Roots and aeration
Roots breathe. A drowned plant dies of suffocation, and the water is only step one.
Roots do not photosynthesise. They are fed sugar from the leaves and they burn it with oxygen, day and night, exactly the way an animal does. Take the oxygen away and the first thing that stops is not growth. It is the pumps that pull nutrients and water into the plant, which is why a suffocating plant looks like a thirsty, hungry one.
One pair of physical constants explains the whole subject.
This is not a rule of thumb from a forum. It is two measured numbers, quoted in the root aeration literature: the diffusion coefficient of oxygen in air is 0.201 cm2 per second at 20 degrees, and in water it is 2.1 times ten to the minus five, a ratio of about nine thousand six hundred, which the paper states as at least ten thousand fold.1
Both bars are drawn to the same scale. The second one is a hairline because it has to be, and that is the entire content of this page.
The diffusion coefficient is only half of it. The other half is how much oxygen is there to move. Air is about 21 percent oxygen, which works out at roughly 280 mg per litre. Water in equilibrium with that same air holds about 9.1 mg per litre at 20 degrees, roughly thirty times less, and the same paper describes the liquid phase resistance as effectively a further thirtyfold greater for exactly that reason.2
Multiply the two penalties together and a water filled pore is on the order of three hundred thousand times worse at delivering oxygen than an air filled pore of the same size. Not slightly worse. An air filled pore is a road and a water filled pore is a wall.
Everything the hobby knows about pots can now be derived instead of memorised. A root sits among pores, and every pore is full of either air or water. Air filled pores connect it to the atmosphere at the surface of the pot; water filled pores do not. Watering fills them all, and the plant is fine, because a root survives brief hypoxia. What matters is how fast the large pores empty again. So: drainage holes matter because they let large pores empty. Compaction kills because it destroys large pores first and large pores are the only ones that ever hold air. A saucer left full is dangerous because it refills from below. Bark, pumice and expanded clay work because their pores are large, rigid and drain by gravity. A pot far too large for its plant is a real risk, because a big volume of wet medium with too few roots in it stays wet for days.
The soil physics literature has a long standing working threshold that root function declines sharply below roughly 10 percent air-filled porosity, and container mixes are targeted higher than field soil precisely because a container has a perched water table and nothing deep to drain into. Treat it as a soft threshold. It moves with temperature, with root density and with species, and the very precise figures that circulate for an unspecified plant in an unspecified pot are more precise than anything anybody has measured.
How a plant actually drowns, in eight steps.
Water is not toxic to roots. Roots grow in water for years, which is what hydroponics is. A reservoir with an air stone in it is not a drowned plant; a pot of saturated peat is. Here is the difference, in order.
The pores fill and stay full
In a well drained mix in a pot with a hole, the large pores empty within minutes to an hour and air rushes back in. In a compacted, fine, water retentive mix, or a pot standing in a full saucer, they do not.
The dissolved oxygen runs out fast
There is very little of it, about 9 mg in a litre, and the root and every aerobic microbe around it are consuming it. Because of the diffusion penalty, replacement cannot arrive from the surface fast enough. Warm makes it worse, because respiration roughly doubles for every ten degrees while the supply does not. Warm and wet is far more dangerous than cool and wet.
The energy budget collapses
With no oxygen the mitochondrial electron transport chain stops and the cell falls back on fermentation. Aerobic respiration yields 36 to 38 ATP per glucose. Fermentation yields 2 to 3.3 The root loses something like nine tenths of its income overnight, and it triages: growth and active transport are discretionary, so they go first.
The cell chemistry goes wrong
Pyruvate goes to lactate, the cytosol acidifies, and the acidity switches the pathway over to acetaldehyde and ethanol. Cytoplasmic acidosis, not alcohol, is the documented determinant of flooding intolerance.4
The root stops drinking
This is the step nobody explains and it resolves the most confusing symptom in the hobby. Water enters a root largely through aquaporins, protein channels that are switched on and off by phosphorylation, which costs ATP, and whose function is sensitive to internal pH. Under hypoxia both go wrong at once, and the reviewed conclusion is direct: acidosis and ATP depletion impair aquaporin phosphorylation and water transport through those channels is inhibited.5 A waterlogged root cannot drink while sitting in water. Root hydraulic conductivity falls, the leaves keep transpiring, and the plant wilts.
