Growing marijuana in plant pots

Growing marijuana in soil is, by far, the most common type of crop throughout the world. In this post, we are interested in how to optimize our soil in a simple and fast way.

Which type of soil should we use for growing marijuana plants?

Humus for cannabis

Humus for cannabis

We recommend you to use, whenever possible, a soil specially developed for marijuana plants, which will offer much better results than any classic gardening substrate.

In fact, using a soil specially developed for the cultivation of cannabis will be an easy way to optimize the crop regarding both the structure (drainage, aeration …), PH, and nutrients (fertiliser) contained. Canna Bioterra, for example, provides excellent results.

How to enrich the soil before its use?

There are several organic fertilisers to enrich the soil before its use. This will allow you to reduce the amount of liquid fertiliser used in the crop; actually, not using any fertiliser is possible if you use large containers.

For example, you can do the following mixture:

  • 100L of soil for marijuana plants
  • 10-20L of Vermicompost (Humus), a natural complete and very balanced fertiliser.
  • 2-5L of Bio Super Mix, a mixture of organic fertilisers from Plagron, which has many nutrients, trace elements and beneficial microbial life.
  • 1 kg of bat guano, a source of phosphorus (P) and elicitors which will enhance the natural defenses of the plant (resin)
  • 1 kg of wood ash (barbecue, fireplace, etc.) source of Potassium (K) and coal for the microbial life.
  • 200 gr. of Nutrihemp, a natural fertiliser based on algae, source of Nitrogen (N) and amino acids.

These are slow release fertilisers, what means that they will degrade gradually in the soil over the course of several weeks. Thus, the risk of over-fertilization is low, in contrast to liquid fertilisers which are assimilated much faster.

You can also improve the structure of the soil by adding:

  • Coconut fibre (20%) and/or Perlite (10%), to increase the drainage capacity and aeration of the soil.
  • Polymers, to ensure a small reserve of water and thus reduce the frequency of watering.

Clay Pebbles on the bottom of the pots aren?t useful, even if this technique is very frequently used. They reduce the amount of available soil and raise the PH if they aren?t correctly prepared a few weeks before.

Marijuana flowering in soil

Marijuana flowering in soil

How to make good use of the microbial life in the soil?

Marijuana roots in symbiosis with the microbial life

Marijuana roots in symbiosis with the microbial life

One of the most significant advantages of growing marijuana in soil is using the active beneficial microbial life of the soil (bacteria and fungi) in natural symbiosis, which perform two important actions:

  • Break down large organic molecules and dead roots into easy-to-assimilate nutrients for the plants.
  • Protect the roots from attacks of harmful bacteria or pathogenic fungi (vectors or triggers of diseases)

The microbial life of the soil goes unnoticed by large part of the marijuana plant growers, while this directly influences in the development of the plant, increasing the quality and quantity of the yield.

You can add some strains of carefully chosen bacteria or fungi in order to significantly improve the quality of the microbial life of the soil. These bacteria will act in symbiosis with the roots of the plants: microbial life will receive the needed sugars for its development through the nutrients provided by the waterings.

The most well-known and probably the most efficient one is called Trichoderma Harzianum, from the family of the Mycorrhizae. We can find it in products like Micoplant (microscopic fungi). BioMagix from GHE is a mixture of Trichoderma and beneficial bacteria. You can also add numerous fungi and favourable bacteria using Bactoforce from House&Garden or bacteria from Bactohemp.

Trichoderma Harzianum (beneficial fungus)

Trichoderma Harzianum (beneficial fungus)

Once this microbial life is introduced and active, it will be necessary to have caution: do not use any bactericidal or fungicidal product that could get in contact with the ground: H2O2, Silicon, Propolix, Sulfur ?

Note that letting the tap water used to water our plants to rest for a few hours is also very important. This removes the chlorine through evaporation, since this is added in a gaseous form to the tap water to eliminate all bacteria and fungi contained in it.

