What is the best type of soil for autoflower cannabis? 

Growing a great crop of autoflower cannabis requires attention to detail regardless of the growing medium you use. If you’re going to grow in soil, you should understand how to prepare (or purchase) the best soil for autoflowers. 

The best soil for growing autoflowers in any environment should drain well and dry out on a daily basis. This will allow the plant to maintain a high metabolism and growth rate. Additionally, as a general rule-of-thumb, the best potting soil for autoflowers consists of about 65% soil and amendments and 35% aeration materials like coco coir and perlite for a 2-to-1-ratio.

This differs from what you may find in the stores, since most of the soils you’ll find in nature are one of the following:

  • Clay: Higher pH (over 7.0) due to dense mineral & nutrient content; very fine particles that stick together, causing poor drainage and holding less oxygen.  
  • Loam(y) – Moderate-high pH(6.0-7.0) with ideal mineral & nutrient content, medium fine particles that hold water & oxygen well. These typically have good drainage and are a balanced mix of all 4 types but with higher clay content.
  • Silt(y) – Moderate-low pH(5.0-6.0) with lesser mineral content than loam soil; medium-coarse particles that drain well and hold oxygen. This is also a balanced mix of all 4 types but with higher sand content.
  • Sand(y) – Low pH (under 5.0) due to its low mineral & nutrient content. This soil has coarse particles that don’t stick together and has excellent drainage with high oxygen levels.
Creating the best soil for growing marijuana auto plants
Autoflowering plants in potted plants with soil

Creating the best soil for autoflowers

To reduce the chances of making costly mistakes, we need the right ingredients to make the best soil for autoflowers. I suggest using a simplified super soil recipe (or purchasing super soil here) so that you only need to use plain water (preferably filtered of chlorine) without additional fertilizer to feed your plants.

Which ingredients make the best super soil for autoflowers? Start by looking at some of the base ingredients used for making soil today. We will look at the best nutrients for autoflowers in soil so we can craft the best water-only soil for autoflowers to use for your grow. If you want to grow your own autoflowers from seed to harvest? Check out our legendary grow bible for a full guide!

Autoflower base ingredients

Coco coir (coconut fiber) is a biodegradable medium made from the fibrous husks (pith) of coconuts.

Coco coir can hold a lot of water and oxygen while maintaining its structure, making it very versatile for use as a growing medium.

It is durable, easy to use, rot-resistant, and fairly inexpensive, with a slightly acidic pH of 5.5-6.5 Some growers use it as a stand-alone medium with perlite or clay pebbles for aeration, but this method also requires regular use of water-soluble fertilizers.

person holding coco coir in a plant pot
Coco coir soil medium

Perlite is made from heating and expanding sand or volcanic glass into a semi-solid puffed structure similar to puffed wheat cereal. This allows perlite to be both lightweight yet structurally sound enough to create aeration and adequate drainage in potted plants. Perlite is one of the main ingredients of almost all bagged and bulk soil types. Perlite can also hold water and nutrients on its irregular surface area.

scooping perlite grow medium
Perlite grow medium

Vermiculite is a softer, more porous version of perlite made by heating and expanding mica, a softer type of rock. It is useful for its ability to absorb water, nutrients, and air. It is used in hydroponic wick applications and outdoor growing containers to hold extra moisture, especially in drier climates. 

vermiculite up close
Vermiculite grow medium

Biochar is made from slow-burning (charring) organic materials like wood at elevated temperatures in a process that uses little to no oxygen (pyrolysis). The resulting charred solids will improve a soil’s ability to hold water, absorb nitrogen, and increase soil microbial activity. Biochar is slow to break down and has been proven to sequester carbon.

A great crop starts with the proper genetics for your situation, a balanced soil mix, and an ideal growing environment. This will help ensure a successful cultivation effort and a rewarding harvest. ILGM also has their own curated fertilizers and grow kits for autoflowers that can enhance your harvest! Check out our selection below!

Marijuana Fertilizer

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Nutrients for autoflowering plants

Autoflowers won’t require the type of nutrient-dense soil mix used for photoperiod cannabis cultivars with longer growth cycles. That type of soil mix can potentially cause nutrient burn and stunt or slow plant growth. Remember that autos are native to areas with poorer soil and climate conditions. It’s important when creating the best soil to use for autoflowers that it meets their needs without causing harm. The best soil medium for autoflowers will have adequate aeration and absorption and won’t contain an excessive amount of fertilizer and nutrients.

