Library › Part C · Article 27 in the series · Level 2 · 12 min read

Growing media I: air, water and particle size

Standfirst: After reading this article you will be able to describe a growing medium as solid, water and air sharing one volume, define total porosity, container capacity, air-filled porosity and bulk density, read a water-retention curve, and explain why the bottom of a drained container is always saturated. You will also be able to estimate how much air a slab or a pot of a given height holds.

Before you read: Anatomy of a hydroponic system, Water in the plant, Dissolved oxygen.

A medium is three phases sharing one volume

Every growing medium, from a stone wool slab to a pot of peat and perlite, is made of three phases: solid particles, water and air¹. The solid phase is fixed once the medium is in the container. The other two share the volume that is left, so every litre of water that drains or is drunk by the roots lets a litre of air in². The grower’s job is to keep both shares adequate at once: water for the plant between irrigations, and air for the roots, for the reasons given in Dissolved oxygen.

Four numbers describe the share. Total porosity is the fraction of the volume that is not solid. Container capacity is the water left after free drainage, as a percentage of the volume⁷. Air-filled porosity, also called air space, is total porosity minus container capacity: the pores that drained and now hold air⁷. Bulk density is the dry mass of the medium per unit volume, in grams per cubic centimetre¹.

Bilderback and colleagues, of the substrates laboratory at North Carolina State University, give the ranges that make most potting substrates easiest to manage: total porosity 50% to 85%, air space 10% to 30%, container capacity 45% to 65% and available water 25% to 35% of the volume (23% to 35% in the same paper’s table; the ranges come from Yeager and colleagues’ 1997 best-management guide)⁷. They add that no universally accepted standard exists, and that air space and container capacity depend on the height of the container as well as on the medium⁷. The rest of this article explains why.

Particle size decides pore size, and pore size decides the share

Wallach calls particle size distribution the most fundamental physical property of a porous medium, because it sets the internal geometry, the porosity and the way the medium holds and releases water¹. The mechanism is capillarity: water is held in a pore by attraction to the particle surfaces, and the height of capillary rise is inversely proportional to the pore radius, so the narrower the pore, the stronger the suction needed to empty it¹. When suction is applied to a saturated medium, the largest pores empty first and progressively smaller ones follow, until at high suction only the very narrow pores still hold water¹.

A coarse medium therefore has large pores that drain under gravity alone: much air, little water, fast drying. A fine medium has small pores that gravity cannot empty: much water, little air. Mixing sizes makes a mix denser, not better, because small particles fill the spaces between large ones; components that differ widely in particle size give a mix with higher bulk density and lower total porosity, water-holding capacity and air-filled porosity than components of similar size¹.

Peat shows the effect in one table. Prasad and Maher sieved peat into size fractions: the fine 0 to 3 mm fraction had low air-filled porosity and high water-holding capacity, while the 6 to 12 mm fraction had an air-filled porosity well above the level recommended for pot plants⁴. In Scharpf’s mixtures of sieved peat fractions, reported in the same chapter, root growth of bean seedlings was affected at air-filled porosities below 12%⁴. Builder’s sand is the extreme: Bilderback and colleagues give it about 36% total porosity and 9% air space, so by their definition it holds 36 − 9 = 27% water after drainage (our subtraction), and fine mortar sand in a mix fills the pores between bark particles and cuts the air space⁷.

The water-retention curve

The water-retention curve connects everything above. Its vertical axis is volumetric water content, the fraction of the total volume that is water, written θ¹. Its horizontal axis is suction, the matric potential written as a positive tension, in centimetres of water head or in kilopascals; a column of water 10 cm high exerts approximately 1 kPa². The curve gives, at equilibrium, the water content at each suction¹.

To read it, start at zero suction on the left, where the medium is saturated, and move right. A soilless medium has a shape that field soils lack: the air-entry value, the suction at which air first replaces water, is practically zero, and the curve falls steeply until, at a suction of about 25 to 40 cm, the water content is almost constant¹. Media need that low air-entry value because containers of the usual 5 to 20 cm height must be aerated soon after irrigation, and anaerobic conditions for only a few days kill some roots¹.

The shape differs between media. Fitted drying curves in Raviv and Lieth give stone wool a saturated water content of 0.935 falling to a residual of 0.007 with a steep drop (van Genuchten exponent n = 3.725), and sphagnum peat a saturated content of 0.901 falling far more gradually (n = 1.390)¹. Stone wool has pores of nearly one size, so almost all of its water leaves over a narrow band of suction; peat has pores of many sizes and releases water a little at a time¹. Stone wool retains virtually no water above 5 kPa, so its water buffer is small and a steep gradient of water content forms from the top to the bottom of a slab after drainage³.

Easily available water and water-buffering capacity

De Boodt and Verdonck proposed in 1972 the definition soilless culture still uses⁶: easily available water is the difference between the water content at suctions of 1 and 5 kPa, and water-buffering capacity the difference between 5 and 10 kPa¹. At about 10 cm per kPa, those bands are 10 to 50 cm and 50 to 100 cm of head (our conversion from²). The first band is water a plant pulls out with little effort; the second is a reserve that slows the slide towards stress.