Beneficial microbial life creates its own enzymes to break down organic molecules, so it isn?t necessary to add them in liquid form. Enzymes have been developed to replace and simulate microbial life when not  present (hydroponics etc.).

If you want to sterilize the soil to eliminate all bacteria, fungi, eggs, larvae, insects, mites, etc, you only have to put it for some seconds in the microwave. Don?t forget to add new microbial life once the soil is sterilized.

Sugars (molasses, glucose … ) will greatly increase the microbial activity, as well as the humic and fulvic acids, such as those contained in Diamond Black from General Organics.

What type of fertiliser should be used for growing marijuana plants in soil?

In order to properly help the microbial life, it will be very important to use organic fertilisers (bio) specially designed for marijuana plants, such as:

Of course, it?s also possible to grow in the ground with mineral fertilizers (chemicals), these are directly assimilated by the plant without passing through the microbial life. This micro-life, then, doesn’t have many elements for its nourishing; its development will not be so pronounced than when using organic fertilisers.

This is the biggest difference between organic and mineral fertilisers; the former feed the soil and the plant at the same time, leaving a rich and alive substrate, while chemical fertilisers only feed the plants, leaving a poor and barren substrate. Leaving aside marijuana plants, this problem affects all soil crops around the world.

Growing marijuana in soil

Growing marijuana in soil

Organic fertilisers, when properly used, allow yield as high as mineral fertilisers. However, it’s especially to achieve the best quality in taste that using organic fertilisers is advisable (bio), what provides the crop with richer and more complex flavours than mineral nutrients.

To avoid accumulations of salt or chemicals in the soil, it will be necessary to water without fertiliser intermittently, doing one watering with fertiliser and another without it, or up to two irrigations with fertiliser for only one with water.

Above all, remember that the final flushing of the roots should be performed only with water – without fertilisers – at least 3 weeks before the harvest to be efficient (the longer the root washing period and the cleaner the roots are from nutrients, the better will be the quality of the taste).

Don?t hesitate to read our post about watering marijuana plants in soil.

Is it necessary to correct the PH value in soil marijuana crops?

When using organic fertilisers – and with a PH level from 5 to 8 – it won?t be necessary to correct the PH level. The soil itself has good buffers to regulate the PH, which are responsible of softening pH fluctuations. The ideal PH level for growing in soil is about 6.5.

If you use acids to reduce the PH level, this will destroy this buffer effect, so adjusting the PH in each watering during the entire crop will be necessary.

In practice, a PH value near 6.5 is advisable only when:

  • Using mineral fertilisers to improve nutrient uptake.
  • Using water with a PH range lower than 5 or higher than 8.
  • Growing mother plants, because the natural buffer of the soil is reduced over time.

To lower the PH level in organic crops (bio) we can use citric acid or lemon juice.

Measuring the EC value (Electrical Conductivity) won?t be entirely necessary with organic fertilisers, because the EC meter won?t provide exact measurements of real nutrient concentrations in the soil. Although it should be noted that we can control the specific amount of mineral fertiliser used during a crop and for a particular marijuana strain to have different benchmarks for future crops with the same genetics.

What type of containers should be used for growing marijuana plants in soil?

The roots usually tend to mainly colonize the external part of the pot, so changing to a larger pot during the cultivation is very useful. It will create a much more efficient root system, what will make the watering and nutrition of the plants much easier. The pots should always have holes in the bottom to ensure effective drainage in case of excessive watering.

Marijuana plant in soil in a 7L plant pot

Marijuana plant in soil in a 7L plant pot

Outdoors, a large pot will ensure great yields, this is why growers normally use 30-50L (or larger) containers during the bloom phase. In the case of regular marijuana seeds  you can wait until the beginning of the flowering phase to transplant the plants, when we know if they?re males or females.

Autoflowering marijuana strains don?t follow these rules due to their small size and short growth period. Growing this type of plants directly in the final pot (10-20L) is recommended to achieve best results with automatic strains. (Big Devil XL, Think Different, Flash Seeds?)

