Watering by Growing Medium: Soil, Coco, Semi-Hydro

How often to water is the wrong question until you know what you are watering into. The same plant, the same weather and the same water need completely different schedules in field soil, in coconut coir and in a passive container system — not because one is harder than the other, but because each holds a different amount of water and air, and releases it at a different rate.

This page explains those three situations separately, how to tell overwatering from underwatering in each, and how much drainage you should be seeing. It assumes no instruments beyond your hands and, later on, a basic meter.

The one idea behind all three

Roots need water and air at the same time. A growing medium is a sponge full of pores: large pores drain and fill with air, small pores hold water against gravity. When you water, you fill everything; as water drains and the plant drinks, air returns.

Prolonged saturation is damaging because the air never returns. This is worth stating plainly because it explains why the commonest mistake among new growers is not underwatering — it is watering too often, which is a different mistake from watering too much at once.

A medium with a lot of large pores drains fast, holds little, and must be watered often in small amounts. A medium with many small pores holds a lot, drains slowly, and must be watered heavily but rarely. Everything below follows from that.

Soil — water deeply, water less often

Field soil holds a large reserve. The useful quantity is available water: the water held between field capacity (what remains after free drainage) and the permanent wilting point (below which roots can no longer extract it).

FAO Irrigation and Drainage Paper 56, Table 19, gives available water by soil texture. These are millimetres of water per metre of soil depth — a conversion we have made from the published volumetric figures:

Soil texture	Available water (mm per metre of depth)
Sand	50 – 110
Loamy sand	60 – 120
Sandy loam	110 – 150
Loam	130 – 180
Silt loam	130 – 190
Silty clay	130 – 190
Clay	120 – 200

Be aware that FAO's own introductory training manual publishes wider figures — sand 25–100, loam 100–175, clay 175–250 mm/m — and that its clay range exceeds the one in Paper 56. Two FAO publications disagree. Paper 56 is the more rigorous of the two; the training manual numbers are the ones most often repeated elsewhere.

How much of that reserve you should use before refilling is the scheduling decision. FAO 56 calls it the depletion fraction, and makes it crop-specific: lettuce 0.30, onions 0.30, carrots 0.35, potato 0.35, tomato 0.40, cabbage and green beans 0.45, field maize 0.55. FAO also adjusts it for evaporative demand — the fraction falls when water use is high, because the plant runs into stress faster.

You will often be told simply to irrigate at 50 percent depletion. That figure is a working default from the United States Department of Agriculture's Natural Resources Conservation Service, which states that fifty percent is the normal recommended value for most irrigated crops in its guide, while noting the range runs from 30 to 60 percent. It is a rule of thumb, not a physical constant, and for shallow-rooted crops FAO's figures are tighter than it.

Judging soil moisture without instruments. The USDA publishes a method for this that works with nothing but your hand: Estimating Soil Moisture by Feel and Appearance. You take a sample from root depth, squeeze it, and read the result against your soil's texture group. Dry coarse soil is loose and leaves sand grains on the fingers; dry medium soil breaks apart with no moisture staining; at half-depleted, medium soil forms a weak ball with a rough surface and no water staining; at three-quarters full it forms a ball with light staining and darker colour. Learning this on your own soil is more useful than any schedule, because it calibrates your judgement to the actual field.

The practice that follows: water enough to wet the full root depth, then wait until a meaningful part of the reserve is used before watering again. Frequent shallow watering keeps the top wet, encourages roots to stay near the surface, and leaves the plant with no buffer when a hot day arrives.

Recognising too much water and too little

These look similar from a distance, which is why they are so often confused. Both produce a wilted, unhappy plant. The distinction is in the root zone, not the leaves.

Too much water — the medium stays wet for a long time after watering; the surface may stay dark between waterings; growth slows without an obvious cause; the plant may wilt despite wet medium, because damaged roots cannot take water up. A sour smell from the medium is a strong sign. In containers the pot stays heavy. Risk rises sharply in warm conditions, which is directly relevant in Thailand: research on root disease in soilless systems notes that roots colonised by Pythium at cooler temperatures can remain symptomless yet "develop severe visible decay within hours when temperature rises to 24–28 °C", with peak virulence of one species above 30 °C.

Too little water — the medium is dry at root depth when you check it; containers feel light; plants wilt in the afternoon and recover overnight; leaf edges dry and become crisp rather than soft; in coir and peat, the medium may pull away from the container wall, after which water runs down the gap and straight out of the bottom without wetting anything.

The check that settles it: put a finger, or a stick, into the medium at root depth — not the surface — before deciding. In containers, lift the pot. The weight difference between a freshly watered pot and a dry one is large and easy to learn.

If the symptoms point to a nutrient problem rather than a water problem, our fertilizer glossary defines the terms you will meet.

Coconut coir — water little and often

Coir behaves unlike soil in two ways that matter: it holds much less total water per unit volume than a metre of field soil, and it is chemically active in a way sand and perlite are not.

