What is the best soil for growing autoflowers?

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There are a few key factors that need to be thoroughly thought out before starting any cannabis grow op. Genetics is usually where most cultivators begin, and with the recent surge in autoflowering options, it’s not just which strain to choose, but also if you want to go down the traditional or autoflowering route. Almost all of the current top-shelf strains come in both options, and in the past 4 or so years, we are seeing more and more growers make the switch over to autos.

What is the difference between autoflower and photoeriod strains?

The short answer is that autos are bred to flower no matter the light conditions, while regular photo strains need a specific amount of light and dark periods in order to do so. This can make autos an easier choice for novice growers who have less experience with environmental controls and, and for those just looking for a simpler overall cultivation method

Cannabis goes through two main growth stages – the vegetative growth stage, and the bloom or flowering growth stage. The majority of the size and plant structure is grown during veg, and then the growth focus is switched to bud production during the flowering stage.

Traditional, photoperiod strains need about 18 hours of light per day to stay in the vegetative growth stage. Once you are happy with the size of the plant, you simply swap the light timing to 12/12 light to dark, and this forces the plants into flowering.

This is not the case for autoflowering strains. Thanks to the ruderalis genetic lineage , autos will switch from veg to flower growth no matter the lighting schedule. This usually happens at around the 4-week mark, with most auto strains taking less than 11 weeks to go all the way from seed to harvest.

But no matter if you are growing autos or photoperiodic strains, deciding on which style of cultivation is what most growers should really focus on first.  There are three main options to choose between –

  • Soil
  • Hydroponic
  • Coco-coir

Since you have ended up here, we are guessing you are keen on using soil as your main substrate for an autoflowering marijuana grow. Let’s dive headfirst into the mucky, dirty, downright amazing world of organic gardening and look at all the best soil options available for autoflowering cannabis.

defoliating autoflower cannabis plants

RELATED ARTICLE – Is It Okay To Defoliate Autoflowers?

Defoliation is a yield boosting technique, but can you use it when growing autoflowers?

Using organic soil for autoflowers

Anybody who knows anything about weed has heard, over and over again, that hydroponics produces the best weed. And while it is true that hydro methods allow for explosive plant growth and flower production, the very best buds in terms of flavor, aroma, and potency still come from Mother Nature herself. There really is no substitute for the depth and nuance in terpene production that organic gardening methods offer.

But what is soil exactly?

Soil is a mix of minerals and organic matter, gases, liquids, and organisms that together support life. It can include a huge range of substances, but most often contains a blend of

  • Clay particles
  • Silt particles
  • Sand particles
  • Decomposing organic material such as leaves, twigs, bark, etc.
  • Humus – decomposed plant and animal matter that adds fertility to the soil.
  • Micro-organisms that break down the organic material and make it bio-available for plant life to uptake

With hydroponic methods, you as the cultivator supply the plant with the exact nutrients (in the exact amounts) it needs to grow and mature. Organic cultivation works differently. The soil itself is full of beneficial micro-organisms that help break down organic matter into a usable form, releasing all the essential nutrients as they do so. This process offers the full range of macro and micro-nutrients to the plants, making organic soil perfect for autoflowers. With organic cultivation methods, plants only uptake the nutrients they need – making overfeeding issues much less common.

As autos usually need a lighter feed schedule than photos, nutrient burn is an issue that novice growers often run into, especially when using hydroponic cultivation methods. It’s almost impossible to overfeed or burn with organic soil, as the beneficial bacteria in the soil help keep nutrient levels and pH balanced.

What is the best soil mixture for indoor autoflowers

We have tried and tested many varieties of soil mixtures to grow autoflowers and there are lots of great variations. The key is to make sure your blend has enough nutrients, provides good drainage and also allows for some water retention. With that in mind our overall favourite blend and what we think is the best soil for autoflowers that you can make at home yourself is this:

  • 2 Parts worm castings
  • 1 Part coco coir
  • 1 Part perlite
  • 1 Part compost

What are the different organic soil additives

Organic soil blends come in a host of options, and cannabis-specific blends are sometimes a little overly convoluted. But there are a few common soil additives that you should look for in any premade soil mix. You can also make your own soil mixture which we will explain later in this article.


Main nutrients in compost: Phosphorus, potassium, nitrogen (varies based on type of compost).

