Buying the right type of soil is the most important step for successful cultivation. Although this should not be a difficult task it can turn into one, especially if you have no horticultural experience and don't know what to look for. This soil guide outlines the basics about horticultural potting soil and what you need to know about your Mandala plants (or cannabis in general) and soil in order to make the right choice*.
* Mandala Seeds shall not be responsible for the use of any information herein presented and you must make your own decision as to its suitability and completeness for your use. We are not responsible for the use of a product mentioned and any effects or consequences it may have on your cultivation.
Product Information in Europe
Product Information in North America
The pitfalls of the industry...and low quality
What plants need
Choosing the right container size
Frequently Asked Questions (FAQ)
If this basic info is not printed on the bag itself it should be available on the company web site or at least easily requested from their customer service by email/phone.
Before you buy any type of soil you should have this information! Without it you are completely in the dark about the product.
In Europe there is the "International labelling according to EN standards".
The members of the European Peat and Growing Media Association, EPAGMA, have agreed and committed themselves to start using commonly agreed labelling standards on their professional horticulture products. The harmonised standards will provide consumers across Europe with more complete and transparent information.
The labelling provides the following information:
Major constituents: e.g. sphagnum peat
Electrical conductivity: e.g. 50 mS/m (± 25 %)
pH value: e.g. 5.5-6.5
Quantity by volume at time of filling: e.g. 70 litres
Recommended use: e.g. for potted plants
Amount of fertiliser added: e.g. 14-16-18 NPK-fertilizer, 2,3 Kg/m3
Producer: company name
Product Information in North America
Unfortunately, there is no industrial labelling standard in North America. But a list of quality potting soil can be found at the web site of the Mulch & Soil Council (www.mulchandsoilcouncil.org). In 2003, the MSC launched its Product Certification Program. Products that pass the requirements of the certification program and comply with ongoing audit testing of products in retail markets earn the badge of the MSC certification logo for their packaging.
Click here to read more on the MSC certification:
Click here to see a list of products that have earned the certification:
Make sure you choose a product that has been check-marked as "Premium Potting Soil" in the certificate logo. You don't want to buy landscape soil or mulch!
The pitfalls of the industry...and low quality
Don't believe the advertisement on product quality before you actually have opened a bag, seen what the soil looks like, or used it for a test grow. Words are not going to help grow your plants - only good soil will. Even established companies with a good reputation sometimes have a production error and churn out thousands of soil bags with the wrong composition.
When you open a bag of soil it should not contain the following:
What plants need
Cannabis requires a certain pH range in order to thrive. The pH regulates nutrient uptake. If the pH is too acidic or alkaline it blocks nutrient availability. pH is therefore important in plant growth because it affects the availability of plant foods and prevents the spread of soil borne diseases. Every plant species on our planet requires a particular soil pH for healthy growth. Soils are known as "sweet" if they are alkaline and "sour" if they are acid.
When you go to a garden center one of the first things you notice in the soil section is that there are different types of soil for various plant species. There is soil specifically for roses, geraniums, rhododendron, bonsai plants, cacti, lawns, orchids, palm trees and so on. If you compare these soil products you will find that many differ in pH. So the first thing you would look for are the products which have the correct pH for cannabis. These are products with a range of 5.5-6.8 pH.
Generally, soil composed for potted household plants falls into this category. Manufacturers add limestone to standard potting mixes to ensure that the pH is stable and in the correct range.
The next step is to study how many nutrients the product provides. Nearly all soil products are pre-fertilized. This is very important because the point of growing in soil is that the plants have a sufficient storage of nutrients at their disposal. Do not buy a product that has little or no nutrients! Some products are clearly marked as "lightweight formula" or "light". They do not contain enough fertilizer and can only be used in small pots to start seeds or root clones. Cannabis is a fast growing plant and needs a good supply of minerals for vigorous growth.
In Europe there is an industrial standard among soil manufacturers especially for potted plants that are "salt tolerant" (ie. do not fall into the category of plants with very low nutrient requirements). This standard mix provides enough nutrients for indoor cannabis in the first 4-5 weeks of growth using a 4-5 Liter/1 gallon container*. After this period, all you need to do is repot into larger containers to provide fresh soil for further plant growth and flowering.
