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Soil Guide
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 cannabis and soil in order to make the right choice.
Product Information in Europe
Let's begin with what the manufacturer has to tell you about their product. Every quality soil brand should contain the following information:
- the composition of the soil (ie. the raw materials)
- how much of the major nutrients it contains (N-P-K & Mg are standard)
- the pH value
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:
http://www.mulchandsoilcouncil.org/ProductCert/Consumer/AboutCert.html
Click here to see a list of products that have earned the certification:
http://www.mulchandsoilcouncil.org/ProductCert/Consumer/Certifiedpps.html
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:
- a strong unpleasant odour such as sulfuric or moldy
- a high % of non-composted materials (bark, wood chips, twigs, gravel, coal, etc.)
- a visible amount of sand
- substantial amounts of coco coir, perlite, vermiculite, or hydro correls
- a mainly coarse, fibrous, dense or clumpy texture
What plants need
pH
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.
Nutrients
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.
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 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.
Drainage
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.
Recommended brands
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.
Some of the best soil products in continental Europe are Compo Sana Universal, Compo Sana Seedling Mix, and Klassman KT2 Potting Soil. Compo Sana is widely available in many garden centers, shopping malls, hardware stores across Europe. Visit their web site for more info on a dealer in your area.
In the UK the products of John Innes are comparable in quality. Use JI Seed Compost or Compost No. 1 for germination/rooting cuttings and JI No. 3 for regular potting soil. A popular choice among UK growers is Westlands Multi-Purpose Compost with added John Innes. This is probably the best value for money in the UK.
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
For germination:
- Scotts Seed Starting Mix
- Miracle-Gro Seed Starting Mix
- Fox Farm Light Warrior
Avoid products that have:
1. "Extended release" or "controlled release" fertilizer (not suitable for a fast growing plant like cannabis)
2. "Lightweight formula" (not enough nutrients to support long-term growth...use only for germination or rooting clones)*
3. Instructions such as: “it is essential to begin fertilization within 7 days after planting”.*
* only suitable for seedlings
* Don't forget:
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!
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.
Frequently Asked Questions
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 soon.
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, hydro correls?
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. Especially harmful is to fill the bottom of the container with coarse materials such as hydro correls. 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. 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.
Another factor is the high pricing on Dutch soil brands... they are 2-3 times more expensive than premium horticultural potting mixes.
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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