Marijuana plant grown in smart pot

Marijuana plant grown in smart pot

It?s not advisable to grow several plants per pot, because both both branches and roots will compete for the available space. If you want to do it, use large containers (growing trays) and leave the longest possible distance between plants.

Plastic pots weigh less and consume less space than clay pots, so they are more comfortable to use. Square pots are more efficient than round ones. Finally, white pots are better to protect the roots from the summer heat.

The oxygenation of the root system is, unfortunately and too often, the limiting factor when growing in soil, so we should avoid pressing the soil. We should also scratch the surface of the substrate before watering for a better absorption of the irrigation water.
To improve the oxygenation of the roots in the ground we can use smart pots, which allow maximum aeration of the substrate.

How to transplant your marijuana plants?

Transplant without stress

Transplant without stress

During the transplant we must avoid to stress the plant, particularly in the root area.

Transplanting is much easier when the ground is relatively dry, so doing it between irrigations is advisable.

Make the transplant as follows, from a small pot (A) to a larger container (B):

Put some soil on the bottom of the larger pot (B), and place the small pot (A) in the center, add soil on the sides and until filling the pot. Press gently the soil (B) and then remove the smaller pot (A).

In this way, we have a hole of the size of the smaller pot (A) in our larger container (B). Remove the plant from its pot and place the rootball in the hole (B).

Add a some soil on the surface to completely fill up the pot (B). Water the plants with root stimulator (this product contains vitamins that reduce the stress caused by the transplant) but without fertiliser (the new soil contains all the necessary nutrients for some days/weeks)

Is it possible to use the same soil several times?

We recommend you not to use the same soil for several crops, since its internal structure is degraded over time (Oxygenation, drainage, buffers … ) and it contains residues of fertilisers or products from previous crops.

However, you can always use it again in your garden or for other home plants (Ficus, etc).

What are the advantages of growing in soil?

Marijuana plants grown in soil

Marijuana plants grown in soil

First of all, growing cannabis in soil allows us to use organic fertilisers in combination with beneficial microbial life, achieving great results with top grade flavours.

The second great advantage is its relative tolerance to mistakes, thanks to its powerful buffer effect. For this reason, it?s recommended for novice growers who begin growing marijuana plants, because in this way they can learn the basics of cannabis farming without the need to use expensive hydroponic systems, which are also more difficult to control.

Soil will remain for a long time as the most commonly used substrate for growing marijuana plants, and let us hope that this easy and natural growing method will allow you to get the best from your plants !

May 10, 2015 | Outdoor cannabis growing
17 Comments


17 comments on “Growing marijuana in plant pots

  1. Papa Indica

    I see that you’re saying to “completely fill up the pot”. Many people, including myself, would say that this is not a good idea. Watering a plant with the soil all the way to the top of the container is an extreme pain in the rear-end. Leaving the soil level down a couple of inches from the top of the pot makes watering much easier. Leaving that last couple of inches has absolutely no negative effect, and it makes watering a much more manageable task, not to mention the savings in soil per container. My cousin was filling his pots to the top for a long time, after I finally convinced him to do it the other way he felt foolish for not doing it my way all along. It used to take him forever to get all of his plants watered, now it’s a breeze. I use three gallon pots and I’ve had very big beautiful plants that never came close to being root-bound, (none of my plants are ever root-bound), so give it a try, I think you’ll find it easier.

  2. Robert Witbracht

    I’m 86 years old & have never tried growing weed plants before,although I have grown about every other kind of fruit and
    veggy. I have a space about 25X15 ft with good sun from about 9:00am to3:00pm with “fair” soil. Could I use some old tires
    as a base for each plant and fill each with a “suggested” potting soil to grow transplanted plants? and how much space
    should I allow between each tire? also I need some advice on the mixture for the potting soil. Any help would be greatly
    appreciated. Thank you,
    Robert Witbracht

    1. Dani Alchimia

      Hi Robert,

      Welcome to the amazing world of growing cannabis! If you are an experienced gardener, you won’t have much trouble with this plant. I’ve seen people using tires as a base for plants, no problem about it. You can also use some type of mulching (straw, etc.). About the space between plants: it all depends on the size you want. If you want really big plants, you should leave about 2 metres between them.
      About nutrients and soil: A mix using some light soil and solid organic nutrients works great. For organic crops, we basically add nutrients to the soil and then water with bacteria to enhance microbial life. Both yields and quality are excellent. You basically want a slightly acid media, spongy and with good drainage/aeration.

