Make Your Own Organic Soil Mix
February 20, 2017
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This article was originally published in 2015 and appeared in Garden Culture Magazine UK, Issue 7
Soil is like water. Both sustain life as we know it, yet they are so omnipresent that we take them for granted. And due to both their importance and complexity, the limitations of language cannot do them justice.
The soil is under our feet at all times, and can also be purchased in a bag at the hardware store. Soil is the primary basis by which we grow food, and the same field can also be subjected to the littering of our poisons. But beyond it all, the soil may very well be the most important substance on Earth. Soil creates and sustains all of life. Soil allows farming, the act of rebellion that catalyzed human specialization from hunting and gathering to society at large, and that started the human experiment more than 10,000 years ago. We’ve come a long way since then, and with good reason, as there are much more mouths to feed with human population growing exponentially in modern times. But we are using more topsoil than we are creating, and we are collectively utilizing soil for all the wrong reasons.
We must respect the soil, not use it as a sponge; even certified organic practices can result in tremendous damage and pollution to the land. Modern farming has become more a creature of synthetic profit, than a source of nourishment for people. USDA data shows food losing nutrient density, and we are experiencing a global degenerative and autoimmune epidemic. But the good news is that we can do something about it.
The growing Food Movement is about creating personal agriculture. This means eating with our ideals and growing at least one thing that we eat. Modern property development obliterates the landscape leaving very poor soil behind, so many home gardeners turn to containers or raised beds. Estimates say that it takes 1000 years to create an inch of topsoil, but fortunately for modern gardeners, we don’t have to wait nearly that long. The easy route is to buy potting soil. There is merit to letting the experts do it for you, but it can get expensive when your gardening habit gets serious. Just a little under thirteen gallons of good organic potting soil can cost $20 (£16).
Many who are looking to invest in serious quantities of soil are making their own soil mixes. Not only is it possible to calibrate a custom soil mix to the crop that you are growing, but given sufficient scale buying the raw ingredients, and formulating the soil yourself costs much less than buying the ready-made version.
It’s actually not as hard as you think, with some intention and practice you can create, and even reuse, your own soil capable of sustainably supporting thriving gardens, and producing increasingly substantial yields. The first thing to think of when making a soil mix is microbes. Microbes manufacture soil, no different from construction workers on a job site. It is the grower’s responsibility to bring the correct building materials to the garden.
Any attempt at making or reusing soil without prioritizing biological inoculation and diversity is like trying to brew beer without adding the yeast or making kombucha or vinegar without a mother. The microbes define the process. So it is in the soil. Source a farm-based biological inoculant, and consider brewing compost tea to concentrate the process. Microbes from a natural environment will always be stronger and have more life experience than lab-based, and you will automatically get a greater diversity of microbes in your mix. Any biological product that can name the microbes in the product is a limitation because we are only aware of a small percentage of microbes found in natural living systems.
In the end, diversity is king. Use compost from your friend’s back yard, worm castings, scrape topsoil from the forest, and buy some premium compost from the garden store. Remember, microbes self-organize, so you cannot mess it up.
Once you have your microbes lined up, it is time to consider the soil mix itself. Popular base materials are peat moss and coir fiber, but it is often possible to source local bulk mixes out of varying materials. The popular bulk soil base in our area is pine bark and turkey manure. Not the best, but it provides cheap volume for the base of the mix that we are going to value-add.
It’s not that making a soil mix is inherently difficult, but that if you don’t do it right it simply may not work the first time. Meaning, it is possible to put together a soil mix that lacks total fertility, like trying to use a budget Big Box fertilizer in hydroponics, the plant cannot grow without at least minimum essential nutrition.
This is generally accomplished through ensuring the ingredients used are as diverse as possible. This means don’t make a soil mix composed of peat moss, rice hulls, and fish meal – and expect your garden to produce.
Instead, make a soil mix of peat moss, rice hulls, worm castings, bat guano, rock dust, farm-based compost, fish meal, alfalfa meal, whey, yucca, kelp meal, and as many other meals as you can muster given the crop that you are cultivating. Use a little bit of a lot of things, the more the merrier. There is strength in diversity.
By providing diverse food sources for the microbes you will inoculate into your mix will create a highly fertile environment for roots to form and feed, but take some time to consider the nutrient balance of the ingredients you are using. For example, you wouldn’t want to have a phosphorous-heavy mix (bone meal, CalPhos, guano) for a crop of basil that you are growing vegetatively, or use too much high NPK ingredient (guano, fish) for light feeders like lettuce. It will take some practice to calibrate your fertility properly in your soil mix, but plants don’t
lie, they will give you constant feedback.
You will also want to investigate the relative concentration of the mix you are creating. For instance, if you evaluate the differing nutritional requirements of lettuce versus tomatoes, you will see that lettuce wants a fertilizer concentration of around 600-800 ppm, while tomatoes desire anywhere between 1700-3500 ppm. This is quite a substantial difference.
A “ppm”, or “parts of ions per million of water”, is the measurement for fertilizer concentration. Imagine a granule of table salt being dissolved in water into a Na and a Cl- ion. Each ion would be a “part” in a ppm, and plants eat these ions created either through solubility or through biological decomposition.
Osmosis is the phenomenon that sees water travel from the lower concentration to the higher concentration through a water permeable membrane in order to equalize concentrations. The root is an osmotic gradient, so this force is at play in roots when it comes to fertilizer concentrations. If a plant has more ions inside than it does outside of its roots than healthy transpiration can occur. But if there are too many ions outside relative to inside the root water is then sucked out of the plant resulting in the plant prioritizing, and shown in the leaves – edges “burning” and becoming necrotic.
Considering this, it becomes clear that all purchased potting soils have to be calibrated to the lower end of this fertilizer spectrum. In other words, if a potting soil formulator created a recipe that resulted in a fertilizer concentration of 2000 ppm tomatoes would love it, but the lettuce would be severely over fertilized resulting in dead plants if not amended.
When taking this into account for your soil mix you may want to keep the higher NPK items out of the mix and feed with them over time in the soil as a fertilizer. Think of the organic fertilizers as the building materials for your microbial construction workers, and as a crutch for results and plant nourishment until your soil food web is ready, and can take over the fertilization responsibility.
The lack of focus on microbes is one of the major problems with gardening techniques like square foot gardening or lasagna gardening. They are great templates for beginning gardeners, but they do not focus on microbes, and people end up with beautifully spaced gardens that cannot sustain themselves over time or immature soil where they can read the copy on the front page of the newspaper when they turn their soil over. Organic matter does not just melt, it is biologically digested by a team of micro-organisms that move micrometers in their
lifetime. If we don’t bring them to the party, they simply are not there!
In the forest, consider that microbes don’t eat the leaves, they eat what the microbes make of them. And trees grow to enormous size and strength in the forest with zero fertilizer. The power of microbes cannot be understated.
You will find that by focusing on biological strength and diversity, the more the natural processes take over, and the more mature your soil becomes, the less responsible you will feel to feed the garden with fertilizer. This is particularly intriguing when it comes to reusing soil. Next issue we will discuss the merits and techniques of re-using your potting soil, so you can take your personal agriculture to an entirely new level.
Top image courtesy of Ramblings from Jewels.
He now works as a consultant in his new project Be Agriculture where he helps new and seasoned growers take their agronomy to the next level. What we think, we grow!