By Luke Sumpter

Reusing Old Soil To Grow Cannabis

Unless there has been a disease or invasion of some kind, there is no need to buy new soil every time you want to pot a cannabis plant or repot existing plants. Simply reuse the old stuff. Revitalising and reusing old soil is easy and a great money saver.

Marijuana is a notorious glutton for nutrients of all kinds; nitrogen-hungry during vegetation and calcium and magnesium-demanding during flowering, cannabis will always leave soil depleted. With that said, there are numerous benefits cannabis provides as well; the roots fluff out and the overall soil structure is improved upon. Such is one of the many upsides to growing cannabis.

  • One Step Ahead

By keeping soil healthy to begin with, you can prevent the need for its radical rehabilitation prior to reusing. Regular fertilising, composting, and mulching of potted plants with ensure healthy soil at every stage of its life. When well-cared-for soil maintains its health, friability, water penetration and retention, it needs little modification when reused.

Companion planting also helps keep soil healthy, vital, and disease-free. When doing this, however, it’s advisable to plant in a bigger pot than necessary to allow room for companion plants to grow. A simple alfalfa, clover, and marigold mix will provide manifold benefits to plant and soil health.

  • Old Soil Can Be Reused

Uncared for and stressed soils will require amending prior to reuse. The soil may become hydrophobic, completely exhausted of nutrients and minerals, or with a collapsed structure. Soil that has been left to bake in the sun will certainly be leached of nutrients and will need to be modified.

Reusing Old Soil To Grow Cannabis
  • Make Sure To Be Clean

If your soil was invaded by a pathogen or is quite unhealthy for whatever reason, the best thing to do is simply put the soil in a bag and discard. Mixing it into garden beds or working it into compost could spread the pathogen to your whole garden. Sometimes, diseases can linger in soil even after you think they’ve left. This is true for several species of mite, fungus, gnat, and rot. Reusing the soil will certainly infect the new occupant. Be sure to wash your hands before touching healthy soil and plants. Wash the pots as well in warm, soapy water to ensure the vector has been scrubbed away.

  • Weed Not Weeds

Pasteurising old soil is a way to get rid of a number of unfriendlies. Grass and weed seeds, insect eggs, and fungal spores can all be killed by putting the old soil in a black garbage bag, then leaving it to bake in the sun for an afternoon. The heat will pasteurise the soil to be ready for use. It will also kill most of the friendly biota, so you are committed to soil-amending once you decide to pasteurise. Bake in the sun before amending so you don’t kill all the friendlies you add.

How To Reuse Potting Soil

By rehabilitating your used growing medium, you can create nutrient-rich recycled soil that provides everything new plants need to remain healthy and produce rewarding harvests.

Check out the tried and tested methods below—and choose your favourite—to clean, revitalise, and successfully reuse your soil.

Use Enzymes To Clean The Soil: The Long Approach

You’re probably aware of the vital roles microscopic substances play in the soil. A whole host of bacteria and fungi are constantly at work devouring organic matter and producing plant-available nutrients.

Both types of these microscopic life forms produce enzymes that break down substances around them to create food. Enzymes are essentially mini machines made of proteins that carry out specific chemical reactions.

Just like bacteria and fungi use enzymes to process organic matter in the soil, so too can cannabis growers!

Contrary to common belief, plants don’t actually directly mine nutrients from the soil. Instead, they recruit an army of microbes to do the dirty work. Like true capitalists, plants then trade sugars (which they create during photosynthesis) in exchange for the nutrients that fungi are able to free up in the soil.

Over time, the soil starts to become depleted of these key nutrients, which is why growers should always amend their growing medium from time to time. Detritus (general waste and debris) also starts to build up in the soil after a growing cycle.

The components of dead microbes, pieces of roots, and insect parts begin to accumulate. Enzymes also help to speed up the rate at which these decompose and are turned into accessible nutrients.

After finishing a grow and harvesting a plant, you can tap into the potential of leftover soil by adding enzymes to unlock untapped nutrients and amendments.

Here’s how to rebuild used soil using enzymes:

  1. Remove the old root ball from the soil.
  2. Select an enzyme product that is plant-safe and not phytotoxic.
  3. Apply the substance to your growing medium.
  4. Add in some amendments, such as worm castings and humic acid, to reintroduce vital minerals into the medium.
  5. Allow the medium to rest for three months so the enzymes can properly catalyse the organic material within.
  6. Agitate the mix every now and then to aerate the soil.
Remove Roots And Worms

Boiling And Amending: A Faster Approach

Enzymes work well, but it means waiting a few months until you can use that batch of soil again. If you need soil much sooner, instead of heading to the shop, use this faster approach to get growing posthaste.

