While creating some videos about potting soil (links below) I reviewed a lot of commercial products, looking at both ingredients and marketing claims. What a mess out there! It’s no wonder gardeners are confused when they go out an buy a potting mix. I created this post to try and bring some clarity to the situation.
I will go through a process of selecting a good product while pointing out the important issues and showing you what marketing gibberish you can ignore. This discussion applies to potting soil for houseplants, outdoor containers and starting seeds.
Potting Soil vs Potting Mix vs Potting Compost
What is the difference? I covered that in detail in this video. It depends on where you live, but in short, in the UK potting compost is soil based with added peat or decomposed wood. Australia uses the term potting mix and it’s wood based with no soil. In North America we are totally confused and use both mix and soil, interchangeably. Normally they are peat-based with no soil.
In this post, I will use both potting mix and potting soil interchangeably to get more google hits 🙂 My focus here is on soilless material.
What Does Potting Mix Do For Plants?
This will surprise many of you. It allows plants to grow in what amounts to a hydroponic system which provides support and air. That’s it. We add water and fertilizer. The media is inert and even when it decomposes it adds virtually no nutrients.
Once you understand this, selecting a good potting mix becomes easier.
Selecting a Base Potting Soil
The first criteria is to select your base product and there are three common options; peat, decomposed wood and coir.
Any of the tree options work. If you have access to peat, I would use it since that is the historical standard in horticulture and we know it works well.
Peat and peat moss are almost the same thing. It has been and still is the gold standard in North America. It was the standard in the UK as well but they are moving away from it and using decomposed wood because of environmental concerns.
Use of peat in horticulture is not nearly as big a problem as the internet would lead you to believe. You can read more about that here, Peat and Peat Moss – the True Environmental Story
Peat slowly decomposes but adds almost no nutrients.
This can be made with a variety of wood products including bark, and is partially decomposed wood. It provides good drainage, lots of air and holds water. It provides few nutrients, in part because of the high C/N ratio.
Coir is ground up coconut husk and is playing a bigger role in replacing peat because people are under the incorrect belief that it is a more eco-friendly alternative. If you look at the whole life cycle of the product you find that production causes significant harm to fresh water in India and Sri Lanka. See, Is Coir an Eco-friendly Substitute for Peat Moss?
It functions very much like peat.
Additives to the Base Potting Mix
Commercial products add lots of additives to the base material. Some of these are beneficial to plants. Others add value but are not needed and some add no value at all. Additives are used mostly to differentiate one product from another. Ignore the claims and understand their real value.
It is common sense to think that plants need lots of different things in their media, but it’s not true. Many orchid mixes include several additives and none are needed. I grew orchids in plain bark for 20 years and then switched to plain coconut husk for the next 25 years. I’ve grown hundreds of species and won regular show awards. Remember epiphytic orchids grow on plain bark in nature.
Charcoal absorbs nutrients which is a good reason not to use it. It is not needed.
Earth Worm Castings
There is a strong unfounded belief that worm poop has special powers. It is just a form of organic fertilizer.
Organic fertilizer does add nutrients, but they also cause a problem in pots. You never know how much nutrient they add. If they decompose faster they add more. If they decompose slower they add less. So you never know if you are under or over feeding your plants.
The “organic” part adds nothing to the plants. The base of your potting mix is already organic matter – adding more does not change things.
Many products now add fertilizer and that feeds your plants for a couple of months which sounds great, but it is a problem. Is it overfeeding or underfeeding your plants? Does it include calcium and magnesium? If you use soft water you probably need added calcium and magnesium. If you use hard water you don’t want any added.
The manufacturer of the media does not know what kind of water you will use, but you do. So it is much better for you to add the fertilizer you need.
Ignore any added fertilizer and supply your own.
Limestone is added to compensate for the acidity of peat moss. Decomposed wood and coir are less acidic and don’t need it.
I tested the pH of peat moss using my tap water which is quite alkaline and the pH of peat rose after a couple of days. If you have similar water you don’t really want limestone in your mix.
The limestone is also not good if you are growing acid loving plants. It is better to add some yourself if you need it.
Meals: Alfalfa, Kelp and Feather
These are all slow release organic fertilizers and as discussed above for worm castings, they don’t really add much value to a mix.
