Systems · Intermediate · 3 min read

Let soil dry, hammer coco: why watering depends on the medium

Roots need oxygen as much as water, and oxygen moves about 10,000 times more slowly through water than through air. In soil, which holds water in fine pores, letting the pot properly dry at times pulls air back into the root zone; the founder lets soil pots dry right out between waterings. Coco holds plenty of air even when wet, so the house approach is the opposite: hammer it with small, frequent fertigations that keep the root zone moist and the salt level steady. Different media, different physics.

Supported

Draft: awaiting expert review Updated 2 November 2026

VegFlowerSoilLiving soilCocoRockwoolDrip

Short answer, in plain words. Roots need two things from the pot: water and air. In soil, water fills the same tiny spaces air needs, so the founder lets soil pots properly dry out at times to let the roots breathe. Coco holds lots of air even when it's wet, so the house approach flips: hammer it with small, frequent feeds that keep it moist and the salt level steady (founder's field experience) ^[6]. Same plant, different physics.

Why do roots need air?

Roots respire, and respiration needs oxygen. Oxygen diffuses roughly 10,000 times more slowly through water than through air, so a root zone whose pores are full of water runs short of oxygen fast; low oxygen injures roots and invites root disease ^[1]. Established.

That's the whole argument: the pores that hold water are the pores that would hold air. How a medium splits its pore space between water and air decides how you should water it ^[2].

Why let soil dry?

Soil and peat-based mixes have many fine pores that hold water tightly and drain slowly. Kept constantly wet, they stay low on air. Supported.

A dry-back lets roots and evaporation pull water out of the pores, and air is drawn back in behind it ^[2].
House position: let soil pots properly dry out at times, then water thoroughly (founder's field experience) ^[6]. Judge by pot weight, not the calendar (see how often to water).
Bonus: a drier surface between waterings discourages fungus gnats, and less time waterlogged means less root-rot risk.

Why hammer coco?

Coco coir holds water and a lot of air at the same time, thanks to its coarse, fibrous structure ^[3]. Roots breathe even when it's wet. Supported.

The bigger risk in coco is the salt level. Water potential, how hard the root has to pull to get water, gets harder for two reasons as a substrate dries ^[4]:

Matric: less water left, held more tightly by the substrate.
Osmotic: the same salts in less water means a higher EC.

In coco both rise together during a dry-back. High-frequency fertigation, several small irrigations a day with a complete solution and some runoff, keeps water easy to take up and the root-zone EC steady ^[2]. House position: hammer coco (founder's field experience) ^[6]. For shot sizes, timing and dry-back targets, see crop steering.

Side by sideSoil vs coco: how to water each

Soil and peat mixes

Fine pores, water blocks air

+Let pots properly dry at times
+Water thoroughly when clearly light
+Fewer gnats, less root rot

−Kept constantly wet: low oxygen

Coco coir

Holds air even when wet

+Hammer it: small, frequent fertigations
+Steady root-zone EC
+Water always easy to take up

−Let it dry: salts concentrate, roots struggle

What about drying down on purpose?

A mild, controlled drought late in flower increased inflorescence dry weight and cannabinoid content in a cannabis trial ^[5], and it's house-endorsed with the caveat that every cultivar responds differently (founder's field experience) ^[6]. That's a deliberate short stress at the end, planned and measured, not a reason to under-water the grow. In coco it's done with planned dry-backs (see crop steering).

Wet, then breatheSoil watering, the house rhythm

Water thoroughlyUntil a little runs out
Pot gets lighterRoots and air pull water out
Air moves back in
Clearly light? Water again

Source: ^[1]^[2]; house position ^[6]

Common mistakes

Watering soil on a schedule so it never dries.
Letting coco dry out like soil, then feeding a big dose into a salty, dry root zone.
Small sips in soil that never wet the whole pot.
Forgetting runoff in coco, so salts build up.
Confusing a planned late-flower dry-back with chronic under-watering.

Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.Decay of roots by pathogens such as Pythium, Fusarium, Rhizoctonia or Phytophthora, usually triggered by low oxygen, heat and waterlogging around the roots.Peat is partially decomposed sphagnum moss from bogs, the base of most potting mixes; it holds water and nutrients well but is acidic and needs lime.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Dryback is the drop in a substrate's water content between irrigations, usually expressed as a percentage of its content at full saturation.Small dark flies of the genus Bradysia (family Sciaridae) whose larvae live in wet growing media, feeding on fungi, organic matter and young roots.Overwatering is watering so often that the medium stays saturated, starving roots of oxygen, which causes drooping, slow growth and root disease.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.High-frequency fertigation feeds small volumes of nutrient solution many times a day, keeping the root zone consistently moist and fed so small pots can produce large plants.Fertigation is delivering fertiliser dissolved in irrigation water, so plants are fed every time they are watered rather than through solid amendments.The weight of cannabis after drying (and usually trimming), the basis growers use for yield; it is typically about 20-25% of trimmed wet weight.A cannabinoid is any compound that binds the body's cannabinoid receptors or shares the typical cannabis terpenophenolic structure; the plant-made ones are phytocannabinoids such as THCA and CBDA.Underwatering is letting the medium dry too far between waterings, so the plant loses turgor and wilts, with limp, papery leaves and crisping edges if repeated.

Questions this article answers

Should I let my soil dry out between waterings?

Yes, at times. GenetixBay's founder is big on letting soil pots properly dry out, because soil holds water in fine pores and oxygen moves about 10,000 times more slowly through water than through air. As the pot dries, air is pulled back into the root zone. Water by weight: when the pot is clearly light, water thoroughly, then let it dry again.

How often should I water coco?

Often. The house approach is to hammer coco with high-frequency fertigation: several small irrigations a day with a complete nutrient solution and some runoff. Coco keeps plenty of air even when wet, so roots breathe, and frequent feeding keeps the root-zone water and salt level steady. Letting coco dry out concentrates salts and makes water harder for roots to pull in.

Why do roots need to dry out?

Roots don't need to dry; they need oxygen. In soil, the pores that hold water are the same pores that would hold air, and oxygen moves about 10,000 times more slowly through water than air. A permanently wet soil starves roots of oxygen, which invites root rot and fungus gnats. A dry-back lets air back in. In coco, air stays available even when wet.

What is water potential in the root zone?

Water potential is how hard the root has to pull to get water. Two things make it harder: a drier medium, which holds its remaining water more tightly, and a higher salt concentration, which pulls water the other way. As coco dries between feeds, both rise at once. Small, frequent fertigations keep water easy to take up.

Does drought stress increase cannabis potency?

Mild, controlled drought late in flower can. In one trial, controlled drought stress before harvest increased inflorescence dry weight and cannabinoid content. GenetixBay endorses a mild late dry-back with the caveat that every cultivar responds differently. That's a deliberate, short stress at the end, not a reason to under-water the whole grow.

Sources

Drew MC. 1997. Oxygen deficiency and root metabolism: injury and acclimation under hypoxia and anoxia. Annual Review of Plant Physiology and Plant Molecular Biology 48:223-250. doi.org/10.1146/annurev.arplant.48.1.223
Raviv M, Lieth JH, Bar-Tal A (eds). 2019. Soilless Culture: Theory and Practice, 2nd ed. (physical properties of substrates, water availability, irrigation). Elsevier. doi.org/10.1016/C2015-0-01807-5
Abad M, Noguera P, Puchades R, Maquieira A, Noguera V. 2002. Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants. Bioresource Technology 82(3):241-245. doi.org/10.1016/S0960-8524(01)00189-4
Taiz L, Zeiger E, Møller IM, Murphy A. 2015. Plant Physiology and Development, 6th ed. (water potential and water uptake). Sinauer Associates..
Caplan D, Dixon M, Zheng Y. 2019. Increasing inflorescence dry weight and cannabinoid content in medical cannabis using controlled drought stress. HortScience 54(5):964-969. doi.org/10.21273/HORTSCI13510-18
GenetixBay founder. Field experience (first-hand account; GenetixBay house position)..

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Why do roots need air?

Why let soil dry?

Why hammer coco?

Soil and peat mixes

Coco coir

What about drying down on purpose?

Common mistakes

Questions this article answers

Sources

Related

Watering: how much, how often, by medium and pot size

Drain-to-waste drip and crop steering (generative vs vegetative dryback)

Growing in coco coir

Growing in soil (potting mix with bottled feed)

Root rot and Pythium in cannabis

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