Are soil requirements different when growing autoflowers outdoors? 

The best soil mix for growing autoflowers should suit the cultivars being grown and the environment they’ll be grown in. It’s important to properly assess the area you will use for growing outdoors.

For example, a small yard with a fence or tall surrounding foliage may block the wind or add shade versus a farm or open field which has less restriction. 

If you’re uncertain about your conditions or the best method for growing autoflowers in soil, use  

Growing autoflowers outdoor in soil
Autoflower seedling outdoors

An aerated mixture and add or top dress with worm castings or a denser mix if it dries too quickly. 

Photoperiod cultivars grown outdoors can have slightly different soil aeration requirements than those grown indoors because they’re subjected to more temperature variation and extremes. Conditions that dry plants quickly, such as wind and bright sunlight, are more present in an outdoor growing environment, especially during the long days of the peak summer season.

Outdoors, the soil mix can be a bit denser, with slightly less aeration. A taller plant in a heavier container will also have an easier time staying upright when windy conditions prevail.

How can you tell if your soil is healthy or not? 

The two main factors that most affect the overall health of your soil and plants are the amount of water and the amount of food (fertilizer) they get. Cannabis plants need the right amounts of each to thrive, but overdoing it will cause harm and potential problems.

Healthy soil should absorb water and hold it in the organic matter. At the same time, it should drain excess water easily and fairly quickly within a 24-hour period. This will keep the microbiology alive and maintain a functioning root zone. Healthy soil maintains an even balance of oxygen and moisture in the root zone and has a pleasant earthy smell. 

Unhealthy soil tends to have a rotten (sulphur) smell, usually caused by overwatering and/or overfertilization. This drowns the microbiology and displaces the oxygen in the root zone, creating anaerobic conditions (no oxygen) which end up killing the roots and eventually the plant. 

Proper watering is key

It is critical that your soil mix has enough pore space in it to maintain healthy oxygen levels. Overfertilization can cause damage to plant tissue, soil microbiology, and soil texture. 

Proper hydration (watering) is one of the toughest concepts for a new grower to master, as it’s nearly always learned by trial and error (mostly error if nobody is teaching you). As a landscape installer, indoor plant technician, and cannabis cultivator, I’ve seen more plants die from drowning than for almost any other reason. 

Watering autoflower marijuana plant
Watering autoflowering cannabis plant

If you’re unsure you gave it enough water, you tend to give it a little more. The plant starts turning yellow as the soil’s nitrogen evaporates. Is the plant sick? It’s turning yellow. Well, I better give it some more water. More is better, right?? WRONG

The advice I would give to any new grower is to eventually learn, by feel, the proper weight of your plant In its container after you have watered it correctly. When in doubt, always err on the side of caution. It’s much easier to save a plant that is wilting than one that is drowning! 

The benefits of organic soil when growing autoflower seeds 

If you can’t make the best living soil, make the best organic soil for autoflowers.

Using organic soils for your auto flower cannabis seeds will give your plants a slow & steady release of nutrients. This will allow them to maximize their growth rate after transplanting and reduce the need for supplemental feeding. It will reduce the risk of nutrient burn and will help ensure a quality harvest with minimal effort and interruption. If you’re search for the best autoflowering seeds with guaranteed germination, hop on over to ILGM by clicking the button below!

bergman holding autoflower plant

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Common amendments used in making organic soils and living soils for autoflowers

Bat Guano is the accumulated excrement of seabirds or bats. Guano is a highly effective fertilizer due to its high nitrogen, phosphate, and potassium content, all key nutrients essential for plant growth. It is used as an organic fertilizer and soil conditioner. It has a high amount of organic matter. Bat guano feeds soil bacteria and is high in nitrogen, which gives plants a quick boost of green growth.

bat guano fertilizer
Bat Guano fertilizer

Worm Castings are the end-product of the breakdown of organic matter by earthworms. Worm castings are a nutrient-rich organic fertilizer and soil conditioner in a form that is relatively easy for plants to absorb. Worm castings (also called vermicompost) are sometimes used as an organic fertilizer. The earthworms grind and uniformly mix minerals in simple forms so plants only need minimal effort to obtain them.