Cells die, and then letting it dry hurts too
Cortical cells starved of energy die first. When air finally returns to a tissue that has been without it, there is a burst of reactive oxygen species and a spike in acetaldehyde. Post-anoxic injury is why a plant sometimes looks worse two days after you finally let it dry out.
The root advertises itself to the pathogen
In a commercial system where every step could be instrumented, the cascade ran root zone hypoxia, ATP depletion, aquaporin closure and reduced hydraulic conductance, membrane lipid peroxidation, then a shift in root exudates towards glutamic and aspartic acid that attracts Pythium zoospores by chemotaxis, then encystment on the compromised root surface.6 The rot is not bad luck. It is the predictable consequence of the oxygen state.
Root rot
Pythium and Phytophthora are not fungi. They are oomycetes, water moulds in a different kingdom, which is why fungicides aimed at true fungi often do nothing to them. Their infective stage is a swimming cell with a flagellum, and it needs a continuous film of free water to travel through the pores to a root. A pot that drains does not provide one. Container air-filled porosity is directly related to the development of Phytophthora root rot, and that relationship has been measured in container media rather than asserted.78
Water is step one of eight. Calling it death by water is like calling a house fire death by matches.
A correction we owe you
The popular version of this story is that the roots are poisoned by the alcohol they make. The primary literature does not support ethanol as the killer. Ethanol diffuses rapidly out of the cells, which makes it more of a carbon loss than a toxin, and tolerance across species does not track ethanol sensitivity in the way that story requires.9 The better supported culprits are cytoplasmic acidosis, lactate accumulation, the energy shortfall itself, and acetaldehyde with reactive oxygen species when air comes back. Say the root suffocates and its chemistry goes wrong. Do not say the root gets drunk.
Why the symptoms look exactly like thirst.
| What you see | If it is underwatered | If it is suffocating |
|---|---|---|
| Wilting | No water available to take up | The root cannot take water up. The channels are shut and the roots are dying |
| Yellowing lower leaves | Nutrients pulled out of old tissue under stress | Active uptake switched off. Nitrogen is mobile, so it is withdrawn from old leaves |
| Crisp brown tips and edges | Water deficit at the leaf margin | The same water deficit at the same place, for the opposite reason |
| No new growth | Energy and water limited | Energy limited |
They match because the proximate state is the same: the shoot is not getting water or nutrients. In one case the pipe is empty. In the other the pump is broken. Then the loop closes, because the owner sees a wilting plant and reaches for the watering can.
The instruction is not to water less. It is this. Before you water a wilting plant, put a finger in the medium. If it is wet and the plant is wilting, water is not the problem and more of it will finish the job. Then check three more things, in order of how much they tell you: smell the medium, because anaerobic media smell sour or swampy and that smell is a direct readout of oxygen status; look at a root, because a rotted one is brown and soft and its outer sleeve slides off the core; and watch the recovery, because a genuinely thirsty plant perks up within hours and a suffocating one does not.
Some plants can sit in water. Most cannot, and you cannot tell by looking.
If oxygen cannot reach a root through the medium, there is one other route: down through the plant. Aerenchyma is a set of gas filled channels running lengthwise through the root, and inside them oxygen is diffusing through air rather than water, which is worth the four orders of magnitude above. Some species build the channels by killing cortical cells deliberately, others by separating them during development without dying.
| Root porosity, gas as a share of root volume | In aerated conditions | Under oxygen deficiency |
|---|---|---|
| Upland, dryland grasses | 1 to 8 % | 3 to 27 % |
| Wetland grasses | 8 to 45 % | 15 to 55 % |
Adventitious roots, measured in grasses and crops.10 A wetland root can be more than half air by volume. A dryland root tops out around a quarter even when it responds, and many species barely respond at all. Getting the oxygen down the pipe is only half the problem: wetland species also seal the sides of the mature root with suberin and lignin so the oxygen is not lost into the mud before it reaches the tip.11 None of it is free, either. The tissue converted to air was doing nutrient uptake.
