Substrates compared: coco, rockwool, soil and hydroponics
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Substrates compared: coco, rockwool, soil and hydroponics

This paper compares the substrates for cannabis roots and shows how to select one. It gives information about the water and air in each medium, the effect of each medium on the nutrients, and the tolerance for errors. It is not necessary to know about substrates before you read this paper.

Basic11 diagrams9 sources~11 min to read
In this paper
01 Purpose and scope02 Definitions03 Evidence and limitations04 Porosity: more water gives less air05 Buffering of nutrients in each substrate06 Number of uses, cost and effect on the environment07 Substrate selection08 Setup and procedures for each substrate09 Troubleshooting10 Expected results and limitations11 References
Start here

Purpose and scope

A substrate (a growing medium) is the material that contains the roots of a plant. It holds the plant in position, and it is the reservoir for water, air and dissolved nutrients. This paper compares the five media that cannabis growers use: coco, rockwool, peat, living soil and deep water culture (DWC). It is not necessary to know about substrates before you read this paper.

At the end of this paper, you will know the two properties of a medium that have the largest effect on the result. The first property is the quantity of water and the quantity of air that the medium holds. The second property is the tolerance of the medium for your errors. When the tolerance is high, an error in the nutrients or in the water has a small effect on the plant. With this information, you can select the correct medium for your setup.

The position of the roots1In a solid mediumcoco, rockwool,peat and livingsoil hold theroots→2In water and airDWC: roots inwater withnutrients andoxygenFive alternatives in two groups. This paper shows how they are different.
Figure 1. The alternatives are in two groups: the media that hold the roots in position, and water culture. In water culture, the roots hang in the nutrient solution.
Who this paper is for

This paper is for a person who selects a first medium and does not know about substrates. It is also for a person who wants to know the cause of the results in a setup. The coco crop-steering paper gives full information about the accurate control of coco, after you select it. You can use the two papers together.

Terms

Definitions

Two values give the condition of a medium when it is wet. The other terms are about nutrients. It is not necessary to know all these terms at this time. It is sufficient to know the basic information about each term.

Water-holding capacityThe part of the pore space that contains water after the medium drains. When the value is higher, the medium stays wet for a longer time.
Air-filled porosityThe part of the pore space that contains air after the medium drains. Water and air fill the same pore space, and the total does not change. Thus when the quantity of water increases, the quantity of air decreases.
EC (electrical conductivity)The quantity of dissolved nutrient salt in the water, in milliSiemens per centimeter (mS/cm). A higher EC is a stronger feed with more salt.
pHA measurement of acidity on a scale of 0 to 14. The best pH range in the root zone of cannabis is approximately 5.5 to 6.5. For accurate control in soilless media, the range becomes smaller: 5.8 to 6.2.[4]
CEC (cation exchange capacity)Some media hold nutrient ions. They absorb the ions when the feed is strong and release them slowly when the feed is weak. The capacity of a medium to hold ions is its cation exchange capacity, in meq/100g.[2] When the CEC is high, the medium buffers your errors in the feed. But when the CEC is high, raw coco also removes some ions from the feed, and not other ions. Refer to the definition of Buffering below.
BufferingWhen you make an error in the dose, a buffered medium absorbs a part of the effect before the effect goes to the roots. Thus a buffered medium has a high tolerance for a feed that you mix incorrectly. An inert medium, for example rockwool, does not absorb the effect. Each change goes directly to the roots.
DrybackWhen you let the medium become dry between two irrigations, but not fully dry, the plant receives a weak stress signal. This signal causes the roots to go deeper in the medium. The quantity by which the medium dries between two irrigations is the dryback. Dryback is the primary control in crop steering. The coco paper gives more information.
One total pore space, divided between water and airWhen the medium is saturated, the part for water increases. The roots get less oxygen.Water-holdingAir0%50%100%
Figure 2. The bar shows the total pore space, and the size of the bar does not change. When the quantity of water in the medium increases, the part for water becomes larger and the part for air becomes smaller. The roots use this air to get oxygen.
Water-content cycle for one dayFill to field capacity, hold it, make a controlled dryback, and do it again. The dryback size is the steering control.correct bandtoo dry0255075100offP3 minonP1FCP2offwater content %
Diagram. Each medium has a cycle from wet to dry each day. The shape of the cycle changes with the quantity of water that the medium holds.
pH and nutrient availability (soilless)most nutrients are available at approximately pH 5.5-6.5lockoutgoodbestgoodlockoutpH 4pH 5pH 6pH 7pH 8When the pH is not in the green band, nutrients precipitate or the roots cannot absorb them. The nutrients are in the tank: lockout.
Diagram. The pH is different in each medium, and the buffer is different. These control the availability of nutrients.
Quality of the information

