Library › Part C · Article 29 in the series · Level 1 · 15 min read

Choosing a medium: stone wool, coir, perlite, expanded clay, peat and others

Standfirst: After reading this article you will be able to compare the growing media sold for hydroponics on measured properties rather than on claims, say which property makes a medium suit a net pot, a propagation block, a drip slab, a bucket or a wick system, prepare stone wool and coir before planting, and work out how much medium a set of containers needs.

Before you read: Growing media I: air, water and particle size.

What a medium has to do, and the five things to compare

A soilless medium must provide oxygen, water, nutrients and support to the roots, just as soil does, and in hydroponics the nutrient solution supplies the water, the nutrients and part of the oxygen⁵. What the medium adds is mostly physical, and Resh gives the mechanism: water is held on particle surfaces and inside pores, so the smaller the particles, the closer they pack and the more water they retain, and porous particles store water inside themselves; because a medium must also drain, excessively fine materials are avoided, since they hold too much water and stop oxygen moving⁵.

The previous article defines the physical quantities used below: total porosity, water held after drainage, air-filled porosity and bulk density (Growing media I). Two chemical properties also matter (Growing media II): the pH a medium brings with it and whether it buffers pH, and the cation exchange capacity (CEC), the quantity of positively charged ions the medium can hold on its surfaces; organic materials in general have a higher CEC and buffer capacity than mineral ones³. The fifth thing to compare is life history: whether the medium arrives sterile, how long it lasts, and what happens to it afterwards. Industrially made materials (expanded clay, perlite, stone wool, vermiculite, polystyrene) arrive sterile because of the high temperatures used to make them³.

As a yardstick, Maucieri and co-authors ask of a pot substrate a porosity of at least 75%, with 15% to 35% of the volume as large air-holding pores and 40% to 60% as small water-holding pores, and an optimum bulk density of 150 to 500 kg/m³; very light materials such as perlite (about 100 kg/m³) are not suitable alone for plants that grow tall³.

The media, one by one

The comparison table below carries the tabulated numbers; these paragraphs say what they mean in use.

Stone wool (rockwool) is spun from molten basalt, limestone and coke into fine fibres (the sources in hand disagree on their thickness: about 0.5 mm¹, 0.05 mm³), bound with a phenolic resin and a wetting agent into slabs and blocks¹. It holds a lot of water at low suction and virtually none above 5 kPa, so its water buffer is small, the bottom of a slab stays wet (just 4% air at 1 cm above the base) while the top dries, and the recommended slab height is 7.5 to 10 cm¹. It has no cation exchange and no buffering of pH, water or nutrients, which makes it easy to steer and, in the words of the Raviv and Lieth chapter, “unforgiving” of management errors^1,4. The data sheet of one named product, the Grodan Classic slab, states a water content control range of 55% to 70% under practical conditions and calls it a single-year slab⁷.

Coir is the fibrous middle layer of the coconut husk, about 75% fibre and 25% fine coir pith; husks are often softened in brackish water, which raises sodium and chloride². Its air-filled porosity at 1 kPa suction spans 8% to 55% (the high value is for chips) and its easily available water 3% to 41%, so “coir” names a family of very different materials². Its CEC, 32 to 95 cmol/kg, is described as high and similar to peat². It is sold as pith, as chips sliced from husks, as fibre and as blends, usually compressed into bricks: 1 kg expands to about 14 L, a volume ratio of about six².

Perlite is a volcanic glass expanded 4 to 20 times by heating to 760 to 1,100 °C; it has strong capillary action and holds 3 to 4 times its weight in water, with 13.6% of its volume as available water in a 0 to 4 mm grade¹. It is neutral, without buffering or nutrients, and at low pH there is a risk of toxic aluminium release¹; the pH should not fall below 5.0 for that reason³. The dust irritates the airways, so wet it before handling⁴.

Vermiculite is a layered clay mineral exfoliated at 1,000 °C into light, accordion-shaped granules that hold 3 to 4 times their weight in water; unlike perlite it carries a permanent negative charge, a CEC of 15 to 21 cmol/kg and some buffering of pH and cations¹. The Maucieri tabulation gives a far higher CEC, 80 to 150 meq per 100 g, the same unit as cmol/kg³; both are printed because the sources disagree. Its structure collapses easily and is ground to powder by compression, so it is not suitable for long use, and it cannot be steam-sterilised because it disintegrates when heated¹. It waterlogs easily⁴.