      Hope it helped!

  3. Christofer

    hello, Dani Alchimia
    I have a difficult one. Do you know if it’s beneficial for auto’s to use bactohemp(bacili ect) and piranha(trichoderma) together at the same phase of grow, are these two simbiotics?
    Should i use them on 1-2 week or it’s more useful to dilute and use the second at the phase of grow, 5-6 week. To many quests at once.
    Thank you.

    1. Dani Alchimia

      Hi Cristofer,

      Sorry for the late reply, I’ve been away a few days. Sure you can use bacteria and fungi together. Actually, we should all use them!!!

      I use trichoderma during the first 2-3 weeks of growth (together with bacteria) to enhance root development, also after every transplant. Once in flowering, I continue using bacteria until the final flush, they greatly enhance nutrient uptake. Microbial life makes the cultivation easier and improves the quality of the buds. A must for any grower!

      Hope it helped. 😉

  4. christofer

    Thank you, for your reply, i have seen great improvement from using the fist one, i hope the second should cover all the microbial phasma, Its an organic root booster.

    1. Dani Alchimia

      Hi christopher,

      I’m amazed with fungi and bacteria. I’ve performed a whole crop watering exclusively with bacteria, vitamins and amino acids. Just added some organic solid nutes to the soil at the beginning of the crop, that’s all. Results are impressive!!!

      Best vibes!

  5. kristina

    Hi, I have question, I’m getting ready for outdoor and I’m just curious about using rice bags. There really big totes. There white, have good drainage, and really inexpensive. Everybody keeps telling not to use them, so I would just like to know your thoughts. Thank you

    1. Dani Alchimia

      Hi kristina,

      From what I read, many poeple use rice bags as pot plants, and almost everyone reports the same problem: they’re not resistant to outdoor conditions, and they quickly deteriorate when wet. Still, they seem to last for at least 6 months, so perhaps you could use them. If they start deteriorating, you can always put the bag in another container, no major problem.

      Since, if there is a problem it will be easily fixed, I’d give them a try. Perhaps in just some of your plants, and if it works then next year you can put all your plants in rice bags.

      Hope it helped!

  6. Sid Weaver

    I wanted to make a comment on a much overlooked subject… The drainage holes at the base of your home made plant container. Whether you are using a solo cup or plastic container I have learned the hard way that holes too small or jagged will cause wet feet . That is the condition where the roots at the bottom of the planter remain wet because of poor drainage. I kept thinking I was over watering when in fact the bottom of the cup was not draining enough. If you are drilling or burning your drain holes you could leave burrs for soil or perlite to become trapped causing water to dam up. Plants start to droop with the top of the pot showing dry soil. The solution is to carefully enlarge or slot the holes with a razor blade. This had me confused for quite awhile. So I have found that drainage holes need to be larger and smoother than I had originally thought I hope this might help someone.

    1. Dani Alchimia

      Hi Sid,

      Thanks a lot for your contribution, I had the same problem many years ago and it was also due to small drainage holes. 😉

      All the best!

  7. Ste

    I’m going to do a soil grow in 25 litre pots should I start the seeds in there or transplant them because the moist soil in the 25 litre pot could cause some damage to the roots or am I just over thinking it?

    1. Tim

      Hi Ste, thanks for your question, and no, you’re not over-thinking it by any means.

      How to germinate and whether to transplant or not is a matter of personal preference. Some growers like to sow their seeds directly in the final container, thus allowing unrestricted growth of the tap-root from the beginning and also avoiding any possible stress resulting from transplanting. On the other hand many growers germinate in jiffy pellets or a small pot and then transplant into an intermediate sized pot before moving to the final container, arguing that in this way the roots grow more densely throughout the whole of the planting medium, thus using the substrate more efficiently and achieving greater yields.

