Best Potting Soil for Container Gardens

Container gardening has become extremely popular and there are all kinds of potting soils you can buy. Gardening in containers is very different than gardening in the ground so it is important to understand the special requirements for potting soil.

If you are looking for a simple solution, where cost is not a factor, and you don’t mind watering a lot, use a name brand potting mix. Almost all of them will produce healthy plants.

If you want a better solution – keep reading.

What is Potting Soil?

Garden writers and manufacturers use all kind of terms and it can get quite confusing. I am going to use the term potting soil to differential it from soil which is found in the ground. Some people call this a soilless mix or soilless potting media – they are all similar enough to consider them the same. It is more important to understand the material than get hung up on a name.

In our discussion, potting soil will refer to the material you use in containers.

Raised beds are also containers, but since they contain much more material, are designed to be more permanent and sit on the ground, they have their own special requirements. You can read about soil for raised beds in Soil for Raised Beds – Which One is Best.

Requirements for Good Potting Mix

There is no one ‘best’ potting mix. You can make potting mix with a number of materials and they will all grow great plants. A good potting mix will have the following properties.

Lots of Air

Plant roots like lots of air – more than you think. Excellent soil, even in the garden, is 25% air. A good potting mix will be airy, providing lots of spaces between particles. Commercial products call this “light and fluffy”.

Hold Moisture

The biggest problem with containers is that they dry out too fast. A potting mix that holds more water will need to be watered less frequently. That is good news for you, but it is also good for plants. Plant roots don’t like dramatic changes in moisture levels. They prefer constant moisture all of the time and soil that dries out slower will meet that requirement.

What About Weight?

Weight is important, but in some cases you want the potting mix to be heavier, and in others you want it lighter.

If you garden on a balcony and you have lots of pots you want less weight. If you routinely move the pots around or you move them to a garage for winter, you also want less weight.

On the other hand, containers with tall plants need more weight or they will fall over in the wind. Smaller containers also benefit with more weight in windy locations.

You can adjust the weight of the mix by using different ingredients.

Hold Nutrients

Containers get watered a lot and each time you water you will wash some nutrients out the bottom of the container. A potting mix that holds onto nutrients tighter will lose nutrients slower. Obviously, this is better for plants, but it will also reduce the cost of fertilizing.

Ingredients in Potting Soil

Potting soil usually contains a variety of ingredients. Even commercial products that are sold as a complete container mix, are a mixture of ingredients. Unfortunately, most products don’t tell you what is in the bag, making it very hard for you to figure out which one is best.

The following are some of the main ingredients that can be used.

Peat Moss and Choir

The bulk of most commercial products consist of either peat moss or choir (milled coconut husks). Both of these products are very light and airy. They do decompose over time, but neither material provides significant nutrients for plants.

They hold a lot of water, but can be very hard to re-wet if they dry completely.

Some people say that harvesting peat moss is not an environmentally sound practice, but that is not true. Have a look at Peat and Peat Moss – The True Story, for more details.

Peat moss is acidic, which is great for some plants, but it is too acidic for most. It is one reason people add lime to it before planting. Adding some soil can also reduce the acidity.

The choice between peat moss and choir probably comes down to your location. Here in Canada, peat is a good option because we are very close to huge peat reserves and it is cheaper than importing choir from India. In other parts of the world, choir might be a better choice. It is important that we all reduce transportation effects on the environment.

Mineral Aggregates

There are several inert fillers including sand, perlite and vermiculite. These are stable and do not decompose quickly, although vermiculite does tend to fall apart over time.

Sand is very poor at holding nutrients, but it does add weight to the mix. It is also relatively inexpensive.

Perlite and vermiculite hold lots of air and are very light weight materials. Vermiculite absorbs and holds nutrients.

All three materials will increase the porosity of the mix, providing better drainage and more air.

Perlite has the annoying habit of floating on water, so it tends to float to the top of the container. If you water too much, you end up with perlite everywhere. Vermiculite compresses easily as you press down on the soil, decreasing its airiness.

Organic Components

In this class of material I include things like compost, leaf mold and aged manure. Commercial potting mixes usually contain very little of these.

