With so much information and so many potting mix recipes out there, it can be tough to know which soil amendments you should add to your mix.
To simplify this process, I’ve put together some quick facts about the most common soil amendments used in potting mix with my personal recommendation about which to use and which to avoid.
We will cover perlite, pumice, orchid bark, horticultural sand, LECA, sphagnum moss, vermiculite, and horticultural charcoal.
Table of Contents
- Potting Mix Amendments I Recommend:
- Potting Mix Amendments that Might Work for You:
- Potting Mix Amendments I Do NOT Recommend:
- Bonus information About Peat Moss and Coco Coir (2 Common Base Ingredients):
- Related Posts
Potting Mix Amendments I Recommend:
Perlite is one of the best amendments to add to your potting mix, because:
- It’s inexpensive &
- It adds air pockets to the soil
The more coarse the perlite is, the better it is at adding air to your potting mix
What are the downsides?
Some of the most available types of perlite are teeny tiny, which means that they aren’t nearly as good at adding aeration.
Perlite breaks down fairly quickly in the potting mix which means it will need to be replaced every year or two.
Perlite is not a renewal resource. It is volcanic glass mined from the earth and then exposed to high temperatures so it pops like popcorn.
Pumice is another fantastic way to add aeration to your soil, because:
- It is super porous, which adds lots of places for air in potting mix
- It takes a long time to decompose so it doesn’t need to be replaced very often
As with perlite, the more coarse pumice is, the more aeration it is able to add.
What are the downsides?
Pumice is not easy to find in some areas. I rarely see it for sale at my local greenhouses so I have to buy mine online when I do want it.
Pumice is definitely more expensive than perlite. I’m not sure if this is because it is less available or perhaps because it doesn’t deteriorate as quickly, but either way, it will cost more.
Pumice is not a renewable resource. It is produced when a volcano erupts lava that contains enough water and gas to create a bubbling effect as the lava cools into volcanic rock.
Orchid bark is an amendment often used with epiphytic plants or plants that naturally grow on trees. It can also be used with plants that like organic, chunky mixes.
It is a great way to provide aeration and a place for chunky roots to cling.
Some examples of plants that enjoy orchid bark as either the entire mix or as part of a potting mix are Orchids, Hoyas, many Philodendrons and Monsteras, and more.
What are the downsides?
Orchid Bark does deteriorate over time and becomes compacted, so it needs to be replaced. It doesn’t do so super quickly though, so there’s no need to replace it annually.
Orchid bark is not a super renewal resource as it is created from the bark of coniferous trees like pine trees and fir trees, which are being consumed more rapidly than they can be replenished.
Potting Mix Amendments that Might Work for You:
Photo is linked to Amazon and is just one example of coarse sand that can be used with plants.
Horticultural Sand is coarse sand that allows for good drainage and works especially well for those plants that would naturally grow in sandy soils, like many cacti, euphorbias, succulents, palms, etcetera.
The key is to ensure it is coarse sand, because fine sand (like that used for playgrounds and on beaches) will actually compact your potting mix instead of increasing drainage.
LECA (Lightweight Expanded Clay Aggregates)
LECA is an acronym that stands for lightweight expanded clay aggregates. LECA looks like little clay balls and is a medium often used in passive hydroponics and fully hydroponic systems because of its ability to wick water up to plants.
However, it can also be added to a potting mix since it is light and airy (which adds aeration) and also has the ability to absorb some moisture, helping to prevent potting mix from staying waterlogged for extended periods of time.
LECA is also very sustainable. It can be used over and over again simply by boiling the LECA in water. There is a lot of varying advice about how long to boil. I personally bring LECA to a hard boil for a few minutes and then allow the water to cool.
Sphagnum moss is harvested from the same bogs as peat moss. It is actually a living moss on top of the peat moss. It is more sustainable than peat moss because it doesn’t take as long for it to be replenished, but it is still not the most renewable resource.
Sphagnum moss, like peat moss, is very good at retaining water and can become compacted over time.
Some people grow plants in 100% sphagnum moss. Others grow plants in sphagnum moss with some perlite or other amendments added.
Some people also grow plants in a mixture of sphagnum moss and peat moss with other potential soil amendments.
I only use sphagnum moss for propagating cuttings sometimes or for growing plants that arrived in it already until they are ready for repotting.
Potting Mix Amendments I Do NOT Recommend:
Vermiculite is a mined mineral that is heated until it expands, kind of similar to perlite. Unlike perlite, it has the ability to hold on to both water and nutrients while also adding a little aeration.
This can be beneficial or detrimental to your potting mix depending on whether you want the mix to be able to hang on to more water. It can provide too much water retention and keep plants soggy for too long, depending on your growing conditions.
While it does add some aeration, it isn’t as aerating as other common amendments (like pumice or perlite) due to its ability to absorb and retain water.
For this reason, I personally do not use vermiculite in my potting mixes and do not typically recommend it because more often houseplant growers are looking for aeration and not more moisture retention.
Horticultural charcoal is the remains of burnt wood. It has been said to reduce odors and filter out toxins (which is true), but it also filters out nutrients, causing potential nutrient deficiencies.
Horticultural charcoal also has a very high PH, which can cause serious issues for plants.
Plants prefer potting mixes to sit around 6 to 7 PH which is slightly acidic to neutral. Horticultural charcoal’s PH is incredibly alkaline, sitting at a PH well above 10.
As such, I no longer recommend using horticultural charcoal in most setups.
Bonus information About Peat Moss and Coco Coir (2 Common Base Ingredients):
Photo is linked to Amazon and is just one example of peat moss that can be used with plants.
Peat moss is usually the base ingredient in most pre-bagged potting mixes. It is harvested from peat bogs and takes thousands of years to form.
Because it takes so long to form, it isn’t a renewable resource.
I wouldn’t recommend adding peat moss to your potting mix if you are starting with a pre-bagged mix, which is what most people do.
Why? Peat moss, by itself, is very acidic in nature and can cause issues if the PH isn’t adjusted. Most pre-bagged mixes contain limestone or something similar to adjust the PH up. If you are using just peat, you may see poor growth and nutrient issues.
In addition, Peat moss is great at water retention, but not great at aeration. Most people want to increase aeration in their potting mix, so adding peat moss might be both redundant (because it’s already in the mix) and counterproductive to adding aeration.
Lastly, when peat moss becomes completely dry, it is hydrophobic – meaning it repels water. It takes several attempts at watering a fully dry pot of peat moss before the water begins to saturate the peat again.
Some people recommend bottom watering in these cases (which is where you sit the pot in a bowl of water so the water has time to penetrate the peat moss and saturate it).
Coco Coir or Coco Peat
Photo is linked to Amazon and is just one example of coco coir that can be used with plants.
Coco Coir has become a popular alternative to peat moss because it is much more sustainable and environmentally friendly. Coco Coir is created by grinding up coconut shells, which are a natural byproduct of the coconut industry.
It should be noted that Coco Coir is often high in salts, which can be particularly damaging for sensitive plants, and does not include a lot of nutrients like peat moss does.
As such, it is super important to consistently fertilize your plants when using coco coir. It’s also important to take plants in a coco coir mix to the sink and run water through the pot and out of the drainage holes several times to regularly flush out extra salts.
The good news is that coco coir does not become hydrophobic, like peat moss, when dry. It does dry more rapidly than peat moss, however, so you may want to monitor plants that have been recently switched to coco coir closely to ensure they aren’t accidentally experiencing drought.
What are your favorite potting mix recipes or ingredients? Let us know in the comments below!