Confused about soil mix formulas?

When you look at soil mixes, you will find a confusing world of formulas. There are far too many different yet similar mix formulas for you to follow, all claiming that they are better than the next. There are formulas under the heading of Square Foot Garden mix, Sub Cool’s mix, Modern mix etc. How does one make sense of this?

When you look closely at the ingredients of the various formulas, you find the there are three main components- a carbon portion such as peat, a humus portion, broadly referred to as compost and an aeration portion which can be pumice, rice hulls or vermiculite. Often the ratio for these is a third of each. Making up a mix of the three components will, in itself, create a basic mix which will produce good results. This is, in fact, the mix that is often referenced in the square foot gardening approach. Lets look at each of these three categories a bit closer.

You will find various arguments around the use of peat as a carbon source for a soil application. Some people question the environmental sustainability of using peat in an ongoing basis. We have looked at these arguments and concluded that we would continue to use peat in our own mixes. There are huge reserves of peat in Canada and when weighed against the alternative- coconut coir, which must be shipped great distances across the planet- we feel that peat is the unquestionably the responsible product to use.

With your aeration options, you have a number of choices, such as lava, pumice, vermiculite, perlite and rice hulls. Again, the choice of what to use is often dictated by what is available locally in your own area. Vermiculite and perlite are frequently used by the commercial soil industry, as they are readily available. Vermiculite is a mineral much like mica, and is heated to the point that it expands into accordion pellets which are quite stable and do not break down. It has greater water holding capacity than perlite does, but its aerating capacity is not as great.

Perlite is volcanic glass that is also heated to great temperatures. This results in a material with a structure that will hold moisture on its surface, but will not absorb it into the core of the material. This means that it allows the water to pass through the soil more readily than vermiculite does. Consequently the vermiculite is the better choice for water loving plants, and perlite for plants the demand dryer conditions. There are concerns about how perlite floats to the surface of the soil as soon as you introduce water, and how it eventually breaks down.

Pumice is a very light weight volcanic rock that is exceptionally porous and very stable in the soil- it does not break down. It has great aeration capacity, and also provides the added benefit of providing an environment for microbes and microorganisms to populate. Lava is similar, though much denser and heavier by comparison. Consequently, lava as an aeration input adds considerably to the weight of the mix.

Rice hulls are from the rice processing industry. These are the very hard outer shells of the rice kernels, and consequently they provide great aeration and structure to the soil mix. As these hulls are very high in cellulose, they take a fair time to break down. They are not porous however and therefore do not hold the water moisture as much as other materials do.

Care should be taken when it comes to choosing your humus. It is the humus that generally supplies the biology to your mix, so you want as biologically alive and active a humus, or compost, as possible. There are many composting facilities that produce nothing more than a partially broken down organic matter that approximates compost in appearance. This broken down organic matter can provide some nutrients and can help to build the soil mix structure, but if not composted properly, it may not have the quantities and varieties of soil microbiology in it that are critical to establishing a nutrient rich base for your plants. Nor might it have the humic content that comes only with a properly composted process that sees the compost turned regularly and not allowed to go anaerobic.

Bovine compost is generally the preferred choice, as they have a stomach system that processes the organic material far beyond that which a horse does. For similar reasons, worm compost is also one of the best composts to use. The worm gut is designed to very thoroughly break down organic matter and as a result they excrete a very rich compost material that is excellent when used in soil mixes. The discussion on what constitutes a better compost is extensive. Often, it takes careful analysis by microscope in order to determine the differences and qualities of good compost. This frequently is beyond the means of most homeowners. For this reason, it is best to build your own compost piles and to raise your own worms. These projects are not as onerous as one might think and can be done in the corner of most backyards.

Once you have your three main components sourced, it is then time to turn your attention to increasing the potential mineral content of your mix. The confusion begins here though, as the soil builder tries to add various other inputs to insure as complete a nutrient base as possible. We have often had mix recipes sent to us that a customer wanted a quote for that contained a huge array of additional ingredients. There are many gardeners out there who take the ‘kitchen sink’ approach to their soil, mistakenly thinking that the more they put into their soil the better the plants will do. Often, this results in an imbalance of critical nutrients, which does more harm than good. There are certain nutrients that once their balance has been tipped in the wrong direction, it can be extremely difficult to get back in order again. And what is worse, this imbalance of certain nutrients in turn blocks the uptake of other nutrients, resulting in a cascading escalation of problems.

Think of your soil mix as a ‘starting’ mix. Properly maintained and amended, your soil will last for years and only improve as you build its rich content. Add some rock dusts for a broad micro and macro nutrient base that that the soil biology- with time- will transform into plant available nutrients. Kelp is an excellent source of essential micro minerals. A bit of bentonite clay helps to increase the CEC of your soil, while helping to increase moisture holding capacity. But the bottom line is do not get caught up in the hype about some recipe that throws a hodgepodge of exotic inputs together while risking tipping the nutrient balance into dangerous levels. Use a recipe that provides the basics and allows room for expansion over the years.

Here is a proven recipe that you can use with confidence. The ratios here assume that you are wanting to build a yard of soil, which is approximately 200 gallons, or 27 cubic feet. There are lots of volume calculators on the internet that will help you calculate the size and shape of your containers and raised beds. This then allows you to relate your soil needs to the amounts of the various inputs you will require in order mix enough soil to fill your needs.

9 cu ft Canadian sphagnum peat

6 cu ft high quality compost

3 cu ft worm castings

9 cu ft rice hulls

The above is your base, giving you 27 cu ft or 1 cu yard of soil. To this, you should add

27 cups of three different rock dusts such as basalt, carbonatite, wollastonite, glacial rock dust, etc. (71 cups total)

27 cups gypsum

27 cups oyster shell flour

13.5 cups kelp flour/meal

13.5 cups crab meal

This will provide you with a good, all round starting mix that has been used by many growers with good results. The soil needs to be nurtured however- it is a complex matrix of nutrients that requires the development of it’s microbial content and ecosystems. This is where proper watering strategies, cover crops, mulches and top dressing come into play. In fact, gardeners who take pride in their living soil/no till approach find some satisfaction in keeping track of how many seasons they have gotten out of a batch of soil. They understand that this approach builds soils that only get better with time and use. As the biology in the soil builds, the structure of the soil mix itself begins to change. The bacteria in the soil produce organic compounds called ‘biofilm’, a type of glue-like slime which attaches to the soil particles and organic matter. This helps bind the soil particles together into aggregates, which helps to improve the oxygen flow through the soil as well as the moisture holding content.