Physical properties. Published figures vary a lot, and the reason is worth understanding. University of Arkansas teaching material gives coir a water-holding capacity of 73 to 80 percent by volume with air-filled pore space of 9.5 to 12.5 percent, and pH 5.8 to 6.9. But Noguera and colleagues, working on coir fibre waste, describe a material with particularly high total porosity, high air content and comparatively low water retention — close to the opposite.

Both are correct, because coir is not one material. Coir pith (dust) holds a great deal of water and little air; coir fibre and chips do the reverse; commercial products are blends engineered somewhere between. A measured trial blending coir into pine bark showed the trend directly: as the coir fraction rose from 0 to 65 percent, container capacity rose from 39 to 59 percent while air space fell from 30 to 21 percent. When you change coir supplier or grade, your watering schedule changes with it.

Why little and often. The strongest controlled evidence comes from a trial that held the daily solution volume constant and varied only the frequency: higher fertigation frequency produced a significant yield increase, particularly at low nutrient concentration, through better nutrient uptake. The authors propose two mechanisms — continuous replenishment of nutrients in the depleted zone right at the root surface, and better transport of dissolved nutrients by mass flow at higher average water content. One honest caveat: that trial was run in perlite, not coir. The mechanism applies to any low-buffer soilless medium, but the demonstration was not in coir.

It is worth resisting the claim that many daily events are always required. A greenhouse tomato trial in coir irrigated twice a day, at 08:00 and 18:00, for three minutes per event at seedling stage and five to eight minutes from flowering to harvest, and worked. Treatments at 60, 70 and 80 percent of substrate water-holding capacity delivered 0.485, 0.601 and 0.789 litres per plant per day; the middle rate yielded only 3.65 percent less than the highest while using water 29 percent more efficiently. Frequency should follow evaporative demand, not a fixed number copied from someone else's farm.

One commercial guideline expresses this by tying frequency to light: roughly zero to one irrigation per hour at low radiation, rising to four or five per hour at high radiation, with the principle that "drain should not be a goal but a result of a structured irrigation strategy".

Buffering and calcium-magnesium in coir

This is the part of coir growing that is most often explained badly, so we will separate what is established from what is not.

What is established. Coir has a high cation exchange capacity — it holds exchangeable nutrient ions on its surfaces, the way a clay soil does and inert media do not. Published values range widely by source and method: 31.7 to 95.4 meq per 100 g across thirteen coir dusts from Asia, America and Africa; 73 to 117 in coir fibre waste; 39 to 60 in the Arkansas teaching figures. So the defensible statement is roughly 30 to 120 meq per 100 g, an order of magnitude above sand. A single quoted number is not defensible.

Those exchange sites do not arrive empty. Work examining seventeen coir dust sources found high contents of exchangeable potassium, sodium, calcium and magnesium on the adsorption complex, and that the water used for washing affects those concentrations. Chemical analysis of coir dusts found nitrogen, calcium, magnesium and micronutrients low, while "those of phosphorus and potassium were remarkably high" and "saline ion concentrations, especially chloride and sodium, were also high".

The consequence, which is real: when you feed untreated coir, divalent calcium and magnesium from your solution exchange onto those sites, displacing the potassium and sodium already there. Your feed loses calcium and magnesium; the root zone gains potassium and sodium. This is why calcium-magnesium supplementation is routinely needed in coir, and it is a genuine chemical effect, not a marketing idea.

Buffering is pre-treating the coir with a calcium source — usually calcium nitrate — so this exchange happens in the bag rather than in your crop.

What is not established, and you deserve to know it. We searched the horticultural literature specifically for peer-reviewed work validating a buffering protocol and found none. There is no published study we could find that quantifies the calcium and magnesium drawdown from feed in unbuffered coir, or that validates a specific calcium nitrate concentration and contact time. The numbers in circulation come from two places: manufacturers, and a United States patent application, which describes calcium nitrate at 7 to 8 kg per 1,000 litres of low-salinity water, applied at 200 litres per cubic metre of coir, with the initial exchange substantially complete in ten to fifteen minutes and an ageing period of one to twenty-four hours. A patent is a primary document, but it is a commercial instrument and it is not peer-reviewed.

Use those figures if you wish — but know what they rest on. The mechanism is well founded; the recipe is not.

Semi-hydroponics and passive systems

Here we have to be straightforward about the evidence, because it is thin. Searching for horticultural research on expanded clay pebble culture, passive wicking pots and similar systems returns almost entirely civil engineering and constructed-wetland wastewater literature. We found no peer-reviewed horticultural study establishing irrigation frequency, water-holding behaviour or salt management for these systems. Guidance circulating online for them has no published basis that we could trace.

The one rigorous primary source is the non-circulating method developed at the University of Hawai'i, and its principles carry over:

The reservoir is filled once before planting and not topped up. As the plant drinks, the level falls away from the container base and an air gap forms. That gap forms by consumption, and it is the point of the design, not a fault.
Two root populations develop: roots in the air gap that take up oxygen from the humid layer, and roots in the solution that take up water and nutrients.
The primary failure mode is stated directly: "submerging the oxygen roots will cause the plant to ‘drown’". Rain raises the liquid level, submerges those roots, and can kill the plant — which in the Thai rainy season means a passive system outdoors must be covered.
Working solution strength is given as EC 1.5 mS, with growers advised to avoid exceeding EC 3.0 mS.

