by Seshata on 06/03/2015 | Cultivation

How to prepare the soil for an outdoor cannabis garden

Growing Correctly preparing the soil for an outdoor cannabis garden can make all the difference to the quality and size of your eventual harvest. Cannabis has specific requirements in terms of soil quality and texture; here is a brief guide to ensuring all variables are optimized.


Correctly preparing the soil for an outdoor cannabis garden can make all the difference to the quality and size of your eventual harvest. Cannabis has specific requirements in terms of soil quality and texture; here is a brief guide to ensuring all variables are optimized.

Soil texture and composition

Soil 1 Sensi Seeds blog
This is one simple and cheap way to determine soil composition (© colostate.edu)

Cannabis grows best in light, loamy soils that drain well but also retain a degree of moisture. Loamy soils are a mixture of sand, silt and clay in an approximately 40:40:20 ratio. Sand is a major constituent of many soils, and is characterized by granular particles of rocks and minerals that measure 0.05mm to 2mm in diameter; silt is finer than sand, and consists of particles measuring 0.002mm-0.05mm; clay is finer still, and its particles measure less than 0.002mm in diameter. One method of determining soil composition involves shaking soil in a jar full of water and allowing the particles to settle; a more detailed explanatiobn can be found here.

As the average particle size of a soil decreases, the ability of water to drain through it is reduced; in very sandy soils, water drains very quickly, while soils with a high clay content can become waterlogged easily as water cannot permeate through their small, tightly-packed particles. If you are using natural local soil, you can mix extra sand, silt or clay into it to improve its soil draining or retaining capabilities as needed. Drainage and soil stability may also be improved by adding gravel, which in technical terms is rock and mineral particles measuring 2m-75mm in diameter. Larger rocks can be removed where possible to avoid causing obstruction to the roots of plants.

If soil is poor, you may wish to consider buying good-quality commercial soil and mixing it into existing soil, or adding manure, mulch, bloodmeal, bonemeal, or a range of other soil additives designed to improve available nutrients. You can even grow your plants entirely in bought commercial soil, in bags or pots so that they are not exposed to local soil.

Regulating pH of soil

Cannabis prefers soils within a slightly-acidic pH range of 5.5-6.5. If soils are more acidic or alkaline than this, a range of deficiencies or toxicities can result, as nutrients begin to be taken up in ratios that are not optimum and may be harmful to the plant. If nutrients are not taken up in optimum ratios and quantities, your plants will not achieve the maximum quality and yield, and may fail to thrive entirely.

If pH of soil is too high (too alkaline) then an acidic compound can be added to the soil to adjust the pH downwards to the desired window. The most commonly-used ingredient is sulphur, which is converted to sulphuric acid by specialized bacteria in the soil to lower pH; a thorough guide can be found here. If soil pH is too low (too acid) the most common additive is agricultural lime (calcium carbonate).

Sterilizing your soil

Sterilizing your soil by exposing it to steam can kill off many harmful bacteria, fungi and insects, while allowing several beneficial bacteria to remain alive. If purchasing good-quality commercial soil intended for growing cannabis, it is usually unnecessary to sterilize soil, but if using local, natural soil, it may be helpful to sterilize where possible. It may also bring the added advantage of killing off any unwanted weed seeds present in the soil. However, it is a difficult and time-consuming process that is often overlooked, and if it is not feasible to conduct this step, there are other ways to control pests, such as use of beneficial microbes and insects, and organic, plant-based compounds that repel or kill pests without harming the plant.

Soil 2 Sensi Seeds blog
Solarization of soil kills pathogens by amplifying the sun’s rays to increase soil temperatures (© University of California)

There are various techniques for sterilizing outdoor soil. Solarization is one method, and involves thoroughly tilling the soil so that it is broken up into fine pieces, watering and covering with a sheet of clear plastic, which amplifies the heat and light of the sun and allows the soil to reach high enough temperatures to kill off most undesirable microorganisms. Soil must reach temperatures of 46°C (114°F) for four to six weeks to be fully effective; it should be checked and re-tilled regularly to ensure that temperatures are sufficient and consistent.

If soil solarization is not possible due to time constraints, it may be possible to sterilize your soil by using steam. Large-scale agricultural operations make use of expensive, specialized equipment, but it is possible to use cheaper household sources of steam such as a pressure cooker to sterilize soil. There are also methods that have been designed for smaller-scale grow operations; for example, the Hoddesdon grid method is a technique that involves layering tilled loam on a steel grid over a shallow pan of constantly-boiling water so that steam can rise through it. When temperatures reach 82-88°C (180-190°F) throughout the soil, sterilization is complete.

Pots, bags, or holes in the ground?

Soil 3 Sensi Seeds blog
If roots are able to grow unrestricted, they may be able to access the groundwater (© csiro.au)

When growing outdoors, there are several options available: grow your plants in pots or planters, keep them in growing bags (which may be the plastic sack your commercial soil was purchased in, or may be specially-designed bags that are typically made of hessian or breathable plastic), or dig holes in the ground and plant directly into the soil, which you can optimize using the methods outlined above, or use without modification if testing shows it to be naturally optimized for growing cannabis.

If growing in pots or bags, you have the advantage of using commercially-bought soil which is not only optimized for growing cannabis but pre-sterilized to ensure that no harmful microbes are present. The downside is that your plants will be constricted by the size of their container. Pots may also require regular transplants as well as water (which they cannot receive from groundwater as plants in permeable bags or holes in the ground can).

On the other hand, digging holes in the ground and planting your young plants straight into the soil allows them to grow without constraint, and will allow roots to access the maximum groundwater. Thus, they will achieve larger sizes and will require less vigilant maintenance, but may be at increased risk of exposure to soil pathogens and even contamination such as from agricultural run-off.

Comment Section

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staffan fredriksson

how i can recipricate all this valued knowledge, i dont know. a distant dream would be to
share sample of "own" strain from 60+. but for now thank you from s.f +46

17/03/2015

Pop

We have very high temperatureschool here. So we decide to mix our soil 50% potting soil 50% compost 25% perlite 25% sand. We have noticedone it crust over. Our question is should add peat to our same mix to loosen it up more? Also our PH is a 7?

07/03/2017

Scarlet Palmer

Hello Pop,

Thank you for your comment 🙂 Unfortunately, for legal reasons we cannot respond to grow questions on the blog, but we do have the Sensi Seeds Forum where you can ask a thriving community of gardening fans for advice, share your experiences, and see if your question has already been covered. I hope this helps.

With best wishes

Scarlet

09/03/2017

Shantam

I am essentially a lazy man, so I recommend the use of "super-soil" outside. It's a real no brainer, at least for organic growers. The soil has everything a growing and budding plant needs. No need to worry about PH, (everything that you put in is already PH balanced), no need to add any nutrients of any kind, no salts build-up, no flushing at the end, no worries about parts per million etc. etc. Just add water. My kind of recipe! City water might need some treating, but my outdoor gardens get well water which even has micronutrients, minerals and other valuable components in it. The water is alive. I would rather let my soil and water do all the work. Alas, this method is not without some cost and effort. Some ingredients (there are multiple recipes out there) can be costly, (there are lots of organic alternatives) and preparing the soil is labour intensive. Truth is though, I'd rather be swimming or playing guitar.

15/05/2017

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