What’s important is that the ingredients you add need to supply for:
- Water retention
- nutrient retention
- Plant food
- Support for the plant
- Thermal insulation
In order to mix the recipes you’ll need basic equipment:
- Big container
- Smaller container to soak some of the ingredients
- Some smaller containers for measuring
- Access to water
- Screen or mesh
- Trays (if you will be pasteurizing the mix)
- Spoons, forks or some tool to mix things
- Mask to avoid aspiring dust or other particles
- A thermometer (when pasteurizing)
- A conventional or solar oven (when pasteurizing)
Option one: the old proven home-made mix from grandma!
Regular garden soil doesn’t work well in containers because it tends to be become compact and may be “infected” with microorganisms (fungi and bacteria) that have a balance in the garden but will not have the same competition or “food” in a container, so they may “eat” your plants instead. But until a few decades ago, that’s what our grandmas and grandpas used: mixed with leaf mold, good compost or barn manure.
You can use garden soil mixed with good compost (usually 2 parts compost and 1 of soil) if you follow some extra steps to avoid problems: screening and then heat-treating or pasteurizing at 160 to 180 degrees (Fahrenheit)
- Screen the soil/compost with a mesh
- Put both the compost and/or the soil in trays
- In the summer, you can “cook” them with a solar cooked made of cardboard box (you’ll have to put the soil/compost inside a plastic bag to help the process. This may take a few hours to reach the desired temperature
- If you don’t have space, or if you want more control over the process, you can use a conventional oven: preheat it to 200 degrees (Farenheit)
- Put the soil or compost in a glass or metal pan/tray and add water to make it lightly moist
- Cover it with aluminum and add a meat thermometer in the middle to check the temperature
- Put the pan/tray inside the oven and check the temperature every 10-15 minutes. Turn over off when the soil’s temperature reaches 150 degrees. Compost will heat faster (usually a few minutes) than regular soil (about 30 minutes). After the temperature reaches 170-180, you may need to vent the oven to avoid overheating. Sharp odours may indicate you overcooked the mix.
- Allow the mix to sit inside the warm oven for at least 30 minutes. You can also wrap up the mix in warm towels to insulate it outside the oven
- Once the mix is cool, you can save it into a clean container with a lid or use it inmediatly.
Depending on what soil type you have access to, it may be too claylike or too sandy and you may need to add either more organic matter or more sand to improve drainage.
Accessing good garden soil may be an issue if you live in an apartment in the middle of the city. You can always try neighbours, friends or farms. Don’t use random city soil as it may be highly contaminated.
Option two: (for about 36 litres or 9.5 gallons)
- 1 part pre-soaked coir peat to boost water retention
- 1 part vermiculite or perlite to increase porosity. You can also add coarse sand instead, the sand will add drainage
- 2 parts sieved compost (to add organic matter). Compost retains minerals, provides moisture, microbes and plat food
- ½ to 1 cup worm castings (hold and supply nutrients), moisture retention, beneficial microbes and plant food
- Pre-soak coir or newspaper/cardboard in warm water
- Mix equal quantities of pre-soaked above with vermiculite (or coarse sand or perlite)
- Add the sieved compost and worm castings and combine well
- You may need to moisten lightly until you can squeeze a few drops of moisture out of the mix but not too wet
- Check the pH, for veggies the best is between 6.0 and 7.0
- If you need to raise the pH, add 1 gr of lime, to lower the pH add 0.3 grs sulphur, keep checking the pH until you get it right
- If not used right away, store in a container with a lid to avoid moisture loss
- If you decide to add extra nutrients, you can add them now before you use the mix: about 1 cup of rock minerals, slow release organic minerals, etc
Cons: coconut coir is a by-product of the coconut industry, which is not a sustainable one: entire ecosystems are destroyed to grow coconut and the coir has to be transported, burning energy and creating emissions. Vermiculite is a mined mineral and need to be processed and transported too. Some vermiculite may also contain traces of asbestos. Perlite is heated volcanic glass and has similar issues as vermiculite
More sustainable alternatives you can use include: rotted sawdust and leaf mold for water retention and coarse sand for lightening the mixture and improve drainage.
Note: to any container mix you may add fertilizers/nutrients either at the beginning or through the growing season. Remember you can boost OM adding compost tea or organic full spectrum fertilizers. Check these resources as a local source of good soil amendments/fertilizers: