The medium your roots grow in decides how you water and feed for the whole run. This section compares common growing media by how they hold water and air, not by what the bag says. It covers peat-based mixes, coco coir, mineral wool (rockwool) and perlite. It also covers inert media, which hold almost no nutrients of their own. Growers use them in recirculating systems: hydroponic setups where the nutrient solution drains back to a tank and is pumped through the roots again.
Where to start
When this section opened, it had one reference article. Read it first:
Two related topics are in the Fertilizers section:
- Organic vs Synthetic Nutrition: Living Soil & No-Till
- pH & EC: The Nutritional Foundation Every Grower Needs to Know. pH shows how acidic a solution is. EC (electrical conductivity) shows how much dissolved fertiliser salt it contains.
Why the same feeding schedule gives different results in two rooms
The substrate sets three things. First, it sets how often you must water. Second, it sets how strongly the medium buffers, meaning how much it resists sudden changes in pH and nutrient level.
Third, the substrate sets how much damage one mistake does. A forgiving medium tolerates a missed watering or a feed mixed too strong for a while. A medium with little buffering passes the damage to the roots almost at once.
So two growers can follow the same feeding schedule and get different plants, because their roots sit in different media. For this reason, read this section before most threads in the Fertilizers section, not after them.
What to do: Before you copy another grower’s feeding schedule, ask which medium and pot size they used. If theirs differ from yours, expect to adjust the schedule. Measure your runoff (the water that drains from the bottom of the pot) to see which way to adjust.
How a medium holds nutrients: cation exchange capacity (CEC)
Many nutrients dissolve in water as positively charged particles called cations. Examples are calcium, magnesium, potassium and ammonium. Sodium is a cation too, but plants don’t want it.
Some media have negatively charged surfaces. These surfaces hold cations and release them slowly to the roots. Cation exchange capacity (CEC) is how many cations a medium can hold this way.
A medium with higher CEC works like a small nutrient store in the pot. It evens out small feeding mistakes and keeps pH steadier. A medium with almost no CEC passes on whatever you pour in. A mistake then reaches the roots straight away.
In general, peat and soil rich in organic matter have the highest CEC. Coco coir is in the middle. Perlite, mineral wool and clay pebbles have almost none. This is why the last group is called “inert”.
| Medium | Holds water | Holds air | Holds nutrients (CEC) | Room for mistakes |
|---|---|---|---|---|
| Peat-based mix | High | Medium, more if perlite is added | High | Most |
| Coco coir | High | High | Medium; arrives already loaded with potassium and sodium | Medium |
| Mineral wool (rockwool) | High | Medium | Almost none | Little |
| Perlite | Low | High | Almost none | Little |
| Clay pebbles | Low | Very high | Almost none | Least |
What to do: Decide how much buffering you need before you choose a medium. If you are new, or you can’t check the plants every day, choose a medium with some CEC. It gives you more room for error.
Coco coir: wash, buffer, then feed from the first watering
Coco coir is made from coconut husk. Its fibres already hold potassium and sodium on their exchange sites before you buy the bag. Some of this salt comes from the husk itself and some from processing. This causes two problems.
First, loose salt in unwashed coco can burn young roots. Second, coco that has not been buffered trades its potassium and sodium for the calcium and magnesium in your feed. Seedlings and clones then show calcium or magnesium deficiency in the first weeks, even when your feed contains enough.
The four steps below prevent both problems.
- Wash. This removes loose salt. Soak the coco in clean water, drain it and repeat. Measure the EC of the water that drains out each time. Stop when it is close to the EC of your source water.
- Buffer. This fills the exchange sites with calcium and magnesium before the plant goes in. Soak the washed coco in water mixed with a calcium–magnesium supplement (often sold as “CalMag”), at the strength on the label. Then drain it.
- Check. This shows whether the coco still leaks salt or shifts pH. Water it once more and measure the pH and EC of the runoff.
- Feed from the first watering. Use a feed that contains calcium and magnesium. That can be a nutrient made for coco, or your base nutrient plus CalMag at label rate. Coco keeps exchanging cations, so starting on plain water brings back the deficiency you just prevented.
During the run, measure runoff EC a few times a week. If the runoff EC climbs well above your feed EC, salt is building up in the pot. Water with more runoff until the reading comes back down.
Some bags say “buffered” or “pre-washed”. Still run step 3 on a sample, because the quality behind those labels can change from batch to batch.
Translator’s note: Coco coir from local producers in Thailand and Southeast Asia is cheap and easy to find. Quality varies between producers and between batches, and some bags still carry a lot of sodium and potassium. Before you fill a whole room, soak a small sample in clean water and measure the EC of that water. Test every new batch the same way. If one batch reads much higher than the others, wash it for longer or don’t use it.
Translator’s note: Tap water and well water in the region may already contain calcium and magnesium. Measure your own water before you decide how much CalMag to add. Irrigation Water for Living Soil: Chloramine, Hard Water, Softened Water explains how. To check that a supplement is legally on sale in Thailand, see Verifying a Fertiliser Is Legally in Thailand. Ask the official distributor in Thailand, or ask in the Fertilizers section, where to get it.
Peat-based mixes: the starter charge and the pH
Most bagged peat-based mixes come with starter fertiliser and lime already mixed in. This starter charge feeds the plant for the first few weeks. How long depends on the product and how often you water. If you also feed at full strength from day one, your feed adds to the starter charge and can be too strong for young roots.
What to do: Check the bag for added fertiliser, such as NPK figures or the words “pre-fertilised”. For the first two weeks, measure runoff EC before each feed. Give plain water at the right pH, or a light feed, while runoff EC stays high. Start your full schedule once it drops.
Raising the pH of raw peat or acidic soil: lime or dolomite
Raw peat moss is acidic, and so are many tropical soils. Raw sphagnum peat often has a pH of about 3.5–4.5. Without lime, it is too acidic for cannabis roots. In a peat-based mix, growers usually aim for a root-zone pH of about 5.8–6.2. Growers raise the pH with ground limestone. There are two main types:
- Calcitic lime (calcium carbonate) raises pH and adds calcium.
- Dolomite lime (calcium–magnesium carbonate) raises pH and adds both calcium and magnesium.
Choose dolomite if your water and fertiliser supply little magnesium. Choose calcitic lime if you already supply enough magnesium. Both work slowly and need moisture to react, and a finer grind reacts faster. Use the rate on the product label, because the right amount depends on the peat and the pH you want. Mix the lime in, keep the mix moist for a few days, then run a slurry test before you plant.
Testing the pH of the substrate
The pH of the water you pour in is not the pH at the roots. Two simple tests measure the pH in the root zone itself:
- Slurry test: Mix one part moist medium with two parts distilled water by volume, and let it stand for 30 minutes. Then measure the pH of the water.
- Pour-through test: Water the pot as usual and wait 30–60 minutes. Then pour just enough distilled water on top to collect about 50 ml of runoff, and measure the pH of what drains out. Larger pots need more water.
Calibrate your pH meter before each round of tests, because an uncalibrated meter gives wrong readings. Test once before planting to confirm that the lime has worked. Then test at regular intervals and write each result down, so you can see whether the pH drifts during the run.
Pot size and structure set the wet–dry cycle
The wet–dry cycle is the rhythm between one watering and the next. Right after watering, the pores in the medium are full of water. As the plant drinks and water evaporates, air moves back into the pores. Roots need both the water and the air.
How long one cycle lasts depends on two things together. The first is the structure of the medium: how coarse it is and how much water it holds. The second is the size of the pot.
A larger pot holds more water for each root, and its lower part stays wet for longer. As a result, the same medium dries more slowly in a bigger pot and behaves like a wetter medium. In practice, changing the pot size is like changing the medium.
What to do: Lift each pot right after watering, and again when it seems ready for more. This teaches you its wet weight and its dry weight. For two weeks, write down the number of days between waterings for each pot size. You can then water when the plant needs it, not by the calendar. When you move a plant into a bigger pot, see Transplanting Cannabis.
Inert media and recirculating systems
Mineral wool, perlite and clay pebbles hold almost no nutrients and barely buffer pH. The roots get exactly what the solution gives them. In a recirculating system, the same solution passes the roots again and again, so its pH and EC change with every pass.
What to do: Measure the pH and EC in the tank every day and write them down. The changes show what the plants are taking up, and they often show a problem before the leaves do.
Feeding each medium in hot, humid weather
Translator’s note: At 30–35 °C, plants drink more and media dry faster. As water evaporates, the salts left behind become more concentrated around the roots. Small pots, and media that hold little water such as perlite and clay pebbles, feel this first.
Translator’s note: The rainy season brings the opposite problem. When the air is humid, the plant loses less water through its leaves, so the medium stays wet longer. Roots in wet medium with no air are at risk of root rot. Peat-based mixes in large pots hold the most water, so they carry the highest risk.
Translator’s note: Use the points below as starting points, and adjust them by your own runoff readings.
- Peat-based mix: In hot, dry spells, water when the pot feels light, not on a fixed day. In the rainy season, water less often and add perlite to the mix to give the roots more air.
- Coco coir: Coco holds plenty of air, so it tolerates frequent watering. In the heat, water more often and include feed in every watering. Don’t let it dry out completely, because the salt left behind becomes concentrated around the roots.
- Inert media and recirculating systems: In the heat, the tank warms up. Warm solution holds less oxygen for the roots, as explained in Running Hydroponics at 31 °C Reservoir Temperature. During a power cut the pump stops, and perlite and clay pebbles dry out fast. Decide in advance how you will water by hand if the power goes off.
What to do: Keep a simple log for each room in every season. Record the temperature, humidity, days between waterings, feed EC and runoff EC. After one hot season and one rainy season, you will have a watering and feeding schedule that fits your own medium.
Ask your question here
Post your substrate question below this topic. To get a useful answer, include these details:
- the medium, and whether you washed and buffered it
- the pot size
- the temperature and humidity in your room
- the pH and EC of your feed
- the pH and EC of your runoff
With these details, other growers can compare your setup with theirs.