Take your indoor gardening to the next level with soilless potting mixtures. Our guide to soilless gardening covers stone, rockwool, perlite, pumice, and bark.

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What is a soilless potting mixture?

Soil consists of organic matter like dead animals or plants that have decomposed into fine particles. Soils also usually contain some inorganic mineral materials like sand or clay. 

But soil isn’t the only material that can anchor and feed your plant. In gardening parlance, we use the term potting medium rather than soil. Potting medium refers to whatever material your plant is rooted in and draws its water and nutrients from - soil is only one kind of potting medium.

In soilless gardening, ‘substrates’ are used as the potting medium. These can be mineral materials like stone or perlite. They can also be non-decomposed organic material, for example, coconut coir. Often, several substrates are mixed together to create a potting mix fine-tuned to the specific needs of your plant. 

Why switch to soilless mediums? 

Soil-based pathogens can wreak havoc on whole nurseries and gardens because they transmit invisibly. Soilless mediums can eliminate this problem at the source. They are manufactured from scratch in sterile environments rather than taken from natural sources that may be contaminated. If your whole garden is soilless from the start, the chances of bringing pests into your garden are dramatically reduced. 

Soilless mediums also allow the plant to take up both air and oxygen through its roots more efficiently. Plants have to work harder to unbind water molecules from soil particles. This is because soils have greater matric potential - a measure of how readily water molecules adhere to surrounding particles. By comparison, particles in soilless mediums release water much more readily so plants don’t need to work a shard to extract it via its roots. 

Because soilless culture is more water-efficient and resistant to pathogens it has attracted scientific and industry attention over the last few decades as a sustainable agriculture solution. 

While soilless culture is generally more sustainable than soil, peat is an exception to this. Peat is a substrate that forms over thousands of years. This natural supply cannot be replenished at anywhere near the same rate it is removed. Peat environments also provide rare habitats for wildlife so harvesting the material damages biodiversity. 

Do soilless substrates contain plant nutrients?  

Some do and some don't, depending on whether the manufacturer has added them. You may need to add nutrients yourself to the substrate.

Plants obtain some nutrients from the air through leaves but the majority is taken up from the soil through the roots. So, whatever potting material you use, it must contain the essential elements that plants use to grow and maintain themselves. This is especially important just before and during the plant’s growth season, which is usually in spring and summer.

All plants need 16 elements to grow, maintain, and develop. Nitrogen, phosphorus, and potassium are needed by plants in the greatest amounts but other elements must also be present for the plant to stay healthy. Even succulents and cacti, highly adapted to harsh environments, need nutrients in small amounts to survive.

Soil is a naturally nutrient-rich potting medium because it is composed of rotted organic matter which releases essential elements as it decomposes. Inorganic substrates like perlite or pumice are not naturally nutrient rich because they are composed of minerals. 

Unless the manufacturer has already added nutrients to the substrate, you should get a complete fertilizer and add it yourself. A complete fertilizer not only contains the three main nutrients - nitrogen, potassium, phosphorus - but also lime, calcium, magnesium, sulfur, iron, manganese, and zinc.

Which soilless substrate should I use for my plants?

Below you can find out the physical properties of different substrates. These differing properties make each substrate suitable for different houseplant species. 

Choose your substrates based on whether your plant prefers damp soil and whether it requires much fertilizing. For plants that prefer moisture and require a lot of feeding, you should consider substrates that hold nutrients and water well, such as sphagnum moss or short-length coconut fiber. 

You will generally need to mix two or more substrate materials to get a potting mixture that works for your tropical plants. Sphagnum moss and coconut fiber alone will tend to get too waterlogged - you need a dry, rocky substrate to balance this out and allow water to drain through the pot more quickly. The exact proportion of dry, airy substrate to dense, organic, and water-retaining substrate will depend on the plant species. 

Ferns will love a mixture heavy in coir or sphagnum moss, with perhaps a ¼ of the mix coming from perlite to aerate the soil. A monstera will do well in a potting mix that has a more balanced mix: ⅓ bark, ⅓ perlite, ⅓ coconut coir. 

However, some succulents (especially desert cacti) can thrive in a single dry and porous substrate like perlite and pumice, which are both made from volcanic rocks.

Sphagnum moss 

Sphagnum moss retains moisture and nutrients well Sdjurovic commons.wikimedia.org/wiki/File:Sphagnum_sp._IMG_6888%5E.jpg

Sphagnum is a family of mosses found in the Baltic, Finland, Germany, and Ireland. There are 120 different species within the genus. 