Expanded clay (LECA) is clay pelleted and fired at 1,200 °C in grades of 4 to 8 or 4 to 16 mm¹. One study reports 11% to 24% water at 10 cm suction; another found the water content falling only from 48% to 44% between 3 and 50 cm suction, so that easily available water is just 4% and the granules hold a large amount of air¹. Mason describes it instead as having low air-filled porosity and higher water holding⁶; the measured sources say the opposite. Its EC is low unless salts were added during firing, in which case it needs washing, and its physical characteristics were unchanged after 5 years of intensive cropping¹.

Peat is partly decomposed bog vegetation; the raised-bog peats used in horticulture are acid, with high CEC and water retention, and a sphagnum peat at pH 3 to 4 needs about 2 kg/m³ of calcium carbonate per unit of pH rise³. Peat decomposes in use, which raises water retention and reduces air over time, and black fen peat is not suitable as a substrate at all³. Some countries are reducing peat use and extraction because of its environmental effects³.

Pumice, sand and gravel. Pumice has large pores, so three measured pumices had 27% to 41% air-filled porosity but only 2% to 5% easily available water, and new pumice leaches sodium at first¹. Sand is heavy and has low porosity, 0.45 and 0.30 as a fraction of volume for fine and coarse sand, and fine sand in ordinary pots gives aeration problems¹. Beach sand and road sand carry salts, and calcareous (limestone) sand or gravel releases carbonate that raises the pH and ties up iron, so igneous sand and gravel are preferred^1,5. Gravel is simply larger particles, 2 to 15 mm⁶ (5 to 15 mm in Jones⁴), that drain freely, hold little water and are heavy^4,6.

Mixes pair a water-holding component with an air-holding one: sand at 10% to 30% by volume with organic materials, expanded clay at 10% to 35% for aeration and drainage, pumice added to peat for the same reason³, perlite with vermiculite as a germination medium⁴, and chopped used stone wool blended into peat- or compost-based media¹.

Comparison table

Where sources disagree both values are given; a blank cell means no figure was found in a source we read. The³ values are Maucieri and co-authors’ reprint of Enzo et al. (2001), tables 4.2 and 4.3, and the⁴ values Jones’s reprint of Morgan (2003), table 9.14; the¹ and² values come from the studies named on the cited pages.

Before planting: pH, salts and conditioning

Stone wool arrives slightly alkaline and unbuffered. In the past its initial pH was 7.0 to 8.0 and values up to 9.5 were recorded; today 6 to 6.5 is the industry standard, and below pH 5.0 stone wool starts to dissolve, so very low pH must be avoided¹. The conditioning step is a presoak: Resh soaks slabs for 24 to 48 h with nutrient solution of EC 2.5 to 3.0 mS/cm and pH 6.0, which adjusts the pH and wets them uniformly, and cuts the drainage holes only after soaking⁵. Grodan’s handling guide says to saturate slabs at least 48 hours before planting, leave them fully saturated for at least 24 hours, never plant onto dry slabs, and cut a 3 cm drain opening only when the slabs are fully saturated, 24 hours before planting⁸.

Coir has the opposite problem: too much salt rather than too high a pH. When sodium, chloride and potassium are high they must be leached out before use; this was first done with water, but part of the potassium and sodium sits on the exchange sites, so today the leaching is done with a solution containing a cation, usually calcium nitrate, giving “treated coir”². Resh says most coir sold for hydroponics is treated, and that untreated coir should be soaked for a number of hours in good-quality water and flushed several times with new water⁵. The EC of coir products spans 0.12 to 2.32 mS/cm in a 1:1.5 water extract, so the label matters²; one consumer product, the CANNA Coco Brick, is described by its maker as washed and buffered¹⁰. No source we read fixes the number of rinses; rinse until the drain EC stops falling (EC and concentration).

Peat is limed as described above³. For perlite and vermiculite the hazard is at the other end: at low pH both can release aluminium into the solution¹.

Sterility, reuse and disposal

Stone wool is usually discarded after one or two crop cycles, producing about 125 m³ of waste per hectare; it cannot easily be returned to nature, and in the United Kingdom and the Netherlands nearly all used slabs are now collected and remade into new mineral wool or bricks¹.