worm castings fertilizer for cannabis plants
Worm Castings fertilizer

Manure is organic matter that is used as organic fertilizer in agriculture. Most manure consists of animal feces; other sources include compost and green manure. Manures contribute to the fertility of soil by adding organic matter and nutrients, such as nitrogen, that are utilized by bacteria, fungi, and other organisms in the soil. Higher organisms then feed on the fungi and bacteria in a chain of life that comprises the soil food web 

manure for a marijuana soil medium
Manure soil medium

Kelp Meal (dried & ground kelp) is a completely natural, organic product and a source of over 70 vitamins and minerals. Organic kelp fertilizer is valued for its micro-nutrients as well as its macro-nutrients of nitrogen, phosphorus, and potassium. Organic kelp fertilizer is dried seaweed that is used to encourage healthier plant growth, promote greater fruit and vegetable yields, and to overall enhance the general appearance of a plant or garden. 

kelp meal soil additive
Kelp meal soil additive

Mycorrhizal fungi help break down nutrients and make them more available to a plant’s roots through a symbiotic association between the fungus and the plant (“root fungus”). These fungi play important roles in plant nutrition, soil biology, and soil chemistry and will help increase the nutritional uptake and growth rate of your plants. https://en.wikipedia.org/wiki/Mycorrhiza

Bone meal & blood meal are the processed and dried by-products of animal slaughter operations used primarily as a source of phosphorus and nitrogen, respectively. Both have well-known uses in farming & agricultural operations, but in recent years have fallen out of favor with many farmers out of concerns over higher levels of hormones and other substances used on animals that are processed in those facilities. These powdered fertilizers can be quite irritable and dangerous to inhale. 

bone meal soil additive
Bone meal

It is a best practice to use a respirator or fine particle mask to protect your lungs when making or mixing soils and fertilizers, as well as a hat, gloves, eye protection, and long sleeve clothing to keep the dust off your hair and skin. 

What is the best soil for autoflowers?  I often hear this question, and the answer depends on many factors. I prefer to grow my cannabis using organic amendments and practices, so I prefer to use a living soil that I make myself. I have the time and materials to make the best living soil for autoflowers, but you may not, and that’s just fine. Use what you have available to you or what you can afford.

Remember, with a faster growth cycle and automatic flowering, there is less margin for error. Autoflowers won’t have time to recover from mistakes. It’s important that we understand this when deciding on the best soil for growing autoflowers.

Using the best soil for autoflowers will simplify your cultivation effort and make it more enjoyable, as the right soil mix will do most of the work for you. Nature performs marvelously under the right conditions. Cultivation should be a fun and enjoyable experience. It can help you understand the nature around you and within you. Cultivation can be your meditation. Learn more tips on growing marijuana at ilovegrowingmarijuana.com and don’t forget to Let it Grow! 

FAQs about the best soil for auto flowers

How much soil does an auto flower need?

A good rule of thumb is to budget for 1 gallon of soil per 1 foot of vertical growth.

Can you use super soil for auto flowers?

Yes, you can, but you may want to dilute your super soil, or create a pocket of potting soil for transplanting into. That way you reduce the chance of having nutrient burn when using or transplanting, and the plant will have time to adapt and grow into the super soil when it’s ready.

How wet should soil be for auto flowers?

It’s important not to overwater your autoflowers! You want to make sure your soil has plenty of oxygen, especially during the first month of growth as they are not the vigorous growers that photo cannabis cultivars are.

Do bigger pots mean bigger buds? 

Genetic factors will determine the overall size of your plants. Bigger plants and bigger buds usually result from having a bigger light source: Plants grown outdoors in full sun tend to be larger and have bigger buds than plants grown indoors under artificial light. 

How can I make my auto flower grow faster? 

The secret to faster growth is making sure you have the right mix of oxygen and water in the root zone, and not overwatering. Higher levels of oxygen in the root zone will help increase the overall metabolic rate of the plant and allow the plant to achieve a greater biomass (roots, shoots, and leaves).

Proper sizing of growing containers is equally important. If you transplant into a container that’s too big, the plant may grow vertically slower at first because the roots are exploring and searching for the bottom before the plant will add more top growth and weight than it can support. 

Avatar for Jason Bird

Mr Jason Bird

Cannabis indoor cultivation expert

Jason Bird has been cultivating organically-grown cannabis in Portland, Oregon, since 1996 and graduated with an AAS degree i ... See profile

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