In Europe the following nutrient content is standard:
200-450 mg/l N
200-500 mg/l P
300-550 mg/l K
80- 120 mg/l Mg
Soil mix for seedlings:
100-250 mg/l N
100-250 mg/l P
150-350 mg/l K
In North America NPK values are written in %. For example, Miracle-Gro Potting Mix has 0,21% N, 0,07% P, and 0,14% K. This tells you that the mix supports vegetative growth very well and is suitable for a nitrogen-loving plant like cannabis. However, the phosphate % is very low and shows that you either need to repot females into a different soil mix for flowering, or have to carefully fertilise the plants once they are mature enough with a bloom fertiliser that provides enough phosphate. Choosing a better soil mix for flowering is the easier and safer alternative. The same principle is true for potassium or potash (K), although cannabis does not require or prefer as much K as it does P (phosphate) unless you are producing seeds. Your soil mix should therefore have a good balance between the three major nutrients of N-P-K during growth and flowering for optimal results and easy cultivation.
If a product "nourishes plants continuously up to 3 months" this tells you that the mix contains adequate NPK levels for indoor cannabis in the first 4-5 weeks of growth using a 4-5 Liter/1 gallon container. Or for indoor cannabis up to 2+ months using 10 Liter/2 gallon container*.
Do not buy products which state info such as “It is essential to begin fertilization within 7 days after planting and maintain fertilizer applications throughout the course of crop production.” This tells you that the soil has very low levels of nutrients and won’t support your plants beyond the early seedling stage. Pro-Mix BX (not to be confused with other Pro-Mix products) is one such brand that should be avoided. This soil mix has only 70-130 mg/l of nitrogen and 25-80 mg/l of phosphate. These are levels even below standard soil mixes for seedlings. Bio Bizz Light Mix is also a similar product that relies on a standard fertilizer schedule almost from the seedling stage. We do not recommend using this brand. CANNA Professional and CANNA Professional Plus also rely on early fertilizing schedules to sustain healthy growth. This makes cultivation unneccessarily complicated and risky.
Quality potting soil automatically provides good drainage and has an excellent water-air ratio. The composition of the mix is professionally put together to facilitate rapid root growth, sufficient water retention, and optimal drainage. You should avoid soils that are compact and loamy or very low in humus content and too airy. These types of soils are probably specialized mixes for particular plant species, or intended for use in outdoor landscaping and mulching, etc.
Photo: an example of quality potting soil - the texture is fluffy & soft, made from fine-grade sphagnum peat, the white specks are traces of perlite. Notice how little perlite professional potting soil contains (1% of total volume)! One of the most common mistakes growers make is to ruin their soil mix with high amounts of perlite.
A selection of brands in mainland Europe:
(Compo Sana & FloraGard is widely available in many garden centers, shopping malls, hardware stores across Europe. Visit their website for more info on a dealer in your area. Online purchases also possible at Amazon.)
A selection of brands in the U.K.:
There is a selection of soil brands available over Amazon.co.uk . This is a good starting point and, of course, you can also find online horticultural shops or enquire at a local gardening center for a particular brand. Stockists can be googled online or you can enquire directly at a manufacturer.
Soil brands to avoid:
A selection of brands in North America:
Fafard Professional Potting Mix
Kellogg Potting Soil
Supersoil Potting Soil
Sunshine All Purpose Planting Mix
Scotts Potting Soil
Miracle-Gro Potting Mix
Miracle-Gro Organic Choice Potting Mix
Fox Farm Ocean Forest
Fox Farm Happy Frog
John Innes Seed Compost (UK)
Levington Seed & Cutting Compost (UK)
Scotts Seed Starting Mix
Fox Farm Light Warrior
Black Gold Seedling Mix
Photo: product example of a commercial seedling soil mix for germinating seeds.
Avoid products that say:
All values given here are approximate. How long the fertilizer in the soil lasts and when you need to repot depends on the size of your plants and amount of light they are getting. There is no golden rule because there are too many possible combinations of grow scenarios (clones/seed plants, low-light or HPS, short or long veg time, indoor/outdoor, etc.). You have to adjust container size and repotting schedule according to the requirements of your plants!