      Both approaches have their disadvantages too; it’s not easy to keep a 25 litre pot evenly watered when it only has a small seedling growing in it – there’s a risk of over-irrigation or conversely underwatering and ending up with dry pockets of soil in the container. However, each transplant we perform on our plants causes stress of some kind, slowing growth to a lesser or greater degree, although root stimulators like Canna’s Rhizotonic can help to mitigate transplant stress.

      Personally I’ve had great results germinating in small pots or jiffy pellets and then transplanting to the final container. I’ve also tried germinating directly in larger pots and while the germination rate wasn’t ideal and a few seeds were lost, the plants that did survive seemed healthier and more vigorous than the transplants.

      For more information on the subject and a foolproof method of germination, check out the useful ‘How-to germinate’ page on our blog.

      All the best, and good luck with your grow!

  8. Gary

    I have tried most ways direct planting and transplanting. They both have problems. For me two transplanting are the best to grow. Direct planting has at times have a shallow root system and stress with watering. If you start with a tube stock you get a tap root development then transplant to a 200mm pot then into your final pot or ground. You will have a great root system. Bigger container are very important for the final stage. Soil is also important but I experimented with garden soil vs soil mix in the end I had a better crop with garden soil . Could of been a fluke that I don’t know. If you’ve a great root system growing you will have no problems.

    1. Dani Alchimia

      Hi Gary,

      Thanks for your comment. You’re absolutely right,a successful harvest starts with a nicely developed root system!

      All the best!

  9. Stephen

    I am new to growing and have a question. I have purchased Miracle Grow Potting Soil, Composted Manure, Earthworm Castings, Coco Coir, and Perlite for a grow medium. In what ratios should these be mixed to achieve optimum results. I intend to start with White Widow and Sour Diesel. I would appreciate some help, thank you.

    1. Tim Alchimia

      Hi Stephen, thanks for your question. Firstly, if you really want optimum results, then Miracle-Gro isn’t really the way to go I’m afraid. There’s a wide range of different brands making potting soil mixes specifically aimed at cannabis cultivation, with the correct balance of nutrients tailored to your style of gardening, particularly useful for beginners, enabling you to stop worrying about the soil and concentrate on the plants themselves.

      That’s not to say you can’t start with Miracle-Gro, but it’s a long way from the best choice, for one thing, it’s not organic (not to mention it’s made by Scotts/Monsanto and we try not to support that kind of multinational corporation if we can avoid it). And it also comes pre-fertilised so you’ll need to take care of adding too many nutrients to avoid over-doing it and burning your young plants. The amount of nutrients you add will also depend on whether you plan to feed the plants with liquid fertiliser as they grow, or to just irrigate with plain water. If you plan to use fertiliser then you must add less manure/worm castings into the mix.

      So, for the first stages of the plants’ life, when it doesn’t have many needs, you’ll be fine with the plain potting soil, but once it passes the seedling stage it will need more nutrition, so then you’ll have to transplant it into a richer soil. Personally, I’ve never used Miracle-Gro, but I’ve made soil mixes based on organic potting soils that are similar in terms of structure and nutrition levels.

      If it was my soil mix, I’d be only irrigating with plain water, and I’d probably start with something like this and see how it looked in terms of aeration (perlite content) and go from there:

      4 parts Miracle-Gro
      2 parts Coco Coir
      2 parts Perlite
      1 part Manure
      1 part Worm castings

      But it’s important to stress that mixing soil is a process of trial and error, and while trying a mix for the first time may give great results, there’s always the chance it could be too rich, or not rich enough, or too heavy with insufficient drainage and aeration. I know gardeners who’ve been mixing their own soil for years and still can’t get the mix perfect!

      I hope that’s some help for you, please let us know how you get on, all the best and happy growing!

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