That table stops where the measurements stop. Nobody has published root porosity values for any common houseplant. Not Monstera, not Epipremnum, not Sansevieria, not Hoya. Any source that gives you a figure for one is making it up. What can be said is ecological rather than anatomical: a large share of the plants that hobbyists find tolerant of permanently wet roots come from families and habitats where flooding is normal, and many Araceae are riparian, swamp margin or hemiepiphytic plants of wet tropical forest. That is a correlation with a plausible mechanism, not a measurement. So the defensible rule is empirical. Tolerance of saturation is species specific and you cannot infer it from a principle. Your pothos thriving in a jar of water tells you nothing about your Sansevieria.
Half the root system is invisible, and repotting destroys it.
Root hairs are single epidermal cells drawn out into long tubes, and they are where most uptake happens, along with the young tissue just behind the tip. They can account for up to about half the total surface area of a root system, and the classic measurement on a single rye plant put the extra area they contributed at roughly 400 square metres.12
They are also fragile and short lived, destroyed by drying and by mechanical disturbance. That is the physical basis of transplant shock: bare root a plant, shake off its medium, or let the root ball dry during a repot, and you have removed most of its absorbing surface without breaking anything you can see. Disturb the root ball as little as the job allows and keep it moist and shaded for the minutes it is out of the pot.
Root hair development also depends on the medium, which is the real thing behind the hobby's water roots and soil roots distinction. Root anatomy responding to the root zone environment is well established. The stronger claim, that water roots cannot function in soil, goes further than anything measured on a houseplant.
And the sachet of mycorrhizae
Most commercial media start with no mycorrhizal fungi at all. Peat and bark mixes are processed and often heat treated; expanded clay, pumice and rockwool are fired or inert and sterile by construction. So an average houseplant probably has no fungal partner and is doing fine. Phosphorus fertilisation suppresses the symbiosis anyway, because when phosphorus is plentiful the plant stops paying for it, and standard feeds supply plenty. A controlled trial of a commercial inoculum across wheat cultivars increased root colonisation and did not deliver the nutritional benefit claimed.13 Getting the fungus in is not the same as getting a benefit out. Mycorrhizae are real, important and fascinating. Buying a sachet for a well fed plant in an inert medium is not supported.
Feeding in an inert medium is a different discipline, and the reason is one property.
Most nutrients arrive as positively charged ions. Clay minerals and organic matter carry negative charges on their surfaces, so those ions stick lightly and can be swapped off later. Cation exchange capacity is simply a count of those parking spaces. A high count makes a medium forgiving in both directions, because it soaks up an overdose and releases a reserve when you forget. A count near zero means the plant is eating whatever is in the solution right now.
That is the actual reason growers in expanded clay feed a dilute solution constantly instead of a monthly dose. It is not a ritual. Two things it is not, because both confusions are common: exchange capacity is not the same as nutrient content, since a material can have plenty of parking and no cars; and it does not hold negatively charged ions at all, so nitrate leaches freely from every medium, which is why nitrogen is the one that runs out first.
| Material | Exchange capacity, cmol per kg | Bulk density kg/L | Per litre of medium, meq/L |
|---|---|---|---|
| Sphagnum peat | 109 | 0.091 | about 99 |
| Coco coir | 59.8 | 0.073 | about 44 |
| Wood fibre | 3.4 | 0.037 | about 1.3 |
| Sandy soil | 3 to 5 | about 1.55 | about 62 |
| Loam | 10 to 15 | about 1.3 | about 163 |
| Clay loam | 20 to 50 | about 1.25 | about 440 |
| Perlite, pumice, expanded clay, rockwool | treat as zero | varies | zero |
Measured exchange capacities and bulk densities for coir, peat and wood fibre, by ammonium acetate at pH 7.0.14 Soil values by texture from an extension fact sheet.15 Perlite is described in the substrate literature as providing no mineral nutrients and no buffering capacity.16 The conversion to a per litre column is our own arithmetic, because exchange capacity is reported per kilogram and a pot is measured in litres.17 It matters: per litre, peat is not far off a loam and coir is comfortably better than a sand. The common line that coir has no exchange capacity is wrong. And for expanded clay and pumice specifically, there is no published measurement with its method stated. Numbers circulate between retailers. The qualitative claim, negligible and treat as zero, is safe. The specific numbers are not.
