Pumice: What Is It & How Much Do I Need in Soil?
Let's take a deep dive into my absolute best accessory when it comes to houseplants and cultivation! No, it's not the intricate handcrafted clay pots nor is it neem oil - my faithful companion who fights vermin and other annoyances that plagues our houseplants. Instead, look far down in the pot among roots and soil. This is because it is an obscure draining material that many have heard of but that everyone should know about - pumice! Perlite's slightly finer cousin who does the job better in every way.
What is it?
It is a volcanic gravelly material that, like perlite and leca balls, is part of the "draining medium" family. Pumice has long been in the soil-mix business, but in recent years has received a real boost and is now widely consumed by us plant lovers - mainly due to its draining and moisture-retaining properties. If you add pumice to the soil, it ensures that the water is drained without being stored in the soil for too long, and that the soil never really dries out.
Another thing that is very good with pumice is that it creates an airy and fine structure in the soil so that all fine hair roots can crawl out without obstacles. The soil then never becomes a clumpy & hard as it otherwise tends to do after a while. It is also sterile, which means that the stones are free from disease or vermin. If you use pumice, you thus reduce the risk of damaging the plant through overwatering considerably while creating a pleasant and airy environment for the roots to grow in.
Which plants Pumice suitable for?
It depends on what ambition you have with your potted plants. Quite frankly, there is no plant that actively chooses to have pumice in their soil, its not a natural addition. However, most people (me included) feel good about having some draining substrate in the soil - especially if you're an over-waterer! If you have to choose, you can say that plants that want a lot of water and plants that basically do not want any water at all (both in terms of quantity and frequency), and just adjust the amount of pumice in the soil accordingly. If you go from one side of the spectrum to the other, it can be, for example:
All types of ferns
Aroids, ie Monsteras, Philodendrons, etc.
Succulents & cacti
Plants that do perfectly well without draining substrates are those that are not particularly sensitive to either over-watering or under-watering. This category includes geraniums, golden vines, lilies and other easy-care plants that often live in offices and public spaces.
How much should I use?
You can very well grow plants in only pumice if you feel like it but it can costs a lot of money. Some grow both plants and vegetables in hydroponic (ie landless) stations with only pumice as a substrate. It is enough to check on them from time to time while not having to replant the plant because the soil loses its quality. But it takes time, and above all a lot of knowledge! So for us at the hobby level, it is better to run on a smaller scale so as not to constantly have to run and fill the pot with water and nutrients.
A really subjective rule of thumb is to have:
15% pumice for a regular soil mix
30% pumice for all tricky plants such as monsteras and calatheas (maranta included)
A little less than 1/2 pumice and 1/2 soil for ferns and other water-sucking plants
Cacti, succulents & caudexes want a huge proportion of draining substrates. You then mix the pumice stone with something that does not retain moisture - sand, for example.
With that said, the proportion of pumice is really not carved in stone. If you feel like having a little more, you can do it. As long as you remember to add nutrients a little now and then, there is no risk of the plant being damaged, regardless of quantity.
Pumice in a nutshell?
extremely porous, with a foamy appearance
suitable for seed germination
sterile material so great for indoor plants
light & airy material even when retaining water
Check out Supplies & Accessories page on where to get them (Malaysian sellers).