Sphagnum moss is a popular soilless substrate because it retains both water and nutrients well. This is thanks to the large surface area that it packs into its thin, fibrous strands. Sphagnum is useful in include in soilless potting mixes for moisture-loving plants like ferns and orchids. For tropical houseplants, it is often mixed with perlite or pumice. The sphagnum moss will temper the high drainage of these volcanic mineral substrates while giving the tropical plants the rich organic material it needs to thrive. 

 

Rockwool

This substrate is made from molten basalt rock, limestone, and coke. It is made of very fine, light, and densely matted fibers and is sold in rectangular blocks. You can insert plants directly into slabs of rockwool to make ‘cube gardens’.

Volcanoes produce rockwool when they spray molten lava into the air which is then spun into fine threads by the wind. This natural process is now replicated industrially and the rockwool you purchase at gardening stores will be this synthetic version.

The main benefit of rockwool is that, because it is manufactured at very high temperatures, it is almost guaranteed to be sterile. However, rockwool has a high water capacity. This can be useful for plants like ferns or fatsia that love moisture. However, it means that it can become more easily waterlogged than other soilless substrates. This substrate is not suitable for cacti and succulents that need dry conditions. 

Bark

 Bark chips have high porosity and give high drainage @miononame pxhere.com/en/photo/1610592

Chipped tree bark is a common component in soilless substrate mixtures. The chips usually measure 1-2 cm. The inner and outer bark of coniferous and hardwood trees can be used.

Bark has high porosity and provides plenty of oxygen to your plant. This also means that it does not retain water well. A mixture heavy in bark material will drain quickly, especially if it contains larger wood chips. Moisture-loving plants like peace lily, maranta, ferns, and bromeliads can have bark in their potting mix but it should be mixed with a water-holding substrate like sphagnum moss or very fine, short-length coconut fiber. Tree bark is only usually used on its own as a potting medium for orchids. 

 

Perlite 

Cacti can be grown in pure perlite. More often, it is mixed with other substrates OpticalNomad unsplash.com/photos/pQwll5IG-I0

A white, crushed volcanic rock mined mainly along the Aegean coast in Turkey. The rock ground and heated to very high temperatures. During heating, the volume of the perlite expands between 4 to 20 times. 

Perlite can be used on its own for succulents and cacti. For tropical plants, it is mixed with organic soilless substrates like sphagnum moss or coconut coir. 

Coarsely ground perlite holds water a lot more than finely ground perlite. 

 

Coconut fiber (coir)/coconut chips 

A pile of coconut fiber or coconut coir - this material is the hair that covers the husks. Another coconut substrate is chipped coconut husk. Fotokannan commons.wikimedia.org/wiki/File:Coir_fiber.jpg 

Coconut fruit husks provide two kinds of substrate: the coir, which is the hair covering the husk, and chips, which are made by crushing the husk. Both are inexpensive soilless substrate options. Nowadays, coconut coir is regarded as the sustainable alternative to peat. 

Often, gardening coir is treated with fertilizer by the manufacturer. If it is not ready-fertilized, you will need to soak it in a complete fertilizer solution at home.

Coconut coir comes in different fiber lengths. Water and nutrient retention and drainage will vary between these grades. For coconut chips, the larger the particles, the higher the drainage. Coconut chips will offer higher drainage than coconut coir, whose thin fibers will retain water much more. Short, fine coconut fiber coir makes an excellent substitute for peat, a very ecologically destructive substrate. Long-fiber coir will offer drainage somewhere between coconut husks and short-fiber coir.

You can easily mix coir or coconut chips with other soilless substrates - for example, short-length fine coir fiber can be combined with pumice or perlite to increase drainage. This potting coir and pumice/perlite mixture will give a potting medium that balances water retention with oxygen pockets.

Pumice

Garden pumice is a crushed, lightweight, and porous volcanic rock. It can be expensive but gives great drainage. Water passes very quickly through it, reducing the likelihood of waterlogging. 

Pumice products will have different properties depending on how finely they are ground. If ground coarsely, pumice will drain very easily and will not retain water or nutrients as effectively as other substrates. This makes coarsely ground pumice unsuitable for faster-growing, moisture-loving species like ferns. Pumice is also unsuitable for moisture-loving flowering species that need additional phosphorus and potassium just before and during their blooming season: these include peace lilies, azaleas, dieffenbachia, maranta, and coleus. 

However, pumice is absolutely ideal for cacti and some succulents since they require little feeding and dry conditions. These plants will thrive in a pumice substrate combined with a terracotta pot, which draws moisture out from the soil. 

Learn More

• Moss, Bark, Coconut Coir or Rock - What's the best soil for my orchid?
• When and How to Repot Orchids
• Peat Moss vs Sphagnum Moss - What's the difference?
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