So the useful grouping is not soil against soilless. It is three tiers. Buffered media, meaning mineral soil, peat, good coir and compost, where occasional feeding works. Weakly buffered media, meaning bark, wood fibre and heavily perlite cut mixes, where the reserve is small. And inert media, meaning expanded clay, pumice, perlite and rockwool, where the solution is the entire diet and it has to be complete every time.
Complete is the operative word. In soil, a plant fed an ordinary bottle of NPK is not living on NPK. It is living on that plus calcium and magnesium from weathering minerals and exchange sites, sulphur from organic matter, micronutrients from clays, and whatever the tap water brings. In an inert medium, everything comes out of the bottle. Feed NPK only and the plant runs out of calcium, magnesium, sulphur and iron, in roughly that order of visibility, and it shows in the new growth while the old leaves look perfect.18
Calcium is the first to bite because it is immobile. It travels in the transpiration stream and is essentially never recovered from old tissue, so a shortage always appears in the newest leaves and growing points. That also means high humidity can make a calcium shortage worse even when supply is adequate, because it slows the stream that carries it.
Coir is not a drop in replacement for peat, and the reason is calcium.
Coconut husk is naturally rich in potassium, and much coir is retted in brackish water and stockpiled on the coast, so its exchange sites arrive already occupied by potassium and sodium. Measured against peat by the same laboratory and the same method:
| Per kg of dry matter | Coir | Peat | Wood fibre |
|---|---|---|---|
| Exchange capacity, cmol per kg | 59.8 | 109.0 | 3.4 |
| Potassium, g | 1.514 | 0.749 | 0.33 |
| Sodium, g | 0.81 | 0.69 | 0.041 |
| Calcium, g | 5.67 | 15.67 | under 3.0 |
| Magnesium, g | 0.621 | 0.996 | under 0.6 |
This coir carried roughly twice the potassium of the peat and barely a third of the calcium.
Now the mechanism. Ion exchange is not first come first served. Higher charge ions bind more strongly, so calcium and magnesium out-compete potassium and sodium for the same sites.19 Pour a well made nutrient solution onto raw coir and the calcium and magnesium in it are pulled onto the exchange sites, knocking potassium and sodium off into the solution. You fed a balanced feed. The plant received an unbalanced one. The medium did it silently and will keep doing it until every site is saturated.
This is where washing and buffering get confused, and the distinction is the single most useful thing in the whole coir topic. Washing flushes soluble salts out of the water in the pores. It lowers the reading on an EC meter and removes free sodium and chloride, and it does nothing to what is bound to the exchange sites. Buffering soaks the material in a strong calcium solution so that calcium and magnesium take those sites over, then rinses the displaced potassium and sodium away. Washed, triple washed and low EC on a bag are not claims about buffering. If a supplier will not say buffered or pre-charged, assume it is not.
The trap is that the plant looks fine right up until it does not. Potassium excess produces no dramatic symptom, nitrogen and phosphorus are present so the plant is green, and calcium is immobile so the mature foliage you look at stays perfect. The damage appears only in new tissue, by which time the medium has been stripping calcium for weeks. Looking fine is a lagging indicator.
That coir is high in potassium and sodium and low in calcium is measured, and so is the enormous variation between sources and batches.20 That root medium materially changes root zone pH and calcium and magnesium management in containers is measured too.21 The step by step exchange story above is textbook ion chemistry plus a practice that is universal in commercial horticulture, but no published trial isolates unbuffered coir causing calcium deficiency as its own result. It is a mechanism, and it is written as one.
The pH chart everyone reproduces was drawn in 1947, for a different material.
pH does not change how much of a nutrient is in the pot. It changes what chemical form it is in, and only some forms cross a root membrane. Iron is the extreme case: at high pH it precipitates as ferric hydroxides, which are effectively rock. The iron is still there and the plant cannot have it.
