Vermiculite: How to increase the water retention of the substrate

Anyone who has ever worked with a potting soil mix has probably noticed those little “blocks” or chunks of gold in it. Light and airy, these particles are known as vermiculite, an important product in landscaping (and many other industries). Although they are not worth their weight in gold, they are highly revered for their many benefits.


Similar in nature to perlite, vermiculite is formed by weathering or heating biotite. Its chemical formula is real gibberish:

(MgFe, Al) 3 (Al, Si) 4 O 10 (OH) 2 · 4H 2 O

Vermiculite is within the group of phyllosilicates, which contain high proportions of magnesium aluminum silicate mineral that resembles mica in appearance. Commercial vermiculites have approximately these amounts of compounds:

  • 38-45% silicon oxide (SiO 2 )
  • 9-17% aluminum oxide (Al 2 O 3 )
  • 15-35% magnesium oxide (MgO)
  • 8-15% water (H 2 O)

They also contain smaller amounts of other chemicals such as those seen in the previous formula such as iron in its composition and other impurities that have nothing to do with pure vermiculite.

After extraction, the mineral is separated from other minerals and sieved to classify it into different particle sizes. Like perlite, these different sizes comprise the different grades of vermiculite: large, medium, fine, and very fine. The most commonly used sizes are 0-2, 2-4 and 4-8 mm in diameter.

The particles are superheated to about 700 / 800ºC for 1 minute in a process called exfoliation to create elongated particles, like a golden beige-brown accordion with a certain brightness depending on the light. With this process it increases its volume up to 30 times .

Detail of the crystal structure of vermiculite. Photo by: Jungle Rebel

Two characteristics of vermiculite make it have a unique molecular structure and therefore such versatile uses. First, a layered crystalline structure within the mineral provides the hinged plates that cause the material to expand or unfold in a linear fashion when heated. Second, the trapped water turns to steam when heated, forcing the layers to open.


Both treated and untreated vermiculite are used in a variety of industries, for many different purposes. Expanded vermiculite is very versatile and has very interesting physicochemical properties that make it really special: it is light, it is not combustible, it is compressible, highly absorbent, it has a neutral pH, it is inert and it does not react to acids except very strong ones. .

All these properties make it widely used for many different purposes. Here are some examples:

  • In construction it is used to make plaster, insulation and lightweight concrete used for floors and ceilings. It has a very low thermal conductivity (0.053 Kcal / h.mºC), making it a very good thermal insulator. It is also fire retardant, an essential feature as a construction material.
  • It is used for example for reptile nesting because its low conductivity keeps heat very well.
  • Automobile – Used to manufacture brake and clutch linings, gaskets, and rubber seals. When ground, it turns into a powder that can be used in automotive paints and lacquers.
  • Packaging material for the transport and storage of dangerous liquids. Highly absorbent, so it will absorb any leakage that occurs; We have commented that it is not flammable so as a fire retardant material it is ideal in liquids of this type. In addition, it decomposes naturally in the environment and does not damage the subsoil or aquifers.

But what we care about in our subject is what we can use it for in our crops and plants. Vermiculite helps with water retention, soil aeration, and drainage. It is used as the sole growing medium in seedbeds, or as a component of potting soil; it is also used as a soil amendment for garden floors.

Reptile emerging from the egg on a bed of vermiculite. Photo by: Javier Ábalos


If we want to improve the water retention capacity of a potting substrate, vermiculite, as well as perlite, are two fabulous examples.


Vermiculite is used as a growing medium for seed pulling and rooting of propagated cuttings due to its sterile nature and its ability to retain moisture without promoting damping. All plants can be started – from seeds or cuttings – using vermiculite as the only growing medium. When the time comes to plant the plants in pots, some will thrive growing solely in vermiculite, others will prefer other types of growing media for maximum plant growth.

Some indoor plants capable of living in a pure vermiculite substrate can be pothos (these grow where you throw them), ferns also require great moisture retention, bamboo is another very thirsty species and philodendron , of which we have spoken very few days ago.

Seedlings with a mixture of substrate and vermiculite. Photo by: Brianna Privett


When we dig up the bulbs in late autumn we have to store them in a cool and dry place during the winter. You may be wondering, if the vermiculite retains water, then we don’t want to put it with the bulbs, right? Yes, because the vermiculite will absorb all the excess moisture without drying the bulb until the next planting.

It’s like using those “silica gel” or silica gel sachets that come in many products such as shoes, electronic products etc. to absorb moisture. The difference is that vermiculite is natural and non-toxic. The silica gel I think is retreating.


I will always remember when I first saw vermiculite. He had a cat that ate house plants (specifically a ficus and a Brazilian trunk ) to purge herself. Tired of crushing the leaves of my plants, I decided to buy cat grass. Well, the surprise was huge when I opened the container and in it there were grass seeds mixed only with 100% vermiculite. That is, for the germination of the grass it is magnificent and in fact it is used for that.

One of the critical aspects of planting grass from scratch is keeping the seeds moist as they germinate . A layer of vermiculite can be spread along with the seed spread over the ground and then watered well. Vermiculite helps keep water close to the seeds as they sprout. It is a solution that works very well.

Substrates – Vermiculite Substrate 5L – Batlle

  • Vermiculite in addition to easily assimilable water retention provides an excellent retention ratio of easily assimilated nutrients
  • Ideal component for spacing fertilizer applications
  • Vermiculita
  • Made in Spain


Like any element, like any substrate, it has advantages and has some drawbacks that can be solved with other types of substrates such as coconut fiber , perlite, akadama , peat, volcanic gravel … There are many types of substrates, and none are perfect but the combination of a few can generate a perfect substrate. We are all clear that vermiculite, although it has drawbacks, are far surpassed by its virtues.


  • Very high water retention, its main characteristic. The particles take a long time to dry, so the root zone stays moist longer.
  • It is ideal for the germination of seeds or the propagation of plants, since the particles allow the plants to be extracted from the perlite without damaging the root system when transplanting them.
  • It clings well to nutrients ( high cation exchange capacity ) due to its structure.
  • It can be used alone or mixed with other media to create potting mixes.
  • pH close to neutral. This is really important. The pH of a soil can be modified but it is not an easy task. Most crops grow around neutral pH.
  • The fact that it is sterile makes it very suitable for starting seeds.
  • Reusable year after year as it does not break down. It can lose its crystalline structure over time but it takes a long time if mechanically it does not move too much


  • Carbonate compounds promote alkaline reactions, raising the pH in the root zone that will eventually increase the pH of the medium slightly.
  • Some plants that do not need constant humidity will not support this type of substrate and their roots could rot in some cases. There are other solutions.
  • It is an exhaustible mineral. There is a lot, there is no need to worry, but it is not something that can be easily obtained. It is a mining extraction like any other with the impact that that supposes.

As we can see, the balance is very unbalanced in our favor.


Is there really a big difference between vermiculite and perlite? Can they be substituted one for the other? Unfortunately, although they have some similar properties, they act very differently.

The biggest difference between the two is that perlite helps increase drainage , while vermiculite increases water retention.

Perlite can be used to grow plants that benefit from the growing medium drying out completely between waterings such as orchids . Vermiculite is good for plants that prefer to stay moist in their root environment, such as tropical plants or ferns.

They can be mixed , of course. This is what we have commented before, there is no perfect substrate, but the mixture of different compounds with complementary properties can lead us to reach that substrate with perfect physical-chemical and biological conditions. If you need to balance drainage and water retention, a mixture of the two will provide a very suitable midpoint for many crops.


Leave a Reply

Your email address will not be published. Required fields are marked *