Mandala strains are unlike other commercial cannabis plant that you may have grown. Mandala plants grow fast and vigorously. They have strong shoots, abundant leaf canopy, extensive root development. This growth needs to be supported by quality potting soil and the appropriate container size. Most soil cultivation problems are partly due to these two factors being neglected.
Small containers can cause the following problems:
The container size has to be chosen according to the following factors:
Use this general guideline to get started. This is not a bible but simply a standard recommendation. You should adjust the container size according to your setup, observations, and the feedback the plants are giving you.
Select a suitable soil mix. Generally this would be a 'seedling soil mix' especially for germination. This guarantees that the soil is not too heavily fertilized, that is does not contain course materials which can obstruct the sprouting of the seeds, that it is sterile and free of pests, and that it has a proper pH for germination. These are all important factors for maximum success. A seedling soil mix supplies enough nutrients for about one week of growth. You should transplant the seedlings within this time frame to a standard potting soil for further growth.
To plant the seeds in the soil use a seedling tray, small cups (should be black or darkened), seedling pots, or germinate in 1-5 Liter/0.25-1 gallon container if you are confident you won't overwater.
Seedlings grown in trays and cups should be repotted within a few days! Do not let your Mandala seedlings suffer in tiny cups.
Plants from seed: your seedlings should be cultivated in at least 4-5 Liter/1 gallon containers for vegetative growth. Starting 12/12 light cycle for flowering after 4 weeks is the standard procedure. In a properly filled container with quality potting soil you can wait until sexing to repot your females. For some plants one mild feeding of a complete NPK flowering fertilizer will be beneficial for plants that are showing mild symptoms of nitrogen deficiency (loss of leaf shine, a little bit of yellowing in bottom leaves).
For flowering the typical container size is 10 Liters/2 gallons. This works well for SOG. Taller or late flowering plants can be repotted in 12-15 Liters/2.5-3 gallons. It depends on soil quality and gardening experience which size is best.
Clones: clones that are grown in a sea of green style (SOG) are usually cultivated in square 4-5 Liter/1 gallon containers. The square shape is perfect for placing containers in tight rows without empty spaces. Large clones with extra vegetative time will need bigger containers.
Mother plants: keep repotting into a larger container once they outgrow it. Big plants require a 20+ Liter/5+ gallon container. Avoid overgrowth and prune the plants down to a manageable size. To keep the plants healthy over long term they will require mild feedings once in a while. Read our FERTILIZING SMART GUIDE for more information.
Plants from seed or clones: with CFL lamps, fluorescents, LED lamps 90-150W, and up to 150-250W HPS your plants cannot grow as fast or as big. Their nutrient requirements are therefore lower and root development will also be less. It is important to avoid overwatering under low light. Choose the correct Mandala strain! Not all of our strains will respond well under low light.
Seedlings can be repotted into 2-4 Liter/0.5-1 gallon containers depending on how strong your light source is. Before starting 12/12 make sure that the plants are not rootbound or suffering from mild deficiencies. Repot if necessary before switching to 12/12 and give the plants at least a few days to root before changing the light cycle. Low light in combination with rootbound plants and/or deficiencies can otherwise lead to a higher male ratio or male flowers.
Do not combine forcing your plants into small pots with a heavy/regular feeding schedule taken from standard fertilizer charts! Choose larger containers for sufficient root space and plentiful natural nutrients from quality potting soil. This should be sufficient for all Mandala strains grown under low light and you do not need to worry about extra fertilizing. Most often a lot of available space get's wasted by growing in containers that are not wide or tall enough although there is plenty of room in the grow space or homebox.
The size of your plants and temperatures will determine how big the containers need to be. Start with 4-5 Liter/1 gallon and proceed to repot whenever necessary. In hot summer months bigger containers will prevent soil drying up too fast. Put a saucer under pots and water from top and bottom once plants have rooted well. Large plants during peak growth or flowering require at least 20-30 Liter/5-7.5 gallon containers. If you have to keep container size smaller due to space constrictions then you should plant late in the season, choose short Mandala strains, or prune the plants 1-2 times for shorter growth.
Experienced growers can experiment with controlled time-release fertilizer in granules for outdoor/greenhouse containers. This allows you to grow large plants in containers without the risk of early yellowing and other deficiency symptoms. Read our FERTILIZING SMART GUIDE for more information.