On oxygen, reported thresholds from the wider literature: growth suppression in tomato and cucumber at 1 to 3 mg/L dissolved oxygen, and a commercial recommendation of 5 mg/L minimum in hydroponic production. The review reporting these makes its own honest point, which we will repeat: no crop-specific dissolved oxygen threshold is actually established, and it calls this a research gap.

Salt accumulation is the unavoidable weakness of any system with no drainage. Nothing leaches, so as water leaves by transpiration and evaporation, everything dissolved stays behind and concentrates. The physics is not in doubt; published flushing intervals are. The EC ceiling of 3.0 mS above is the only quantitative anchor we could find.

Runoff — how much should drain, and why any at all

New growers often treat water running out of the pot as waste. In any container or soilless system it is not waste — it is the mechanism that stops salt accumulating in the root zone. As the plant takes up water, dissolved salts are left behind; the only way out is to push some through.

The published figures converge tightly:

Virginia and North Carolina Cooperative Extension: generally 15 to 30 percent, scaled to water quality — applied water above EC 2.0 dS/m calls for a leaching fraction of 0.3; EC 1.5 for 0.2; below EC 1.0 for 0.1.
A review of soilless cropping in southern Europe: free-drain control with a "high (20–30 %) leaching fraction to avoid salt accumulation in the root zone".
The greenhouse tomato coir trial cited above measured 25 to 30 percent in practice.

A correction worth making, because the figure circulates widely: we found no primary source recommending a 50 percent drain fraction as good practice. Figures in the 30 to 60 percent range appear in one commercial guideline, but there they describe traditional practice being superseded by tighter control, not a target. Treat 15 to 30 percent as the defensible range.

Note also how water quality drives the answer. The worse your water, the more you must drain — which ties this page directly to irrigation water quality, pH and EC. A farm on clean rainwater and a farm on saline borewell water cannot run the same schedule.

Reading the runoff. Measure EC of what drains out and compare it with what you put in. Higher out than in means salts are concentrating — increase volume or leaching fraction. Lower out than in means you are over-leaching and wasting fertiliser — reduce it. Roughly equal is steady state.

A repeatable container method is the pour-through: 30 minutes to 2 hours after irrigation, pour 120 mL over a 1-gallon container, or 360 mL over a 3-gallon container, and collect what comes out. Target EC 0.5 to 1.0 dS/m under liquid feed, not exceeding 2.0 dS/m for most bark-based substrates.

One scheduling formula, from Virginia Cooperative Extension, adjusts the next irrigation from the last measured result: time adjustment = last irrigation length × (target leaching fraction − measured leaching fraction).

Downloads

Ten principles of watering (PDF, 5.5 MB — Vietnamese)
Optimal pH range for crops (PDF, 1.9 MB — Vietnamese)

Thai and English editions of both documents are being prepared.

Related guides

Sources

Allen, R.G. et al. (1998). Crop evapotranspiration, FAO Irrigation and Drainage Paper 56 — Table 19 (Ch. 7), Tables 22 and equations (Ch. 8). fao.org
FAO. Irrigation Water Management: Introduction to Irrigation, Training Manual 1, Ch. 2. fao.org
USDA NRCS. Estimating Soil Moisture by Feel and Appearance, Program Aid 1619. usda.gov
USDA NRCS. Pennsylvania Irrigation Guide (2018) — management allowed depletion.
Evans, M.R. Greenhouse Management Online, Unit 07: Substrates, University of Arkansas. uark.edu
Noguera, P. et al. (1997). Acta Horticulturae 450:365.
Abad, M. et al. (2002). Bioresource Technology 82(3):241. doi.org
Verhagen, J.B.G.M. (1999). CEC and the saturation of the adsorption complex of coir dust, Acta Horticulturae 481:151.
Basiri Jahromi, N. et al. (2020). Water 12(2):362. doi.org
Silber, A. et al. (2003). Plant and Soil 253:467. doi.org
Effects of Nutrient Solution Application Rates on Greenhouse Tomatoes Grown in Coir (2024). Plants 13(6):893. doi.org
US Patent Application 20220227680, Buffered Coco Material (2022). patents.justia.com
Kratky, B.A. A Suspended Net-Pot, Non-Circulating Hydroponic Method, VC-1, CTAHR, University of Hawai'i. hawaii.edu
Dissolved oxygen limitation and Pythium root rot in strawberry NFT systems (2026). Frontiers in Plant Science. nih.gov
Owen, J.S. Jr. et al. (2019). Leaching Fraction: A Tool to Schedule Irrigation for Container-Grown Nursery Crops, Virginia Cooperative Extension SPES-128. vt.edu
Massa, D. et al. (2020). Agricultural Water Management 241:106395. doi.org
Bilderback, T.E. (2001). Using the PourThru Procedure for Checking EC and pH for Nursery Crops, NC State Extension. ncsu.edu