Rockwool is the favored growing medium in hydroponics because of its good water retention, aeration, porosity, and malleable nature. It is the recommended growing medium for both beginners and veterans alike. However, are there other alternatives, as good or even better?
The most viable rockwool alternatives in hydroponics are 1) jigsaw foams, 2) perlite and vermiculite, 3) expanded clay pellets 4) coco coir, 5) rice hulls, 6) potting soil 7) gardening soil 8) gravel 9) sand. The growing medium must be apt to the hydroponic technique being used and the plants being grown.
There’s a whole other world of growing mediums beside rockwool which may be cheaper, better, or have qualities you want for your special medium mix.
Table of Contents
- 1 What is Rockwool?
- 2 Why is Rockwool Preferred?
- 3 The 9 Alternatives to Rockwool
- 4 Is Rockwool Safe?
- 5 Takeaways
- 6 Sources
Rockwool is mineral fiber insulation that has its application in the construction industry. It’s a mixture of volcanic rock and slag by-products of the metal industry wherein it is melted and spun into fibers.
It is a breathable material that lets moisture and air pass through hence the material found its way to the gardening and horticulture world. The material allows the optimization of advanced root zone techniques for greater plant growth and yield.
Rockwool is a fantastic way to start seeds. It has an ideal ratio of water retention and aeration. It can hold moisture but can retain enough air space (at least 18%) to promote optimal root growth.
Due to its porous and fibrous nature, it also drains excess water yet can retain an adequate amount of moisture at the bottom. Roots can easily permeate rockwool, allowing the root system to reach the nutrient-rich water. This is an important factor to consider in both germinating seeds to growing plants in full.
It is easy to cut, shape, and incorporate in any hydroponics system, regardless of size and setup. The material is easy to fit in a net cup and can be reused by simply treating the used rockwool with diluted bleach.
Rockwell can be used for all types of plants and crops such as flowers, melons, lettuce, peppers, strawberries, and herbs, among others.
The greatest advantage of rockwool is its simplicity and effective nature. It does its job excellently and has wide application on any hydroponic system regardless of technique used.
Despite how commonplace rockwool is, there are other alternatives still available because almost anything can be used as a growing medium in hydroponics. These alternatives are equally viable in growing hydroponically.
In general, the properties that make rockwool a top choice in hydroponic are good water retention with respect to good aeration. There must be a balance between these two properties.
The application of a growing medium must be in concurrence with the hydroponic technique or system being used and the plants being grown. Some systems have different watering tendencies, some plants have different watering requirements, and the growing medium must be adjusted accordingly to these conditions.
Jigsaw foams are the same foams found in children’s playgrounds or nursery schools. These can be cut in such a way that they can provide support to a plant. A pudgy cross shape can easily provide support to seedling or full plants on a circular opening/
Pool noodles are used in swimming pools for training, waterplay, and recreation. Hollow pool noodles are used in the same way as jigsaw foams but are better suited for longer and larger plants.
Pool noodles and jigsaw foam sheets are made from ethylene-vinyl acetate, the same compound found, hot-glue, floats, adhesives, baby teethers, and cosplay costumes. EVA is nontoxic because it does not contain BPAs, heavy metals, or chlorides. Likewise, studies have found that exposure to EVAs do not raise any health concerns.
These alternatives are best used for NFT systems. But this does not mean its application is limited only to NFT systems as its application depends more on how the system is set up rather than what the technique the system is using.
Perlite and vermiculite are light volcanic rocks and are often mixed together to combine their distinct properties.
Perlite has poor water retention but good aeration. It looks like small white popcorn. It is lightweight, odorless, porous, and has optimal pH levels of 6.0-7.0.
Vermiculite has excellent water retention and looks like little worms when they absorb water. It looks like brown or light brown flakes. It is fairly similar to perlite but is often used as a secondary component to create a highly customized growing medium.
For more on their differences check our full vermiculite vs perlite guide.
This is why these are often mixed. It creates a mixture that has the properties of both while mitigating the cons of each.
Most techniques will grow well with this growing medium but is primarily used by hydroponic growers using Dutch buckets systems.
Expanded clay pellets, also known as hydroton pellets, are baked clay pellets which come in different sizes and diameters. They look like porous clay balls. Due to their shape, they provide good aeration but dry out fairly easily.
An entire grow bed full of hydrogen can become heavy so weight should be a consideration. The materials in your system should be strong enough to bear the weight.
These are best for ebb and flow techniques or techniques which lets the grow medium be consistently hydrated such as deep water culture or drip system.
Most techniques will work well with this growing medium.
Coco Coir is simply processed dry coconut husk. Coconuts are the fruits of tropical coconut trees. These large fruits are covered by thick, protective husks that protect the seeds inside.
Coco coir can come in different forms however they share similar properties. It is inert meaning that they have no nutrients in them. They are also somewhat expensive depending on which part of the world you are in.