The are lots of variations of compost from bark, vegetable, peat, loam, organic and you can even buy all purpose compost from most garden centers. Compost if basically decomposted organic matter that has been broken down over time to make a nutrient rich soil.

Compost is great because you can either make it at home yourself for next to nothing, or purchase it in any garden center. If you make your own compost at home you can adjust it to contain whatever organic matter you like from vegetables, manure to dead leaves and twigs.

Compost is a great additive for growing autoflowers because…

  • It contains macro nutrients NPK.
  • Its affordable.
  • It improves soil density.
  • It reduces risk of pests and disease.
  • It improves water retention.

Worm castings

Main nutrients in worm castings: Phosphorus, potassium, magnesium and calcium.

Worm castings, or Vermicompost, are a totally organic form of fertilizer that is produced by earthworms. It is essentially the manure produced as earthworms digest the organic substances in the soil, and it is one of the most nutrient-dense fertilizers on earth. Worm castings are rich in essential minerals such as calcium, magnesium, and nitrogen – all key ingredients for growing healthy cannabis plants. Although worm castings are often used as an additive to ordinary soil, it can actually be used solely on its own to grow cannabis and provides a healthy blend of nutrients, water retention and drainage.

Worm castings fertilizer in white scoop

Bat guano

Main nutrient in bat guano: Nitrogen and phosphorus.

After worm castings, bat guano (bat droppings) is the number one soil enricher used in organic cannabis cultivation. It is fast acting, contains very little odor, and can be used before planting or during the grow as a top dressing.


Main nutrients in manure: Nitrogen, phosphorus and potassium.

Regular manure, usually sourced from bovine (cows), is also a very commonly used additive in organic soil mixes. It is high in nitrogen, phosphorus, and potassium – key macro-nutrients for cannabis growth. Though it can be applied directly to the soil before planting, it should really be composted first to reduce the risk of burning your plants. We will discuss this further in a later section.

Bone meal and blood

Main nutrients in blood & bone meal: Nitrogen, phosphorus and potassium.

Both bone and blood meal are excellent organic soil additives, supplying essential nitrogen and phosphorus to the plants. They are both “slow-release” fertilizers, meaning that they are slowly broken down in the soil over a period of time, releasing their nutrients as and when needed. As the name suggests, these are produced from non-vegan sources, so if organic vegan gardening is what you are looking for then skip any products containing these.

Soy bean meal

Main nutrients in soy bean meal: Potassium and calcium.

If you want to get away from using animal products then soy bean meal is an excellent alternative that provides your autoflowers with all of the nutrients they need. Soy bean meal is much less likely to attract nasty critters too, which is one of the downsides to using bone and blood meal, especially if you are growing outdoors. One thing to note though is that soy bean meal doesn’t have as high a concentrate of some of the vital nutrients such as nitrogen, so you will need to use more of it so supply the same nutrient levels.

soy bean meal used for growing autoflowers

Kelp meal

Main nutrients in kelp meal: Potassium and magnesium.

Kelp meal is the perfect vegan, sustainable substitute for blood and bone meal. It is produced from dried seaweed, as is absolutely packed to the brim with all the good stuff we want populating our organic soil mix. It contains an abundance of macro and micro-nutrients, as well as vitamins, minerals, amino acids, proteins, and enzymes – all key ingredients for top-quality cannabis growth.

Mycorrhizal fungi

Main nutrients in Mycorrhizal fungi: Nitrogen and phosphorus.

Mycorrhizal Fungi works a little differently from the other additives listed above. Instead of adding to the nutritional value of the soil, it helps to increase the effective use of nutrients already in the mix. Mycorrhizae Fungi forms a symbiotic relationship with plant roots, colonizing and producing a web of fine root hairs that help to absorb water, minerals, and other nutrients.

Other additives to improve soil quality


Perlite is a volcanic glass material that is used to aerate the soil and increase drainage. It is almost essential when growing cannabis in pots, as it prevents waterlogging by massively increasing the drainage efficiency of the medium and increases oxygenation levels at the root zone.

perlite on a metal scoop


Biochar is a specific form of charcoal that is produced by a process known as pyrolysis – when organic matter is heated in an oxygen-free environment. Biochar is abundant in carbon, and it is highly porous, with lots of nooks and crannies that are perfect for microbial colonization. It also helps to prevent nutrients from leaching, increasing their availability to the plants.