Evidence and limitations

Three types of information

We want each paper to be correct. Thus we examine each paper for information that is not solid. This information comes from the view of one grower, from a small number of sources, or from grower methods that have no controlled test. We identify this information, and we do not show it as solid.

Frequently there is no paper for the decision that you must make. Then we use the reports of other growers and the methods that work in the rooms that we know. This information can help you, but it is not the result of a laboratory test. Use the methods that work for your plants, for your room and for your meters. If a table does not agree with your crop, use the data from your crop and record the difference.

Solid
Tests of plants, specifications or many sources that agree show that the information is correct
  • Air-filled porosity and water-holding capacity are different for each type of medium.
  • Soil is a buffer for pH and nutrients. Inert media are not buffers.
Grower method
Many growers use this method in their rooms. Start with it, then change it for your room.
  • Easy methods for a new grower to select a medium for tolerance of errors or for control.
Weak
The view of one grower, a small number of sources, one test, or a method that works in one room
  • The pH of a living soil corrects automatically in a small number of minutes, for all types of water.
  • One percentage of air-filled porosity (AFP) for all rockwool products.

If you find an error, make a report. We will correct it. Make the report on GitHub, with the name of the paper and the information that is incorrect. If you have a source, include it: Make a report of an accuracy problem. The regulations, labels and licenses of your area always override each procedure in these papers. The label weak in a paper shows the points where the risk of an error is highest.

Primary information 1

Porosity: more water gives less air

Oxygen is as necessary for the roots as water. If a medium stays saturated, the roots do not get sufficient oxygen and root rot can occur. If a medium does not hold sufficient water, it becomes dry and causes stress in the plant. The five media are at different positions between these two conditions.

Rockwool has a total porosity of approximately 96%, but the air-filled porosity is only approximately 11%. The water-holding capacity is approximately 91%. Thus rockwool holds a large quantity of water and releases it easily.[7] Peat has a total porosity of approximately 90 to 95% and a higher air-filled porosity of 18 to 25%.[9]

Coco coir is different from the other media. Saturated coco coir can keep approximately 22% air, and saturated rockwool keeps approximately 10%. As a result, it is not easy to apply too much water to coco.[2] DWC is at the end of this range. The roots hang in nutrient water and get oxygen only from a pump.

Air-filled porosity when saturatedMore air at the roots when saturated gives less risk of too much water.0815223022%Coco coir21%Peat11%Rockwool
Figure 3. Coco keeps the most air when it is wet. Thus it is not easy to apply too much water to coco. Peat has more air than rockwool.[2][7]
After irrigation, when the rockwool drains, approximately 80% of the volume is solution, 15% is air and 5% is fiber.[7]
MediumTotal porosityAir-filled porosityWater-holding capacityAir when saturated
Rockwoolapproximately 96%approximately 11%approximately 91%approximately 10%
Coco coirapproximately 94 to 96%highhighapproximately 22%
Peatapproximately 90 to 95%18 to 25%highmoderate
The primary fact

When there is more water, there is less air. A medium that keeps air around the roots when it is wet has a high tolerance for too much water.

The parts of a substrateSolids give structure. Pores contain water and air.water 35%air 53%Cocowater 30%air 66%Rockwoolsolid 18%water 45%air 37%Peat mixtureAir-filled porosity gives root oxygen. Water-holding capacity is the buffer. The ratio shows if the medium is wet or has more air.
Diagram. A substrate contains solids, water and air. The ratio of the porosity that holds water to the porosity that holds air shows if the substrate is wet or has more air.
Primary information 2

Buffering of nutrients in each substrate

An inert medium, for example rockwool, contains almost no nutrients. The EC of the medium is very low, and its pH is alkaline: approximately 8. Thus the feed that you apply is the feed that the roots get.[7] The control of the feed is accurate, but the tolerance for an incorrect mix is low.

