Soil and Fertilizer

Caja fertilizerThe soil (or substrate) is one of the most important aspects of a successful garden. It is like the foundation of a building, except it also provides nutrients, water, a home for beneficial micro-organisms, and a place for the roots to stay cool and oxygenated. We’ve put together this short overview to get you started and growing without getting too technical or complicated.

Soil/Substrate Characteristics

There are many different types of soils and soil inputs you can use. The Caja works best with a substrata having three main characteristics: clean and 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 mictornutrients that plants need to flourish. In general, organic matter will make up 1/4 to 1/2 of the total soil volume. Over the course of 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 with a soil rich in organic matter – after all, the plants need to eat!

The plants also need good structure that provides bulk, provides a strong anchoring point for roots, and to facilitate the wicking up of water from the reservoir. This structural and bulking material should be lightweight and allow air and water to penetrate throughout. The two most common sources of sturctural materials are coconut coir (cocfiber or cocopeat) and peat moss (sphagnum peat). While these materials provide very little in terms of nutrition, they are slow to break down and allow the substrata to be used again 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. Pumice can be used as well. These are rock minerals that are lightweight, resist compaction, while also providing bulk and structure to the soil.

Creating Your Own vs. Purchasing a Commercial Mix

The next step will be to decide if you want to create your own soil blend using a mix of different ingredients, or purchase a commercial mix that incorporates all three of these main elements.

Creating your own soil blend is the ability to 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 better drainage and thus will include additional perlite or coconut coir. For larger garden installations (20+ boxes), you can save money by buying in bulk and mixing the ingredients yourself.

A commercial potting soil blend is a great for those looking for a quick all inclusive option. Soils such as a seed starting mix, or many other indoor growing 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 by fertilizing partway through the growing cycle.

Avoid digging up the dirt/soil from your backyard to use in your Caja. It is not designed for use in a container growing system, and the structure and weight will probably be too heavy while providing poor drainage, and poor moisture retention. Backyard dirt is most likely full of dormant weed seeds and can harbor plant predators and harmful fungi and bacteria. A final and maybe even most importantly, 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.

Once you have your soil/substrate and assuming the plants and soil are free of soil-borne pests and diseases, you should reuse it 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 releasing nutrients back into the soil for the next round of plants.


Once you have selected your soil, the next step will be to add in a granular fertilizer and some dolomite. Use a granular fertilizer, with grains that will gradually break down to provide your plants with 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. A liquid fertilizer 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. Two certified organic fertilizers we like to use are Myke Tomato Food 5-6-8 + calcium and Gaia Green All Purpose 4-4-4 fertilizer.


Dolomite or dolomitic lime is calcium magnesium carbonate with a chemical composition of CaMg(CO3)2. These are essential nutrients especially to plants like tomatoes and peppers, and they help to neutralize overly acidic soil. If you are using primarily peat moss as the structural component in the soil, adding in dolomite is an almost essential step. 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 material and suppliers.

Caja fertilizer addition Worm Castings Loose