The rainbow chart with the bands that swell and pinch descends from a paper published in 1947, and it was drawn for mineral agricultural soil.22 A recent study measured macronutrient solubility against pH in four peat based substrates and set out to build a more sophisticated representation than the charts in current use, concluding that real behaviour is more complex than those charts allow: potassium availability depended on the substrate and its exchange capacity, phosphorus fell at high pH with insoluble calcium phosphates forming, and which liming material was used changed the outcome.23 The deepest limitation is that it is a solubility chart being read as an uptake chart, and tissue concentrations do not always follow solubility.
The working range is also not the one most people quote. For peat and coir based soilless substrates the optimum for nutrient availability across most species is described as roughly pH 5.5 to 6.024, against about 6.0 to 7.0 for mineral garden soil. Run a peat or coir mix at 6.5 and iron availability falls away for iron-inefficient species long before it would in a field. Reproduce the chart if it helps someone picture the idea, label it as a mineral soil diagram from 1947, and never make a specific recommendation for a soilless substrate from it.
What actually moves pH in a pot, in order: the alkalinity of your irrigation water, which is not the same thing as its pH reading and is usually the biggest driver; the form of nitrogen you feed, since taking up ammonium pushes a proton out and acidifies the root zone while taking up nitrate does the reverse; limestone still reacting in a fresh bag of peat mix; the plant itself; and the breakdown of the medium. In an inert medium all of it happens faster, because there is nothing there to slow it down. That is the trade nobody mentions when selling expanded clay as low maintenance. You gain total control of the solution and you lose the medium's forgiveness.
Salt has to have a way out. Most reservoir pots do not give it one.
Every watering adds salts. The plant takes up water and ions at different rates and is particularly reluctant to take up sodium and chloride, and evaporation removes pure water and leaves every dissolved solid behind. Add salts, remove water, repeat, and the concentration climbs. It is the same process that salinises irrigated farmland, running in a pot on a windowsill.
What high salt does is worth stating precisely, because it is the third mechanism on this page by which a plant standing in moisture behaves as though it is in drought. A concentrated solution outside the root flattens the gradient the root depends on, and past a point water moves the wrong way. The growing tips burn first, before anything is visible above ground. And the classic crispy brown leaf tip is salt arriving in the transpiration stream and being deposited where the water evaporates and the salt cannot go any further, which is exactly why the damage is at the tip and the margin rather than the middle of the leaf. That symptom is almost universally blamed on dry air. Dry air is at most an accelerant, because it raises the delivery rate.
Where it concentrates depends entirely on where water leaves without taking salt with it. A top watered pot with free drainage exports some and crusts the rest at the surface. A bottom watered pot has no export path at all, and the salt accumulates towards the top of the root ball, where a lot of the fine roots are. A self watering or wicking pot has the same problem permanently and by design, and this is the main long term failure mode of the whole category. In a reservoir system, topping up is not maintaining. The reservoir has to be emptied and replaced, not refilled.
Flushing means running a large volume of low salt water through the medium and letting it drain away. It only works if water leaves the pot; pouring water into a pot with no hole moves salt around and adds more. Volume matters more than speed, because a fast pour channels down the side of a shrunken root ball. And a little every time beats a lot occasionally, which is the commercial leaching fraction idea in domestic form. As for the advice to flush monthly, the interval is invented: the rate depends on your water, your feed, your watering volume, the fraction that drains and the size of the plant, and those vary by more than an order of magnitude between two readers. Commercial soilless production commonly targets an electrical conductivity of 1.5 to 3.5 dS per metre25, but that is for crops being pushed for yield, and there is nothing equivalent for ornamental foliage houseplants. Use a trigger rather than a calendar: a crust on the surface or the rim, tips browning without an obvious cause, or months of bottom watering with nothing ever draining out.
What this requires of a pot
Three requirements come out of this page rather than out of taste, and we are recording them as requirements. Drainage geometry is not judged on whether a pot drains but on how fast the large pores refill with air, which makes the height of the saturated layer the controlling variable. Any closed, wicking or reservoir geometry needs a designed answer to the question of how the owner gets the salt out: a drain plug, a removable reservoir, an overflow, or an explicit decision that the vessel is a cover pot and the plant lives in a liner inside it. And a porous wall is an evaporating surface, which makes it a salt depositing surface too, which is what the white crust on the rim of a terracotta pot is.
Sources.