Coco is the opposite of rockwool. Coco has a high CEC of approximately 40 to 100 meq/100g, and this CEC buffers the changes in EC. But raw coco has exchange sites that hold potassium and sodium. When the feed water flows through the coco, the sites release potassium and sodium into the water. At the same time, they absorb calcium and magnesium from the water. Calcium and magnesium are the nutrients that are the most necessary for the plant, and the sites remove them before they go to the roots.[2]

The result is a deficiency of calcium and magnesium, unless you buffer the coco (soak it in a solution of calcium and magnesium) or you use pre-buffered coco.[4]

Living soil has the highest tolerance for errors of all the media. Microbes, organic matter and minerals keep the pH of the root zone at approximately 5.2 to 6.5 for hours to days. This effect occurs when the soil biology is in good condition. Many organic growers do not decrease the pH of the water for usual irrigations. But they do not use very alkaline water, and they monitor the pH if problems occur.

In DWC there is no buffer. The reservoir is the only protection from an error.

Scale of tolerance for errors in each mediumLeft: no buffer. Right: soil corrects the pH. Coco buffers EC. Pre-buffering is necessary for calcium and magnesium.Inert: DWC, rockwoolBuffered: cocoCorrects its pH: living soil050100
Figure 4. The scale goes from no buffer (DWC and rockwool), to the EC buffering of coco, and then to living soil. Living soil corrects its pH after each irrigation.
Select a medium for the work that you do

If you cannot make the feed correct each time, select a medium with a high tolerance for errors. When a medium gives more control, it gives less protection from your errors.

Primary information 3

Number of uses, cost and effect on the environment

The media are very different in the number of crops that you can get from each medium. You can use rockwool again for a maximum of approximately three years, if you do the sanitation correctly. But much energy is necessary to make rockwool, and in some areas the regulations do not let you put it in a landfill. As a result, growers in parts of Europe and Japan use less rockwool.[8]

You can clean coco with water, buffer it again and use it again for some cycles. Then you can make compost from it for garden soil. But most coco moves a long distance to the grower.

You can use living soil again more times than all the other media. You add amendments to a bed of recycled organic living soil (ROLS) between two crops. The bed has no limit on the number of crops, and it becomes better with time.

DWC has no medium to discard. But you replace the nutrient reservoir frequently, and the pumps must have continuous electricity. Peat has a low cost and gives good results. But it becomes available again only after a very long time, and it has a carbon cost. As a result, many growers select coco.[9]

No medium is the best in all columns. Living soil has the lowest waste, but it has the most work at the start.
MediumTypical number of usesEnd of lifeCost groupEffect on the environment
Living soilNo limit (ROLS, with amendments)The soil stays in the bedLow after the setupLowest waste after the setup
Coco coirSome cycles, with buffering againBecomes compost for soilLow to moderateLarge effect on the environment, because the coco moves a long distance
PeatUsually one cycleCan become compostLowBecomes available again slowly. Has a carbon cost.
RockwoolApproximately 3 years, with sanitationSome areas do not let you put it in a landfillModerateMuch energy to make
DWCNo mediumNo medium to discardModerate (equipment and power)Frequent replacement of the reservoir, power for 24 hours each day, 7 days each week
Make a decision

Substrate selection

Select a medium for the quantity of work that you accept. Do not select a medium because it gives the highest yield. The correct medium has tasks that you can continue to do each day.

Select by how much work you accept1Water only, easy setupliving soil orsoil mix withgood-qualitypeat→2Feed and EC/pH checkspre-bufferedcoco, also thefirst step tocrop steering→3Known correct settingsrockwool, afterthe irrigationis stable→4You do constant checksDWC: highestpossible yield,lowest tolerancefor errorsStart at the top. Select the next step only when you can do more tasks each day.
Figure 5. The steps go from the medium with the highest tolerance for errors to the medium with the most work each day. We recommend that most new growers start at the top two steps.
Tolerance for errorsHow much tolerance for errors the medium gives a new grower. A higher value is safer.0258109/10Living soil8/10Peat / soil mix6/10Coco (pre-buffered)4/10Rockwool2/10DWC
Figure 6. Living soil and soil mixes of good quality have the highest tolerance for errors. DWC has the lowest tolerance. Thus DWC is not a good medium for a first crop.
Do not use DWC for your first crop

DWC can cause very fast growth. But a warm reservoir or an air pump that stops can kill a plant in one day.[6] First, use a medium with a higher tolerance for errors. Then you will know the basic procedures.

