drain holes with rocks or potshards. Use paper towels to cover drain holes. It maintains good drainage.
- Huge containers don’t have to be filled all the way with soil. Use an old pot or hanging basket flipped over in the bottom of the container to displace some soil and lighten the load.
- Leave a headspace. Nothing is more annoying that having your water run over the sides of your finished container instead of where the plants need it.
- Moisten your soil well before planting. You can moisten it while mixing or by watering it in the containers a few times before planting. You’ll get more even watering and better root-soil contact.
Next we’ll cover what to put in your containers and how to keep it all looking and producing it’s best.