Perlite can serve for years. In a commercial trial, used perlite from 18.9 L bags was treated with 13.25 L of hot water per bag at up to 93.3 °C, which cut the EC by 43%, disinfected the medium, showed no change in particle size over 8 years, and saved 56% of the cost of replacing it¹. Expanded clay can be washed and sterilised without harm and lasts many years; pumice can be reused practically indefinitely; sand can be steam-sterilised and is very durable; vermiculite cannot be steamed and collapses with use¹.

Coir is biodegradable, though the pith can take up to 20 years to decompose⁵. Maucieri and co-authors rate its environmental impact lower than that of peat (exploitation of bogs) and of stone wool (disposal), and note that organic substrates decompose and compact faster than mineral ones, so they suit shorter crops while mineral substrates suit long ones³.

Matching a medium to a system

The match follows from one property each time.

Net pots in NFT and deep water culture. The roots grow out of the pot into the film or the pond, so the medium only anchors the seedling and must not hold a waterlogged column above the solution. Expanded clay fits because it is free draining, sterile, reusable if sterilised, and already common in pot hydroponics⁴; alternatively a seedling raised in a stone wool cube is moved with its cube, as Resh describes for transplanting⁵. See Nutrient film technique and Deep water culture.

Propagation needs a fine, uniform, sterile material that holds water next to the seed. Stone wool cubes and blocks come in sizes such as 7.5 × 7.5 × 6.5 cm and 10 × 10 × 8 cm, are soaked with dilute nutrient solution of EC 0.5 mS/cm before sowing and EC 2.5 mS/cm at pH 6.0 before transplanting, and larger blocks hold plants longer⁵. Vermiculite is a sowing medium and perlite with vermiculite a germination mix^1,4.

Slabs under drip are small water buffers refilled many times a day, so a medium that holds much water at low suction suits them: stone wool slabs 7.5 to 10 cm high¹ and coir slabs⁵. See Drip and substrate culture.

Bags and buckets give a larger volume per plant and tolerate a coarser, more air-filled medium, such as perlite in 18.9 L bags for tomatoes¹, or expanded clay. Weight decides between them: perlite at about 100 kg/m³ is too light alone to hold up a tall plant³, and sand is added to mixes as ballast against top-heavy plants and wind¹.

Wick and passive systems need water to climb from a reservoir, so capillary action decides: perlite and vermiculite both have strong capillary action¹, and coir holds 80% to 85% of its volume as water³. See Kratky and passive systems.

Worked example: hydrating a 5 kg coir block

A supplier’s specification sheet for a named product, the CocoGrow Lanka “Coco peat 5kg” block, gives: block size 30 × 30 × 12 cm, compressed 5:1, weight 5 kg, moisture below 22%, EC ’less than 0.5 or more than 0.5’ by the 1:5 test method (no unit on the sheet; presumably mS/cm), pH 5.5 to 6.8, and 70 to 75 L after expansion⁹.

Cross-check: Raviv and Lieth say 1 kg of compressed coir expands to about 14 L², so 5 kg gives 5 × 14 = 70 L, our calculation, which agrees with the sheet. The compressed block is 0.30 × 0.30 × 0.12 = 0.0108 m³, or 10.8 L, so 70 L is a volume ratio of 70 ÷ 10.8 = 6.5, this article’s calculation, close to the “about six” of Raviv and Lieth².

How much water to add is not on the sheet. Our estimate: coir at container capacity holds 80% to 85% of its volume as water³, so 70 L of expanded coir holds 0.80 × 70 = 56 L to 0.85 × 70 = 59.5 L. The block already contains up to 22% moisture by mass⁹, that is up to 0.22 × 5 = 1.1 kg, about 1.1 L of water, so add roughly 55 to 58 L to bring the whole block to container capacity, and less if you want it moist rather than wet. CANNA’s instruction for its own brick is to add water 0.5 to 1 L at a time until fully expanded, and that lukewarm water speeds expansion¹⁰.

How many pots: 70 L fills ten 7 L pots or seven 10 L Dutch buckets, our arithmetic. A mass check for the alternative fillers: ten buckets of 11 L need 110 L of medium; at 0.1 g/cm³ perlite weighs 110 × 0.1 = 11 kg, and expanded clay at 0.28 to 0.63 g/cm³ weighs 31 to 69 kg, our calculations from the bulk densities in¹: one is light to lift, the other heavy enough to hold a plant upright.