Microbes and Mycorrhizal Fungi
These are becoming hot garden products and they are now added to more potting mixes. Microbes play an important role in real soil, but potted plants are not growing in real soil. Microbes are not need in our pots and it isn’t even clear that they add any benefit. You can grow almost any plant hydroponically without microbes, even orchids.
If you fertilize, the plants do not need microbes and if you add a good level of phosphate then plants won’t even allow mycorrhizal fungi to connect to them.
These additives add no value, but they don’t do any harm either, except to your pocketbook.
Finally an additive that is worth adding. Perlite increases drainage and air in the media. It will make your base media drier and provide more air at the roots. It also compensates a bit for overwatering – which is a very common problem.
If a commercial potting soil contains perlite it is usually only a small amount. You might want to add some more.
The addition of sand increases drainage. It does the same as perlite, except that it is heavier and therefore makes the mix heavier. That can be an advantage for small tippy pots but it’s not so great for large planters that need to be moved around.
Rather than buying a product that contains sand, add it yourself if it is needed.
Soil seems to only be added to a mix in the UK, where it is a major component. I have no idea why the UK and US are so different in this regard – we grow the same plants.
Vermiculite also makes the media drain better, but unlike perlite it holds more water resulting in a wetter mix. Perlite seems more popular in commercial mixes than vermiculite and since most gardeners have a tendency to overwater, I’d stick with perlite.
What Are the Best Additives?
Which additives do you want in a purchased product? Some perlite or sand is an advantage. The rest add little or no value so add them yourself if and when they are needed.
Marketing Potting Soil
Marketing hype also confuses the purchasing process with a number of words or claims that really don’t mean much and add no value to the product. Ignore all of the following.
I always get a chuckle about this one. Gardeners associate black with good soil. So if you are selling “potting soil”, why not call it “Black Potting Soil”?
The word black tells you nothing about the quality of the product. Ignore it on bags of potting soil and on bags of real soil where the term is used even more.
The word organic has been abused so much that it doesn’t mean much any more. There is no such thing as “organically certified potting mix” but products are labeled as organic.
There are OMRI certified products, but that designation means that the material can be used in organic farming. I am not sure how that helps a gardener growing houseplants or even raising seedlings for a vegetable garden.
The word organic on a bag adds no value.
The main ingredients are either peat, coir or wood – all of these are natural. That means all potting soil is natural even if the bag is not labeled as such.
There is no such thing as sterile soil. It is possible that the mixture was sterilized or that one or more ingredients are sterilized, but packaging facilities for potting mix are not sterile environments. And even it the manufacturer did produce a sterile product, as soon as you place a plant in it, it is no longer sterile.
Most of the ingredients in potting soil are fairly pest free and sterility is not really an issue. Any pests in potting media either get in there during the bulk storage period, during packaging or after you open the bag.
Big Plants and More Blooms
Such labels are quite comical. Remember that the potting media is a hydroponic system that provides very little to the plant. Growth and flowering are mostly determined by temperature, humidity, fertilizer levels, water and light. You can grow big plants with lots of blooms in just about any media provided the other parameters are correct and there is enough air for roots to grow.
There are special blends for plant categories like succulents, African violets, vegetables, and even seed starting mixes. None of these are needed.
There are also some gimmicky blends like “moisture control media”. You don’t need that either. You control moisture by watering correctly.
Some plants do require a drier or wetter environment and I suggest using the base product and amending it when needed with sand, perlite or vermiculite.
What is the Best Potting Mix?
Select the base material. Then look for a product that uses this base and does not have too many additives.
Name brands tend to be better than no-name brands. Really cheap brands are not worth buying, but you also don’t need the most expensive ones. You pay high prices for special additives that are not needed, or for over priced branding.
Once you find a brand that works for you, stick with it. Over time you will get to know it and understand how to properly water it. This is much better than constantly switching brands to save a dollar or two.
Figure out how much you can use over a couple of years and buy in bulk. It will save you money and media does not get old, provided it is kept dry.
What do I use? I have been using ProMix for many years. It is peat-based and has a bit of perlite in it. It now comes with mycorrhizal fungi which add no value. ProMix is also used a lot in the horticulture industry.
For succulents, I add some sand. For plants that like to be on the dry side, I add some perlite. For outdoor containers I add garden soil which translates into much less watering – see video below. But for all seedlings and 95% of houseplants I use straight ProMix.
Will other mixes work just as well as ProMix? I am sure they will. Buy a good product that is available locally and stick with it.