Lieth and Oki report that irrigation practices which avoid water stress in soilless production stay within 0 to 8 kPa, and that stone wool shows stress once 5 kPa is exceeded². Wallach cautions that fixed suction bands ignore how fast the medium can deliver water to the root, and that Naasz and colleagues found thresholds measured directly for peat and pine bark did not match the de Boodt and Verdonck values¹. Treat the bands as a yardstick for comparing media, not as a promise about the plant.

The perched water table: container height decides air

After irrigation and free drainage, the medium at the bottom of a container sits on free water at the drainage holes. There the matric potential is zero, and above it the potential becomes more negative with height¹. At equilibrium the suction at any point equals its height above the free water. Lieth and Oki use a sponge: stand a saturated sponge 10 cm tall on end and let it drain; when it stops, the gravitational potential at the top is 1 kPa, so the matric potential there is −1 kPa, holding the remaining water against that pull².

So the retention curve is also a picture of the container turned on its side: height above the base is suction, and the water content at each height is what the curve gives at that suction. The bottom centimetre is at near-zero suction and stays saturated whatever the medium. This is the perched water table, perched because capillarity holds it above the drainage holes.

A shallow container filled with a given medium is therefore wetter and more airless than a tall one, because all of it sits in the saturated foot of the curve; this is why Bilderback and colleagues say air space and container capacity depend on container height, and measure them in a cylinder of fixed height, 7.6 cm in their case⁷. In stone wool slabs the air volume is just 4% at 1 cm above the base, and the recommended slab height for an optimal water-to-air ratio is 7.5 to 10 cm³. Wallach’s rule is that the air-entry value of the medium should be lower than the container height¹. A saucer of standing water moves the free water, and with it the saturated layer, up into the pot (this article’s inference from the mechanism).

Water moves slowly once the medium dries

The retention curve says how much water is held; the hydraulic conductivity says how fast it moves. Soilless media conduct water fast when wet: saturated stone wool conducts 4.6 cm per minute³. But conductivity falls steeply with suction. From bottom to top of a 25 cm container at container capacity, Wallach reports a fall of about three orders of magnitude for peat and about four for tuff, and concludes that in soilless media generally the conductivity drops by several orders of magnitude over the narrow suction range that de Boodt and Verdonck called easily available water¹.

Two problems follow. Water can be “available” by the curve yet not reach the root fast enough, because water is fully available only while the medium replaces, at the rate of transpiration, what the root removes from its surroundings¹. And rewetting is uneven: the parts of the root zone with most roots dry first, their conductivity collapses, and water applied at the top or bottom cannot spread through them, so a plant can look water stressed while its container is heavy with water, and dry pockets lose roots and concentrate salts². Lieth and Oki’s remedy is to irrigate several times within a few hours to raise the conductivity through the whole root zone². Drip irrigation and substrate culture applies this to slabs.

Measuring the share

The laboratory method is the suction table: samples sit on a bed of fine sand or clay in a water-tight container whose overflow sets the suction; common points are saturation and 2.5, 10, 20, 50, 100 and 500 cm of head, each taking one to two days to reach equilibrium, and air content is porosity minus water content⁵. The North Carolina porometer uses a 7.6 cm cylinder and reports container capacity as the percentage of volume held at drainage and unavailable water as the volume held at 1500 kPa⁷.

A grower can get a usable approximation with a container and a measuring jug. Bilderback and colleagues describe it: cover the drain holes, measure the container volume, measure the water needed to saturate the medium, then uncover the holes and measure the volume that drains⁷. By their definitions, the drained volume over the container volume is the air space at that container height, the water that stayed is the container capacity, and their sum is the total porosity. The result is a “home remedy” figure, less precise than a laboratory core, but it tracks changes in your own medium over a crop⁷.

Worked example: the same medium in a 7.5 cm slab and a 20 cm pot

Everything here is computed by this article from published inputs. The inputs are the drying-curve parameters Wallach tabulates: for stone wool, saturated water content θs = 0.935, residual water content 0.007 (the table’s θf column, read here as θr), α = 0.083 and n = 3.725; for sphagnum peat, θs = 0.901, residual 0.0, α = 0.264 and n = 1.390¹. The table prints no unit for α; it is per cm, since the chapter’s tension head h is in cm (its Fig. 3.8) and α approximates the inverse of the air-entry suction (our reading of¹). The van Genuchten equation, as printed in the same chapter, is θ(h) = θr + (θs − θr) × [1 + (αh)ⁿ]^(−m), with h the suction head in cm and m = 1 − 1/n¹.

Step 1, water content at each height. At container capacity the suction head at height z above the base equals z in cm^1,2, so put h = z. For stone wool the equation gives θ = 0.935 at 1 cm, 0.910 at 5 cm, 0.834 at 7.5 cm, 0.697 at 10 cm, 0.398 at 15 cm and 0.217 at 20 cm (our computation from¹).