Organic material absorbs water, holds nutrients well, and slowly decomposes to add more nutrients. Think of organic matter as being a slow release fertilizer. In the ground this works wonders with plants, but in a container, the frequent watering washes a lot of the extra nutrients out. Even with organic matter added to the potting mix you will probably have to fertilize.

Organic matter tends to be heavier than peat moss or choir, and it is a bit less airy.

Soil

Many people who talk about making potting mix don’t include soil as a component and most commercial products don’t include it. But there are good reasons to include it.

Soil is heavier than just about all of the other material, except maybe sand. If the soil contains a good amount of clay it can be very nutritious, and hold a lot of water. It is also extremely cheap, especially if you dig it out of your own garden.

Adding soil to a potting mix can dramatically reduce watering frequency. And that reduces nutrient leaching.

Some people claim you should not use soil because it contains bugs and weeds. I have news for you – bugs will find your containers even if you start with bug-free soil.  As far as weeds go, they are quite easy to pull out of a container.  This downside of soil is minor compared to its benefits.

Most garden soil does not have enough air spaces and should not be used as 100% of the mix.

Wood Products

Wood products can include bark chunks, arborist wood chips and saw dust. If these products are heavily decomposed they can be thought of as organic components. If they are not decomposed, they should not be used in potting mix because as they decompose they remove nitrogen from the soil. If too much is removed, plants will not grow well.

You can compensate for this with fertilizer – but why make things complicated?

Sterile Potting Mix

Do you need to buy sterile potting mix? Do you need to sterilize your own mix? The short answer is no.

There is no such thing as commercial sterile potting mix – no matter what the bag might say. The mix may be heat treated to kill pathogens and weed seed, but none of the manufacturers package the finished product in sterile conditions. Besides, even if they did, as soon as you open the bag, or plant something in it – it is no longer sterile.

Sterile Soil – Does It Really Exist?

Heat treating your own mix might kill some weed seeds, but why not just wait until they germinate and pull them out? Seems like a much easier way to deal with them.

The Ideal Potting Mix

So what is the best mix? It depends on your native soil, but for most soil this mix will work very well.

50% garden soil

30% peat moss or choir

20% compost

This mix provides a good combination of airiness and water holding capabilities. The mix is fairly heavy so it will hold taller plants in place. Even small containers will not have to be watered daily. The soil and compost will help hold nutrients in place.

The peat moss could be replaced with most commercial potting soil since they are mostly peat moss or choir, but it is cheaper to buy bulk peat moss.

In my own containers, I use about 70% soil and 30 % (peat moss and compost). The larger containers are only watered once a week in summer. Plants don’t grow as fast or large as they could in a more open soil, but the Calibrachoa flower all summer long – what more can you ask for?

If you want a drier mix replace some of the soil with sand.

If you need a light mix to make moving containers easier, use 20% soil and 60% peat moss or choir. You will have to water a lot more.

Potting Soil Do’s and Don’ts

Here are some other suggestions.

Hydrogels (water crystals)

Hydrogels, also called water crystals, are small beads that expand when water is added. They are also found in diapers. Many people on the internet recommend that water crystals, or even whole diapers be added to containers. Science has shown that although hydrogels absorb water, they don’t release it to plant roots. This product should not be added to containers.

Rocks to Improve Drainage

Rocks at the bottom of the container do not improve drainage. All they do is reduce the effective size of the container. Do not put rocks, or clay pot shards or Styrofoam peanuts in the bottom to increase drainage.

Containers With No Drainage Hole

The pot containing plants needs to have drainage holes. If your fancy container does not have holes, either drill some, or use another pot inside of the container and make sure that after it is watered, it is not sitting in water. You can drill holes in almost any material using metal, masonry or glass drill bits. Don’t press too hard while drilling or you might crack some material.

Mycorrhizal Fungi

It is claimed that mycorrhizal fungi are import for plants and that is absolutely true – in the ground. Mycorrhizal fungi provide plant roots with water and nutrients, especially phosphate. And if your container is not fertilized enough there might be some benefit. But most people over fertilize, in which case the fungi provide no benefit. And if you fertilize with too much phosphorus, you will kill the mycorrhizal fungi you added.

Container gardening is more like a hydroponic system. It is artificial. You do not need to follow nature to make it work – it is not a natural growing location.