Do this

Setup and procedures for each substrate

Each medium has one step to prepare the medium. You must do this step. When you do it correctly, the other tasks are easy.

  1. 1
    Coco
    Clean the coco with water. If the coco is not pre-buffered, soak it for 8 to 24 hours in a solution of calcium and magnesium. Then apply feed in each irrigation, at pH 5.8 to 6.2 and a low EC. Apply small quantities of water frequently.
  2. 2
    Rockwool
    Before you transplant, soak the cubes or slabs in a solution at pH approximately 5.5. Dry rockwool has a pH of approximately 8. After this, do not let the rockwool become fully dry.
  3. 3
    Living soil
    Make the bed, or get a bed from a supplier. Let the bed cycle for some weeks. Then apply only water. Do not adjust the pH, unless symptoms occur.
  4. 4
    DWC
    Keep the reservoir at 18 to 20 °C (65 to 68 °F), the dissolved oxygen at 7 to 9 mg/L, and the pH at 5.5 to 6.0. Operate the air pump 24 hours each day, 7 days each week.
Most new growers do not do the step to prepare the medium. For each medium, this step is the difference between an easy start and a problem at the start.[4]
MediumStep to prepareProcedure for feed and waterpH targetDo not ignore
CocoClean with water. Buffer with calcium and magnesium.Apply feed in each irrigation, low EC5.8 to 6.2The pre-buffering step
RockwoolSoak at pH approximately 5.5Frequent irrigation. Do not let the rockwool become dry.5.5 to 6.0Soak the rockwool before you put plants in it
Living soilCycle for some weeksOnly waterDo not adjustDo not adjust the pH of the water
DWCInstall the pump and the temperature controlRecirculating reservoir5.5 to 6.0Operate the air pump 24 hours each day, 7 days each week
DWC reservoir: the zone of riskTemperature, dissolved oxygen, crop and pathogen have effects on each other. 23 °C is not a threshold for all diseases.cooltarget 18–20 °Cless oxygen headroomwarm-water risk14C21C28C
Figure 7. Warm water holds less dissolved oxygen. But the risk of Pythium does not have one threshold of 23 °C for all conditions. Select the range of operation for the crop and the system. Then monitor the temperature, the dissolved oxygen and the condition of the roots.[5][6]
When a problem occurs

Troubleshooting

The usual errors of a new grower are different for each medium. Most of these errors show symptoms of a different problem.

Growers read the problems in the coco and rockwool rows incorrectly as problems with the feed. The cause of these problems is an error in the step to prepare the medium.[4][5]
MediumUsual errorThe effect that you seeCorrection
CocoNo buffering stepDeficiency of calcium and magnesium: orange-brown marks on the leaves, and leaves with a yellow colorDo the pre-buffering of the coco. Add calcium and magnesium to the first feeds.
RockwoolYou put the plants in dry cubesPale plant with pH-8 lockoutSoak at pH approximately 5.5 before you transplant
RockwoolToo much water in a medium with a small quantity of airSlow growth. Wet roots that do not get sufficient oxygen.Apply a smaller number of shots. Let air go into the rockwool between the shots.
Living soilYou adjust the pH of the water or add saltsGrowth stops, and the quantity of microbes decreasesApply only water. Do not adjust the pH.
DWCWarm water, or an air pump that stopsThe condition of the roots can become very bad in a short timeKeep the temperature and the aeration in the range that is correct for your system. Monitor the dissolved oxygen.
DWC temperature and aeration

Monitor the temperature, the dissolved oxygen and the roots. Make sure that a second air pump is available. If the system cannot keep the correct range, add a chiller.

Warm water decreases the headroom of dissolved oxygen. An air pump that stops removes the aeration from the pump. Root rot does not occur because of only one of these conditions. The risk of disease also changes with the crop, the pathogen, the inoculum and the exposure time.[5][6]

Possible results

Expected results and limitations

No medium does the work for you. In each medium, some tasks are not easy, and the media are different in the part of the procedure where these tasks occur. In living soil, most of the work is at the start, when you make the bed. After this, the bed operates with a very small quantity of work from you. In coco and rockwool, the work is the feed and the checks each day. In DWC, the risk is in a small number of parameters, and these parameters must stay in the correct range.