Step 2, the average over the container. Averaging the curve over the height in 0.01 cm steps, with air content taken as θs minus the mean water content, gives (our computation from¹):

The 7.5 cm stone wool slab lies almost entirely in the saturated foot of its curve, at 91% water and about 2% air, which is why a slab of that height is wet and why Papadopoulos and colleagues report only 4% air 1 cm above the base³. The 20 cm pot of the same material holds 65% water and 29% air, inside the Bilderback ranges of 45% to 65% container capacity and 10% to 30% air space⁷. Peat, with its gentler curve, already has 15% air in the shallow slab (our computation from¹).

Step 3, the de Boodt and Verdonck bands. With 1 kPa as 10 cm of head², easily available water is θ(10 cm) − θ(50 cm) and water-buffering capacity is θ(50 cm) − θ(100 cm)¹. For stone wool that is 0.697 − 0.026 = 0.67 and 0.026 − 0.010 = 0.02; for peat it is 0.578 − 0.327 = 0.25 and 0.327 − 0.251 = 0.08 (our computation from¹). Stone wool gives up two thirds of its volume easily and keeps almost nothing in reserve, matching the description of a small water buffer and virtually no water above 5 kPa³; peat gives up less easily but holds a reserve several times larger.

Two cautions. The parameters come from laboratory cores, and the same book says the upper layers of a drained slab are dry³, which is drier than the 0.83 the fitted curve gives at 7.5 cm (our comparison); and the curve ignores roots, uptake and the hysteresis of a slab wetted from a dripper. Use the numbers as the mechanism’s prediction, to be checked by weighing your own slab or pot.

Key numbers

Common mistakes

1. Choosing a medium by its porosity alone. Builder’s sand has 36% porosity but only 9% air after drainage, because its pores are too fine for gravity to empty⁷.
2. Expecting a shallow tray to drain like a tall pot. Suction equals height above the free water at the base, so a shallow container sits entirely in the saturated foot of the retention curve^2,7.
3. Adding fine material to hold more water. Fine particles fill the spaces between coarse ones, so the mix gains bulk density and loses air-filled porosity^1,4.
4. Letting a slab dry out and rewetting it once. Hydraulic conductivity falls by orders of magnitude as the medium dries, so one irrigation cannot spread through the dry pockets^1,2.

Summary

A growing medium is solid, water and air sharing one volume, described by total porosity, container capacity, air-filled porosity and bulk density. Particle size sets pore size: large pores drain under gravity and small pores do not, so coarse media are airy and dry fast, fine media are wet and airless, and mixing sizes packs the mix tighter. The water-retention curve plots water content against suction, and the bands of de Boodt and Verdonck read easily available water and the buffer from it. After drainage the suction at any height equals that height, so the base of every container is saturated and a shallow container is wetter than a tall one filled with the same medium. Hydraulic conductivity collapses as the medium dries, so a dry medium rewets unevenly. The worked example puts a stone wool slab 7.5 cm high at about 91% water and 2% air, and the same material in a 20 cm pot at 65% water and 29% air.

Next in the series: Growing media II: pH, CEC, buffering and microbes.

References

1. Wallach, R. (2008). Physical characteristics of soilless media. In: Raviv, M. and Lieth, J. H. (eds.), Soilless Culture: Theory and Practice. Elsevier, Amsterdam, pp. 41–116. https://doi.org/10.1016/B978-044452975-6.50005-8
2. Lieth, J. H. and Oki, L. R. (2008). Irrigation in soilless production. In: Raviv, M. and Lieth, J. H. (eds.), Soilless Culture: Theory and Practice. Elsevier, Amsterdam, pp. 117–156. https://doi.org/10.1016/B978-044452975-6.50006-X
3. 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. Elsevier, Amsterdam, pp. 505–543. https://doi.org/10.1016/B978-044452975-6.50014-9
4. 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. Elsevier, Amsterdam, pp. 459–504. https://doi.org/10.1016/B978-044452975-6.50013-7
5. Blok, C., de Kreij, C., Baas, R. and Wever, G. (2008). Analytical methods used in soilless cultivation. In: Raviv, M. and Lieth, J. H. (eds.), Soilless Culture: Theory and Practice. Elsevier, Amsterdam, pp. 245–289. https://doi.org/10.1016/B978-044452975-6.50009-5
6. De Boodt, M. and Verdonck, O. (1972). The physical properties of the substrates in horticulture. Acta Horticulturae, 26, 37–44. https://doi.org/10.17660/ActaHortic.1972.26.5
7. Bilderback, T. E., Warren, S. L., Owen, J. S. Jr. and Albano, J. P. (2005). Healthy substrates need physicals too! HortTechnology, 15(4), 747–751. https://doi.org/10.21273/HORTTECH.15.4.0747 (open PDF: https://www.ars.usda.gov/ARSUserFiles/11847/bilderback_etal_2005_Hort%20Tech%2015(4)%20pp%20747-751.pdf

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.