The soil (or substrate) is a critical aspect of a successful garden. Just like a building needs a strong foundation, your plants need healthy soil to provide nutrients, water, a home for beneficial micro-organisms, and a place for the roots to stay cool and oxygenated. We’ve put together this overview to help you choose the best potting soil mix.
Soil/Substrate Characteristics
There are many different types of soils and ingredients you can use. The Caja works best with the following characteristics: nutritious organic matter, good structure, and good drainage.
Organic matter can include homemade compost, worm castings, or composted animal manures (ex. composted duck manure, sheep manure, etc.). This organic matter will provide much of the macro and micronutrients that plants need to flourish. In general, organic matter will make up 1/4 to 1/2 of the total soil volume. During the growing season, the organic matter will break down, shrink, and get used up, leading to a loss in total soil volume. This is normal and to be expected as the plants convert these nutrients into growing healthy leaves, stems, and fruit!
Soil structure allows water to be wicked up from the reservoir and an anchoring point for plant roots. It should be lightweight and allow air and water to move throughout. The two most common materials are peat moss (sphagnum peat) and coconut coir (cocfiber or cocopeat). These materials do not provide much in terms of nutrition. They are slow to break down and allow the substrata to be used over multiple crop cycles and multiple years.
Finally, you’ll want to add materials that facilitate good drainage. Two of our favourite ingredients for this purpose are vermiculite and perlite. These specialized rock minerals are lightweight, resist compaction, and round out the structure of the soil.
Creating Your Own vs. Purchasing a Commercial Mix
For many growers, we recommend purchasing a high quality commercial potting soil mix. You can create your own soil blend using a mix of different ingredients, and the choice often comes down to time, budget, and how precise you want to be with the ingredients.
Creating your own soil blend allows you to select and customize the exact ratio of ingredients and make adjustments depending on the types of crops being grown. For example, heavy-feeding leeks and tomatoes will benefit from a higher percentage of organic material, while herbs will benefit from a lighter soil with higher amounts of perlite or coconut coir. For larger garden installations (20+ boxes), you can save money by buying in bulk and mixing the ingredients yourself.
Commercial potting soils are great for those looking for a simple and effective option. Soils such as a “seed starting mix”, “container garden blend” or other potting soil blends can also be used. They may be low on organic material, but that can be compensated by increasing the amount of fertilizer, adding in extra worm castings or compost, or through midsummer feedings.
Do not dig up and use the dirt/soil from your backyard for your Caja. It is not designed for use in a container growing system as the structure and weight will be too heavy, the drainage will be poor, and the wicking action won’t be very effective. Backyard dirt will also introduce dormant weed seeds and can harbor diseases, harmful fungi, and bacteria. A final and maybe even most important note is that backyard dirt may contain heavy metals or toxic chemicals. Unless you’ve taken a soil sample recently or know exactly how the area has been used, there is a chance the soil is contaminated.
Your potting soil can be reused for many years. In fact, the soil/substrate can get richer over time as the community of microorganisms increases and the old roots of previous crops break down and release nutrients back into the soil for the next round of plants.
Fertilizer
Once you have selected your soil, the next step will be to add in granular fertilizer and dolomite. Granular fertilizers will gradually break down to provide your plants with a slow release of nutrients throughout the growing cycle. Once the soil/substrate is properly hydrated throughout, water will flow throughout the soil bringing nutrients from the fertilizer directly to the plant roots. Liquid fertilizers can be used as a supplementary feeding option during the growing cycle, but due to its fast-acting nature, should not be used as the primary fertilizer. Typically 1lb (~450g) of fertilizer is used per box. We like to use Gaia Green All Purpose 4-4-4 fertilizer.
Dolomite
Dolomite or dolomitic lime is calcium magnesium carbonate with a chemical composition of CaMg(CO3)2. These are essential nutrients, especially for plants like tomatoes and peppers while also helping to neutralize acidic soils. If you are using primarily peat moss as the structural component in the soil, adding in dolomite is essential (unless you are growing plants like blueberries that require highly acidic soils). Look for dolomite with a uniform granular or powdered texture. Typically 1lb (~450g) of dolomite is used per box.
Soil and soil science can be an endlessly fascinating and complex topic of discussion. This page only covers the basics when it comes to planting and setting up your Caja planter. For international projects, we can help you source input materials using local materials and suppliers.