Work each day and the maximum for yield and speedThe work and the risk each day increase from left to right. Height is the maximum with correct settings.025810Living soilPeat / soilCocoRockwoolDWCmaximum
Figure 8. The work and the risk increase from left to right. The maximum also increases, but only a careful grower gets it. A soil crop that you control correctly gives better results than a hydroponic crop that you control incorrectly, each time.

If the feed is accurate, inert media, for example coco and rockwool, can give a better yield and speed than soil. Tests that compare media show this result.[1] The difference is small. In soilless cannabis, an accurate feed has a large effect on the results.[3] But this result is possible only when your environment and your irrigation are stable.

How to become a good grower
  1. Select the problem that you can accept. Each medium causes a different problem. Select a medium when you can prevent the worst problem of that medium.
  2. Do one full crop cycle in a medium before you make a decision about it. You know a medium when you do one full crop in the medium. You do not know it when you only read about it.
  3. Change one parameter in the next crop. Do not change all the parameters at the same time, because you cannot know which change helped.

First, do one full crop. Then make your procedures better. The coco crop-steering paper continues from this paper when you want accurate control of a medium. The pH paper gives information about the pH value. If the pH value is not correct, the feed does not go to the roots.

Related papers
Coco and crop steering
Use water to control the plant
pH control
Keep pH in range for nutrient uptake
Water quality
Source water, RO and alkalinity

References

  1. Xiong J, Tian Y, Wang J, Liu W, Chen Q. Comparison of Coconut Coir, Rockwool, and Peat Cultivations for Tomato Production: Nutrient Balance, Plant Growth and Fruit Quality. Frontiers in Plant Science. 2017;8:1327. doi:10.3389/fpls.2017.01327 https://pmc.ncbi.nlm.nih.gov/articles/PMC5539188/
  2. Abad M, Noguera P, Puchades R, Maquieira A, Noguera V. Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants. Bioresource Technology. 2002;82(3):241-245. doi:10.1016/S0960-8524(01)00189-4 https://www.sciencedirect.com/science/article/abs/pii/S0960852401001894
  3. Bevan L, Jones M, Zheng Y. Optimisation of Nitrogen, Phosphorus, and Potassium for Soilless Production of Cannabis sativa in the Flowering Stage Using Response Surface Analysis. Frontiers in Plant Science. 2021;12:764103. doi:10.3389/fpls.2021.764103 https://pmc.ncbi.nlm.nih.gov/articles/PMC8635921/
  4. Cockson P, Landis H, Smith T, Hicks K, Whipker BE. Characterization of Nutrient Disorders of Cannabis sativa. Applied Sciences. 2019;9(20):4432. doi:10.3390/app9204432 https://www.mdpi.com/2076-3417/9/20/4432
  5. Le DH, et al. Etiology and epidemiology of Pythium root rot in hydroponic crops: current knowledge and perspectives. Summa Phytopathologica. (review) Available via SciELO Brazil. https://www.scielo.br/j/sp/a/cGCh3FGRRzGLVFJf577mypy/?lang=en
  6. Dissolved oxygen limitation and Pythium root rot in strawberry NFT systems: mechanisms, research gaps, and prospects for substrate-free production. Frontiers in Plant Science. 2026. doi:10.3389/fpls.2026.1829367 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2026.1829367/full
  7. Raviv M, Lieth JH (eds.). Soilless Culture: Theory and Practice. Elsevier/Academic Press, 2008. ISBN 978-0-444-52975-6. (Chapter: Physical Characteristics of Soilless Media.) https://shop.elsevier.com/books/soilless-culture-theory-and-practice/raviv/978-0-444-52975-6
  8. Composting of recovered rock wool from hydroponics for the production of soil amendment. Environmental Science and Pollution Research. 2024. doi:10.1007/s11356-024-33041-2 https://link.springer.com/article/10.1007/s11356-024-33041-2
  9. Peat Use in Horticulture. IntechOpen (peer-reviewed book chapter), 2018. doi:10.5772/intechopen.79611 https://www.intechopen.com/chapters/62735

Each citation in the paper, for example [1], refers to an item in this list. Each item is a primary source or an instruction from an authority, unless the item shows a different type. The results of cannabis tissue culture change much with the genotype. Before you use the dilutions, hormone doses and local regulations, make sure that they agree with the primary sources.