- Jimenez, J. de la C., Armstrong, W., Colmer, T.D. and Pedersen, O. (2024). Overcoming constraints to measuring O2 diffusivity and consumption of intact roots. Plant Physiology 195(1). The diffusion coefficients in air and water, the statement that liquid phase coefficients are at least ten thousand fold smaller, the further thirtyfold penalty from low solubility, and the higher ATP requirement at the ion entry points of the root. DOI 10.1093/plphys/kiae046 open access
- Our own arithmetic, and a working threshold. Not a published result. The 280 mg per litre of oxygen in air comes from 21 percent by volume through the ideal gas law, and the 9.1 mg per litre in water from standard dissolved oxygen saturation tables at 20 degrees; the ratio of about thirty independently reproduces the thirtyfold figure stated in source 1. The combined penalty of roughly three hundred thousand is the product of that ratio and the diffusion ratio, and is ours. The temperature effect on respiration is the standard doubling per ten degrees. The 10 percent air-filled porosity threshold is a long standing working figure in the soil physics literature rather than a measurement of any particular plant, and the higher container ranges are in the substrate page's sources 3 and 4. calculated, repeatable, unverified by a third party
- Bashir, Rehman and colleagues (2022). Try or die: dynamics of plant respiration and how to survive low oxygen conditions. Plants 11(2):205. The ATP yields, 36 to 38 aerobic against 2 to 3 fermentative. DOI 10.3390/plants11020205 open access
- Roberts, J.K.M., Callis, J., Jardetzky, O., Walbot, V. and Freeling, M. (1984). Cytoplasmic acidosis as a determinant of flooding intolerance in plants. Proceedings of the National Academy of Sciences 81(19):6029 to 6033. The pH-stat sequence and the finding that acidosis, not ethanol, determines intolerance. DOI 10.1073/pnas.81.19.6029
- Toro, G. and colleagues (2020). Keep calm and survive: adaptation strategies to energy crisis in fruit trees under root hypoxia. Plants 9(9):1108. The aquaporin, ATP and acidosis mechanism behind the loss of root hydraulic conductivity, the post-anoxic burst of reactive oxygen species and acetaldehyde, and the point that ethanol diffuses out of the cells. DOI 10.3390/plants9091108 open access
- Scott, S., Villouta, C. and colleagues (2026). Dissolved oxygen limitation and Pythium root rot in strawberry nutrient film systems. Frontiers in Plant Science. The instrumented cascade from hypoxia to aquaporin closure to a shift in root exudates that attracts zoospores by chemotaxis, the 5 and 4 mg per litre working figures, and the explicit statement that species specific thresholds are unestablished. DOI 10.3389/fpls.2026.1829367 open access
- MacDonald, J.D. and colleagues (1986). Influence of air-filled porosity of container media on Phytophthora root rot of toyon. HortScience 21(4):1010. DOI 10.21273/HORTSCI.21.4.1010
- Relationship of matric water potential and air-filled porosity of container media to development of Phytophthora root rot (1991). Phytopathology 81:936. DOI 10.1094/Phyto-81-936
- Drew, M.C. (1997). Oxygen deficiency and root metabolism: injury and acclimation under hypoxia and anoxia. Annual Review of Plant Physiology and Plant Molecular Biology 48:223 to 250. The classic review behind the whole injury account. DOI 10.1146/annurev.arplant.48.1.223
- Yamauchi, T. and Nakazono, M. (2021). Climate-smart crops: key root anatomical traits that confer flooding tolerance. Breeding Science 71(1), summarising Colmer 2003 and Herzog and colleagues 2016. The root porosity table for upland and wetland grasses. DOI 10.1270/jsbbs.20119 open access
- Ejiri, M., Fukao, T., Miyashita, T. and Shiono, K. (2021). A barrier to radial oxygen loss helps the root system cope with waterlogging-induced hypoxia. Breeding Science 71:40 to 50. The suberin and lignin barrier that stops oxygen leaking out of the mature root. DOI 10.1270/jsbbs.20110
- Root hairs: an underexplored target for sustainable cereal crop production (2024). Journal of Experimental Botany. Root hairs as up to about half of total root surface area, and the Dittmer 1937 measurement of roughly 400 square metres of extra surface on a single rye plant. DOI 10.1093/jxb/erae275 open access