Using the right type of container
The main point is to select a container with maximum volume so that you can fill it with as much soil as possible. In many indoor grow rooms we see nutrient problems caused by wasting a big space underneath the plants because of cultivating in small containers that are not wide enough. Especially tapering pots with a narrow bottom are a complete waste of valuable space and unnecessarily restrict plant growth. Roots grow down...if there is hardly anything at the bottom for water, nutrients, root expansion, it automatically slows down potential growth and flowering. Look at the floor space in your grow room/homebox and ask yourself how much of that space is available for the plants to grow in. If you see as much empty space as containers it's time to act and upgrade your containers!
Read the strain details of the Mandala strain you plan on cultivating. The strain details often contain specific recommendations and information on container size and how big plants from seed grow indoor under 400W HPS and 4 weeks of growth.
Consult the guidelines (above) to see our general recommendation for specific plants and environmental conditions.
Purchase the appropriate containers. Make sure that they have the actual size indicated by the manufacturer. Many containers are considerably smaller than indicated. This can cause a major plant care problem with smaller containers used for clones and young plants. Check the container volume by lining it with a waterproof plastic bag and filling in a measured amount of water. A 4 liter/1 gallon container should be able to carry 4 liters or 1 gallon of water, and so on.
Always fill the container to the top with potting soil. A major mistake is to waste valuable container space by leaving empty space at the top. Potting soil eventually settles down and becomes more compact thereby reducing its volume naturally. If you waste space the plants loose even more potential for root growth and nutrients.
After some weeks you should check if the plants have filled out the container with roots. Lift up the plant from the container to see if it is well rooted.
Repot any plants that are rootbound or getting too large for their container. You can repot Mandala plants at any time from seedling stage to mid-flowering. Plants in the late flowering phase will probably not grow any substantial new roots and repotting may lead to overwatering and pose a danger of bud mold.
Photo: an example of a rootbound plant in a greenhouse. Some slight yellowing has started at the lowest internode...an indication that nitrogen supply is running low and that it’s time to move the plant to a bigger container. During vegetative and early flowering time rootbound plants quickly yellow because nutrients are depleted in the container. Fertilizing is rarely a satisfying long-term solution and rootbound plants should be repotted as soon as possible. Mandala strains have very vigorous root development and require a sufficient container size to support their quick growth.
This photo is a good example of a container offering maximum volume from top to bottom. It has no tapering bottom but offers a very nice broad base for optimal root development. Notice also how the top rim is filled with soil and roots. It shows that the soil was filled intelligently right to the top edge of the container to make maximum use of the available space.
Photos: examples of rootbound plants indoor. Before sexing your plants make sure they are not already cramped in their containers. Rootbound plants suffer from stress and this can increase the ratio of male plants or trigger male flowers on females.
Notice how healthy the roots are in these two photos! Healthy roots are a key factor to successful gardening. Taking care not to overwater, avoiding heat damage or desiccation from direct sunlight (outdoor) and providing quality soil with beneficial microorganisms, ensures a healthy root zone.
Photo: cuttings and seedlings raised in small cups quickly suffer from stunted growth and nutrient deficiency. In this photo the cutting is also growing in a poor soil mix. It contains a lot of coco coir and perlite. This only adds to the lack of available nutrients in such a small container.
Can I use any potting soil for germinating seeds?
Generally speaking, the premium potting soils listed here can also be used for germinating seeds. There are only a few exceptions to this rule such as products containing guano. Fox Farm Ocean Forest is one such example. This soil mix can be too "hot" for seedlings. Do a test with 2-3 seeds in the soil you plan to use. If you are in doubt buy a soil mix for starting seeds or a soil with lightweight formula. The main advantage of soil mixes for seedlings is that they are finer grade and it is easier to fill small cups and plant the seeds. They usually have only half of the usual fertilizer content. That's why you should start seeds in small pots and then transplant into the standard potting soil within a few days.
Should I mix a soil amendment into the potting mix?
A beneficial soil amendment is worm manure or homemade compost. Do not use compost from autumn leaves (they contain almost no nutrients and no nitrogen). Mix 10-15% worm manure/compost thoroughly into the potting soil.