Coco peat is finely ground coconut husk which is highly absorbent and it is becoming a great replacement to peat moss
Coco fiber is coconut fibers and strands which makes it less absorbent but with good aeration.
Coco chips are chopped coconut husks in small tiles. They act similar to hydroton but are more water absorbent while still being aerated.
Most techniques will work well with this growing medium.
Rice hulls are the by-products of the rice milling process wherein the coating of the unprocessed rice is peeled off. Hence, they are inexpensive and used in many developing nations such as the Philippines.
They offer decent water retention and good aeration because the rice hulls do not clump together. Apart from that, rice hulls are also buoyant.
Lastly, they are organic hence biodegradable. Their decomposition is slow however which makes them suitable for longer grow cycles.
Rice hulls are best mixed with other mediums to create a specialized growing medium, custom built for growing certain plants. This principle applies to soil and soilless cultivation.
Potting soil in a garden can work equally well in a hydroponics set up.They have good water retention properties and good aeration because of their varied composition of different materials such as peat moss,, perlite, sand, and/or coconut coir, among others.
They have a light and grainy appearance and are almost “soilless” since they are just a varied mix of the above mentioned materials. Potting soil drains well but still retains some moisture. Furthermore, they are a relatively inert medium which means that they have little to no nutrients in them.
Most techniques will work well with this growing medium.
For soil cultivation, YourIndoorHerbs has an informative article on potting soil and whether or not they can burn.
Gardening soil is a staple in garden beds and conventional cultivation. It is topsoil enriched with organic matter, giving it a rich, loamy, and heavy texture. Gardening soil offers greater water retention at the expense of drainage. These properties are brand-dependent, however.
However, because they have such excellent water retention properties, it is important to not over-water or use systems which tend not to overwater plants. They are also not inert because of the enriched organic matter, meaning that they have nutrients inside them which should be taken into consideration when observing the nutrient balance in a system.
Remember, when we mention gardening soil we do not mean your garden soil. Never take outside garden soil for indoor or hydroponic applications as it will bring a large amount of insects or eggs that might not be visible at first but that can infest your new plants.
Gravel is simple, easy to use, readily available, and cheap. Gravel tends to provide plenty of oxygen to the roots but does not retain water, hence it must be watered regularly. It is heavy however so it is necessary that the system is strong enough to bear the weight.
The best part of gravel is that it is reusable. Simply treat the used gravel by applying diluted bleach after every growing cycle to make sure it is sterilized.
Sand is not widely used due to its lack of inability to retain water and its poor aeration. Sand tends to compact leaving no air available for the roots.
However, sand can be used with other growing mediums to create a specialized grow medium to mix and match. It is also readily available and cheap to acquire which works in its favour.
Sand is better mixed with other mediums and would be better for ebb and flow systems since the medium is periodically drenched and dried.
Rockwool is safe. Scientific studies using magnetometric evaluation have shown that inhaled rockwool matter will not cause significant lung toxicity. The most rockwool can do is cause mild irritation to the skins and eyes when they come into contact with rockwool’s dust and coarse fibers.
While it is known the rockwool fibers can cause some skin irritation, gloves are a solid remedy to an easily avoidable problem. If the fiber dust is a concern, then a simple mask will suffice. Applying moisture to the rockwool will bind the dust to the material making this problem easily remedied.
Rockwool’s sustainability concerns mostly stem from the carbon footprint in the transportation of materials and the energy requirements to manufacture it. Otherwise, the material itself is safe.
- Rockwool is a fantastic growing medium that combines good water retention to aeration ratio. It is recommended for both beginners and veteran growers alike.
- Alternatives such as coco coir, clay pebbles, vermiculite and perlite are all viable as well and will provide similar results to rockwool. Other mediums require mixing with others to become more effective.
- In the end, the chosen growing medium must conform to the needs of the plant and the peculiarity of the used growing technique. It must be adjusting accordingly to these conditions to manipulate water retention and aeration.
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- “Alternative Hydroponic Substrates” by Bridget White in Greenhouse Product News
- “Assessing the Carbon Footprint of the Rockwool Supply Chain: A Real-world Case Study” by Aivazidou et al in 5th International Conference on Manufacturing Engineering (5th ICMEN), Thessaloniki, Greece
- “Chemical and Physical Properties of Various Commercial Garden Soils and their Effects on Plant Growth” by F. Hancock, S. Howington, and C. Currie in University of Georgia College of Agricultural & Environmental Sciences
- “Characteristics of rock wool” by Dr. Lynette Morgan in Canna Gardening
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- “Scientific Opinion on the safety assessment of the substance ethylene-vinyl acetate copolymer wax, CAS No 24937-78-8 for use in food contact materials” by EFSA Panel on Food Contact Materials, Enzymes, Flavourings and Processing Aids (CEF) in European Food Safety Authority (EFSA)
- “What is the Difference Between Potting Soil and Garden Soil?” by Leanne Potts in HGTV