Step 1: Break Down Your Soil

Cannabis plants create large root balls in the soil as they grow and mature. These systems help to give the soil shape and structure, and make the growing medium extremely firm and condensed. At the end of a growing cycle, you need to empty your containers and loosen up all of that compacted soil. Empty your old pots into a large plastic tote and break up the soil with your hands or a gardening tool such as a rake or trowel.

How To Save Money By Reusing Old Soil

Step 2: Remove Roots And Worms

As you break apart and comb through the substrate, you’ll come across stray roots and worms. Take out the roots, throw them onto your compost, and turn them into soil for the following season.

You’ll also want to gently remove the worms and place them into your compost pile or elsewhere in the garden. Far from being pests, these little soil-builders dramatically increase the quality of the soil. They gobble up microbes and organic matter, and poop out castings rich in key nutrients like nitrogen.

Step 3: Pasteurise Your Soil

Pasteurisation will help to kill off any microbial bad guys lingering in your soil. Not all microbes help plants to thrive. Some forms of fungus and bacteria work against the interest of the grower and can cause disease.

Don’t worry; although it might sound complicated, pasteurisation simply refers to pouring boiling water over your soil. Fill your tote with boiled kettle water and completely submerge the substrate. Let it sit for one hour.

Strain off the water and let your soil dry in the sun, preferably in a greenhouse or polytunnel. 

How To Save Money By Reusing Old Soil

Step 4: Introduce New Life To Your Soil

After following the steps above, you’ll be left with clean but lifeless soil. Microbial communities are key to healthy soil. Without them, all you’ll have is an inert mass of organic matter. By introducing beneficial fungi and bacteria, you’ll bring the substrate to life and, in doing so, create much healthier and more productive plants.

Growers are spoilt for choice when it comes to improving their soil. We recommend adding worm castings, and worms themselves, to really kick-start the process.

Next, add a few scoops of mycorrhizal fungi into the mix. These life forms physically fuse to plant roots and shuttle nutrients directly into their system. Add beneficial bacteria such as rhizobacteria to further increase nutrient availability and combat pathogens in the soil.

Step 5: Add Extra Soil

Upon cleaning and rebuilding your soil, you probably lost some volume along the way. Top up your supply with some aged compost, ideally from your own bins, to make up for any lost material, then fill up your containers once more.

Step 6: Transplant Your Seedlings

You’ve done it! Transplant your seedlings into your rebuilt and restored soil and watch them thrive over the course of the coming growing cycle.

How To Save Money By Reusing Old Soil

Tips To Reuse Coco Coir

You can also reuse other growing media to reduce waste, including coco coir. Using similar steps to those mentioned above, you can rehabilitate this excellent substrate and use it for several growing cycles.

Use these steps to reuse your coco coir:

  1. Break up your coco and remove any stray roots.
  2. Soak your coco in an enzyme solution to dissolve smaller roots that can’t be removed by hand.
  3. Wash your coco with distilled water to remove detritus and residual salts from the medium. 
How To Save Money By Reusing Old Soil

4. Sterilise your coco using a 35% hydrogen peroxide solution.
5. Top up your supply with some fresh coco, and get planting! 

How To Save Money By Reusing Old Soil

Tips To Reuse Clay Pebbles Or Perlite 

There’s no need to throw out your clay pebbles or perlite, either. Use the method below to clean them up and prepare them for your next grow:

  1. Remove all dead plant matter and bathe in an enzyme solution.
  2. Let the pebbles or clay sit for 30 minutes, strain, and repeat the process again.
How To Save Money By Reusing Old Soil

3. Place your pebbles or perlite in the sun to disinfect for 2–3 days, preferably under the cover of a greenhouse.
4. Reuse your pebbles or perlite within your soil and hydroponic setups.

How To Save Money By Reusing Old Soil

Other Ways To Put Old Soil To Use

If you don’t want to go through the process of rehabilitating your soil, you can let nature do the work for you. Empty your containers into your compost bin and let the microbes break down the roots and other material over the course of the next year, ready for use further down the line.

Alternatively, you can apply your old soil directly as a top dressing to your raised beds. Break up the soil and rake it into your beds to add organic matter that will eventually break down into available nutrients.

You Can Always Reuse Soil, But You Need To Work For It

By rehabilitating your soil, you are one step closer to creating a closed-loop growing system. You’ll minimise waste, save money, and develop a priceless skill. However, it takes time and effort, but the rewards are well worth the work.

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