I would like to enquire about soil products in my local garden centers. For optimal discretion what type of plant can I say that I’m growing in case I am asked?
Tomato plants are very similar. Both cannabis and tomato plants are fast growing plant species and have similar needs regarding soil nutrients, container size, light. Do not use tomato fertilizer however because this is much too potent.
Do you recommend any particular product that enhances soil cultivation?
Endomycorrhiza is an excellent supplement. It is added very sparsely (approx.1% only) to the soil mix and greatly enhances overall health in the root zone.
Rock dust is another beneficial product. It contains up to 100 minerals! Adding Rock Dust mineral content to a soil increases bacterial activity and promotes new root growth. These extended root systems allow plants to increase their mineral intake, and provide for even stronger, healthier plants. It is used sparsely and is very inexpensive.
Is it necessary to add anything for drainage such as perlite, vermiculite, sand, expanded clay pebbles?
It is best not to add any additional materials to your potting soil. Horticultural grade potting soil is perfectly mixed for the best air-water ratio. Adding more non-nutritive substances depletes the water retention capacity of the soil, it unnecessarily "stretches" the soil and reduces the total amount of nutrients available to the plant, and it creates dry pockets in the container.
Adding a large amount of perlite/vermiculite (some growers add as much as 25%!) is a completely outdated practice from 30-40 years ago when there was only a very small selection of horticultural potting soil available for non-commercial gardeners. It is one of the unfortunate harmful practices in cannabis cultivation that many still cling to although they do not understand the reason or consequences.
Especially harmful is to fill the bottom of the container with coarse materials such as expanded clay pebbles. This is where most roots grow down looking for water and nutrients! If they reach a dry and sterile layer of substrate the delicate root hairs shrivel, valuable space is lost where the plant requires rich soil that stores moisture and minerals.
If you are planting outdoor and your soil is too compact a modest addition of perlite/vermiculite or sand helps to increase drainage. Humus, such as from compost, is the best additive because it also provides many microorganisms and nutrients to the soil mix.
Can I add guano to my potting soil?
NO! Guano is extremely potent and easily burns plants. You should avoid using guano for any indoor cultivation. It can only be used in small amounts for greenhouse or outdoor gardening in the ground and in large containers. Once you mix guano into the soil you can't remove it! For outdoor use it is also best to only apply small amounts onto the top layer of the soil.
I choose a lightweight soil mix for my plants and am worried about early yellowing. What can I do?
Repot into a larger container with the correct soil mix. You can do this up to 30 days into flowering. If there is no space for repotting you have to begin a mild feeding schedule to top-up nutrients. Start with the lowest recommended dosage for organic fertilizers, or with 10-15% of the recommended dosage for mineral fertilizers. Repeat the feeding on a weekly basis if necessary. Use a grow fertilizer to enrich nitrogen supply because this is used up quickly in pots. Observe the plants closely to determine their requirements. Stop feeding if you see that the plants have a shiny and rich green colour. Leaf burn or leaf curl indicates that there is a salt build-up in the soil and you need to stop feeding immediately (don't panic about it and don't flush the pots with water).
What about Dutch soil mixes sold in the grow shops?
Up till now we have not received any satisfying feedback on Dutch soil products except for CANNA Professional. We do not recommend soil products from Bio Bizz or Plagron. Most of them are too strongly fertilized for indoor cultivation and others are too light and require a complicated supplementary feeding schedule with liquid fertilizers. For example, All Mix has an EC of 2,4! That will easily burn your plants and create many complications. Plagron Royalty Mix and Plagron Bat Mix are also "hot" and contain guano. Bio Bizz Light Mix requires a feeding program and is not sufficiently pre-fertilized for natural growth. The point of growing in soil is to have an easy and uncomplicated cultivation and that is not guaranteed with these products.
Can I just take soil from outside (garden, forest) for my indoor/balcony cultivation?
This is a poor choice for several reasons: outdoor soil is infested with pests such as spider mites and these can wreak havoc in the warm and safe indoor environment; you don’t know what nutrients, if any, the soil contains and the plants can quickly suffer from deficiencies; the soil can have the wrong composition and be too compact, loamy, sandy, or full of non-composted materials; the pH may be wrong.
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