- A commercial arbuscular mycorrhizal inoculum increases root colonization across wheat cultivars but does not increase assimilation of mineral nitrogen from crop residue (2021). Plants, People, Planet. Full text open access
- Reuse of coir, peat, and wood fiber in strawberry production (2023). Frontiers in Plant Science, table 3. Measured exchange capacity by ammonium acetate at pH 7.0, bulk densities, and the potassium, sodium, calcium and magnesium contents of virgin coir, peat and wood fibre. DOI 10.3389/fpls.2023.1307240 open access
- Ohio State University Extension, fact sheet ANR-81 (2019). Calculating cation exchange capacity, base saturation, and calcium saturation. The exchange capacity of soils by texture. ohioline.osu.edu/factsheet/anr-81
- Potential of pine bark to replace perlite in coir-based substrates (2025). Plants 14(16):2577. The statement that perlite provides no mineral nutrients or buffering capacity. DOI 10.3390/plants14162577 open access
- Our own arithmetic and our own reasoning. Not a published result. The per litre exchange capacity column is the per kilogram figure multiplied by the bulk density from source 14, with conventional typical bulk densities for the mineral soils, which should be treated as approximate. The account of where salt concentrates in a bottom watered or reservoir pot, and the conclusion that a reservoir must be emptied rather than topped up, is deduction from the mass balance rather than a measured study of such pots. calculated, repeatable, unverified by a third party
- Arnon, D.I. and Stout, P.R. (1939), The essentiality of certain elements in minute quantity for plants with special reference to copper, Plant Physiology 14(2):371 to 375; and Brown, P.H., Welch, R.M. and Cary, E.E. (1987), Nickel: a micronutrient essential for higher plants, Plant Physiology 85(3):801 to 803. The criteria for calling an element essential, and the last one added to the list.
- Wada, N. and Weerasooriya, S.V.R. (1990). Sodium-calcium, calcium-potassium, and potassium-sodium exchange equilibria on a montmorillonitic soil. Soil Science and Plant Nutrition 36(3). The quantification of exchange selectivity behind the divalent over monovalent rule. DOI 10.1080/00380768.1990.10416913
- Evans, M.R., Konduru, S. and Stamps, R.H. (1996). Source variation in physical and chemical properties of coconut coir dust. HortScience 31(6):965. The batch to batch and source to source variation that makes coir not one material. DOI 10.21273/HORTSCI.31.6.965
- Argo, W.R. and Biernbaum, J.A. (1997). The effect of root media on root-zone pH, calcium, and magnesium management in containers with impatiens. Journal of the American Society for Horticultural Science 122(2):275 to 284. DOI 10.21273/JASHS.122.2.275
- Truog, E. (1947). Soil reaction influence on availability of plant nutrients. Soil Science Society of America Journal 11(C):305. The origin of the chart. DOI 10.2136/sssaj1947.036159950011000c0057x
- Huang, J., Fisher, P.R., Argo, W.R., Jeong, K.Y. and Altland, J. (2025). Macronutrient solubility in response to the pH of soilless container substrates. Journal of Soil Science and Plant Nutrition 25:3224 to 3240. DOI 10.1007/s42729-025-02331-0 the publisher's full text was not retrievable during this research. The findings quoted here come from the paper's own published abstract, and any specific figure should be checked against the full text before it is used
- An overview of soil and soilless cultivation techniques (2022). Plants 11(9):1153. The statement that for most species the root environment pH for optimal nutrient availability is between 5.5 and 6.0. DOI 10.3390/plants11091153 open access
- Kenaf core as an alternative soilless growing medium: a review (2026). Plants 15(4):666. The exchange capacity ranges for peat and coir, the 1.5 to 3.5 dS per metre electrical conductivity target for commercial soilless production, and the sodium, chloride and potassium problem in coastal coir. DOI 10.3390/plants15040666 open access
Where these numbers come from. Almost everything quantitative here was measured on crops — grasses, maize, rice, strawberry, wheat and impatiens — and the physics carries straight over, because diffusion coefficients do not care what species they are inside. Each crop figure is labelled where it appears, so you can see which plant it was taken from and read the threshold with that in mind.