Similar to perlite, vermiculite is a naturally occurring mineral that is used to increase water retention and aeration of the soil. It also helps to prevent compaction and enhance drainage.


Made from the ground-up husks of coconuts, coco-coir is a fantastic soil amendment. Coco is a totally inert substance, and it has excellent water retention properties. It also helps to aerate the soil and can hold up to 70% more oxygen than regular soil.

Coco-coir is actually used as a growing medium all in itself and strikes a great balance between soil and hydro cultivation techniques. Check out our full guide to coco-coir right here

autoflower growing in coco coir enriched soil

Using ordinary garden soil for autoflowering cannabis grows

A question we get asked over and over again is whether it is ok to use ordinary garden soil for growing autos. And the answer is…

It depends.

Not all garden soil is created equally, and it may be depleted of certain important nutrients and microorganisms that are vital for healthy cannabis growth.

Signs of good garden soil

What attributes should the best soil for weed have?

  • Color – you want to use soil that is nice and dark. Good soil contains lots of organic matter, which gives it this dark, rich color.
  • Texture – the soil should have a nice crumbly texture and not be too compacted. One aspect that many novice growers don’t fully understand is just how important drainage is. If the soil is overly dense, the drainage will be slowed, and your plants won’t be able to access the oxygen they need. It should be easy to work over and contain a good mix of different textures.
  • Fragrance – The last thing you want is sticky soil. Sure, a soil mix containing fresh manure may be a little pungent, but you don’t want it to be sour or musty.
  • Healthy soil is full of signs of life, both animal and microbial. Dig up a patch and see if there are signs of earthworms and other critters. If it looks like the organic material is breaking down nicely then the microbial life is healthy.
  • The best sign that your garden soil is good to use, is how other plants are faring. If the rest of your garden is doing well, then the soil is most likely in great shape.
healthy soil for autoflowers with nutrient symbols overlaying

How many gallons of soil does an autoflower need?

It really depends on the size you want your plants to get, if you are using a technique such as sea of green where the goal is not necessarily huge yields then a 3-5 litre pot is fine. However, if you are growing high yielding autoflower strains and using techniques such as LST then you should use a 10-15 litre pot size to really get the most out of each plant. Remember that with autos, you want to stress them as little as possible during the entire lifecycle, so always plant them directly into the pot you intend to finish them in to eliminate and chance of transplant stress.

For indoor grows we go with 3-gallon fabric smart pots, and for outdoor settings, we increase that to 5 gallons – just to be sure the roots have all the room they need to develop fully.


Autos need a lighter nutrient-density soil than photoperiods, but they still thrive in top-quality soil mixes.

If you are wanting to buy a ready-to-use soil mix, the best store bought soils for autoflowers in our opinion are: (we are not affiliated with any of these brands – this is the opinion based on advice from our growing team)

Just remember that most of these potting mixes are pretty nutrient-dense, so we suggest mixing in 30% coco-coir and 20% perlite to even things out. For best results, home made super soil using organic compost tends to get the best results.


How deep do you plant autoflowers?

Once you have germinated the seeds and see the tap root emerging, you can plant the seeds directly in the soil about an inch, or one knuckle deep.

Do you need fertilizer for autoflowers?

That depends on your soil mix. Some soil will come packed with all the necessary nutrients, while others will require additional fertilizer. This will change from strain to strain, and even between phenotypes. Let the plants tell you what they need. Keep a close eye on them all the way through the grow, and if you see any signs of underfeeding you can give them a top-up of fertilizer.

Can you grow autoflowers without nutrients?

No, all plant life needs nutrients to develop. Autoflowers are no exception. You can grow them without artificial nutrients, simply by using a soil mixture that contains organic nutrients. You can either make a soil mixture yourself using a variety of the organic additives to enrich your soil, or you can purchase pre-made mixtures and living soils.

When should you start feeding autoflowers?

Again, that depends on your soil mix. You may not need to feed them at all, or they may run a little low on the required nutes as they come into flower. There’s no way to give a blanket answer here, but again, let the plants be your guide.

Joe Musgrave

Joe Musgrave is a keen 420 blogger who writes about all things cannabis. After harnessing his green thumbed skills through years of working on cannabis farms, Joe now shares his knowledge with the rest of the online 420 community.

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