Key numbers

Common mistakes

1. Filling a deep pot with fine sand or fine coir because it “holds water well”. Fine particles pack closely and the bottom of the container stays saturated, so the roots lose oxygen; fine sand gives aeration problems in ordinary pots^1,5.
2. Planting into dry stone wool or cutting drain holes before it is saturated. The slab then never reaches its full capillary capacity, and plants on a dry slab are greatly restricted^5,8.
3. Using untreated coir straight from the brick. Sodium, chloride and potassium on the particle surfaces enter the solution, and water alone does not remove the exchangeable part; a calcium nitrate soak does².
4. Running a very acid solution through perlite, vermiculite or stone wool. Below pH 5 stone wool dissolves, and perlite and vermiculite can release aluminium¹.

Summary

Every medium is a compromise between water held and air kept: stone wool and coir hold around 80% of their volume as water with little air^2,3,4, perlite and pumice hold far less water and much more air^1,3, and expanded clay holds a few percent of easily available water in a heavy, very durable granule^1,3. Chemistry adds a second axis: stone wool arrives alkaline and unbuffered, coir arrives salty with a high CEC, peat arrives acid, and perlite and vermiculite release aluminium at low pH^1,2,3. Each system picks the property it needs: free drainage and weight for net pots, fine sterile water-holding for propagation, water held at low suction for drip slabs, lightness or ballast for buckets, and capillary rise for wicks. Conditioning is part of the choice: stone wool is presoaked at pH 6.0 two days before planting^5,8, and coir is leached with a calcium salt². One 5 kg coir block becomes about 70 L of medium after roughly 55 to 58 L of water, by this article’s arithmetic from^2,3,9.

Next in the series: Light: PPFD, DLI, spectrum and photoperiod.

References

1. Papadopoulos, A. P., Bar-Tal, A., Silber, A., Saha, U. K. and Raviv, M. (2008). Inorganic and synthetic organic components of soilless culture and potting mixes. In Raviv, M. and Lieth, J. H. (eds), Soilless Culture: Theory and Practice, chapter 12, pp. 505–543. Elsevier, Amsterdam. https://doi.org/10.1016/B978-044452975-6.50014-9
2. Maher, M., Prasad, M. and Raviv, M. (2008). Organic soilless media components. In Raviv, M. and Lieth, J. H. (eds), Soilless Culture: Theory and Practice, chapter 11, pp. 459–504. Elsevier, Amsterdam. https://doi.org/10.1016/B978-044452975-6.50013-7
3. Maucieri, C., Nicoletto, C., van Os, E., Anseeuw, D., Van Havermaet, R. and Junge, R. (2019). Hydroponic technologies. In Goddek, S., Joyce, A., Kotzen, B. and Burnell, G. M. (eds), Aquaponics Food Production Systems, pp. 77–110. Springer, Cham. https://doi.org/10.1007/978-3-030-15943-6_4 (open access; tables 4.2 and 4.3 reproduce Enzo et al. 2001)
4. Jones, J. B. Jr. (2005). Hydroponics: A Practical Guide for the Soilless Grower, 2nd ed. CRC Press, Boca Raton. Table 9.14, after Morgan, L. (2003), The Growing Edge 15(2):54–66.
5. Resh, H. M. (2013). Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial Hydroponic Grower, 7th ed. CRC Press, Boca Raton.
6. Mason, J. Commercial Hydroponics, 3rd ed. ACS Distance Education.
7. Grodan (2021). Grodan Classic: product data sheet, March 2021. ROCKWOOL Group, Roermond. https://www.grodan.com/siteassets/downloads/downloads-en/product-data-sheets-en/Grodan-Product-data-sheet-Classic-slab.pdf
8. Grodan. 3-3 Handling and placing of the slabs (grower factsheet). https://editserver.grodan.com/siteassets/downloads/tools--services/english/ts-3-3-handling-the-slabs-en.pdf
9. CocoGrow Lanka. Cocopeat and other coconut based products: Coco peat 5kg, product specification. Sri Lanka Export Development Board e-marketplace. https://www.srilankabusiness.com/emarketplace/media/sellerAdditionalInfo/1593060500C.pdf
10. CANNA. CANNA Coco Brick, product page. https://www.cannagardening.com/canna-coco-brick

This article was drafted with the help of AI tools and reviewed and edited by PonicLabs. Sources are listed above; tell us if you find an error.