With the recent passing of legendary California breeder Subcool, I decided to do a post about super soil, or more specifically, Subcool’s Super Soil. The term “super soil” refers to an organic soil/fertilizer blend that is designed to nourish the plant from start to finish without adding anything but clean water. There are a few commercial super soils on the market, but most growers follow a recipe, and over the years the Subcool recipe has become the gold standard in water-only blends.
Super soils produce intensely flavored flowers without all the maintenance of a hydroponic system. But don’t expect high yields; this method of growing was originally designed for medical patients who didn’t have the ability to work in a garden all day, and who didn’t need an excessively large stash. It’s also a great method for first-time growers, especially those transitioning from traditional outdoor gardening techniques.
One step that cannot be skipped is the cooking process. The raw organic fertilizers in this recipe will burn clones or small seedlings if the soil microbes don’t have adequate time to break them down. We’ll dive deeper into this topic a little later on.
- 8 large bags of a high-quality organic potting soil with coco fiber and mycorrhizae (i.e., your base soil)
- 25 to 50 lbs of organic worm castings
- 5 lbs steamed bone meal
- 5 lbs Bloom bat guano
- 5 lbs blood meal
- 3 lbs rock phosphate
- ¾ cup Epsom salts
- ½ cup sweet lime (dolomite)
- ½ cup azomite (trace elements)
- 2 tbsp powdered humic acid
Here’s the basic recipe, straight from the horse’s mouth, published in High Times back in 2009. This list is enough to fill two large trash cans with super soil concentrate, which will then be “cooked” outside for 30-90 days and combined with base soil for the final planting. Use these ratios as a starting point and adjust the amounts for the total volume of super soil you need. Also keep in mind that Subcool was always tweaking his signature recipe, so this is simply a snapshot of what he was doing at the time.
A good base soil is just as important as the amendments you add to it. Aeration and water retention are key since this is a hands-off method, so choose a mix that is airy with plenty of perlite. Peat moss is the main ingredient in most commercially-available potting soils, but I would recommend using a product that has at least 20% coconut coir. While peat tends to be acidic, coco coir is pH neutral. Coco coir also has a lower cation exchange capacity than peat, meaning it’s less likely to hang on to plant-available nutrients.
Subcool recommends Roots Organics Original Potting Soil, which is a great choice. Another soil brand we’ve grown to love is Royal Gold. They offer coco-based soilless media with varying levels of organic amendments already present. Choose Tupur for a lighter blend or Mendo Mix if you want something that’s a little more fortified straight out of the bag.
Earthworm castings (poo) are a smorgasbord of beneficial goodies for your plants. Along with adding aeration and structure to soil, they contain an array of micronutrients and trace minerals like iron and calcium, among many others. Most importantly, worm castings are an incredible source of plant-beneficial microbes. Studies from top universities have shown improvements in seed germination, rooting, and flower production, along with a reduction in disease and wilt in plants amended with earthworm castings.
Wiggle Worm is a tried and true source of quality worm castings, and it’s readily available in 15 and 30lb bags. You might also be able to find a good source of local worm castings depending where you live.
Steamed Bone Meal
Granular bone meal contains a phosphorus to nitrogen ratio of about 5:1, so it’s great for balancing out phosphorus levels in organic mixes that tend to be high in nitrogen. It also contains some calcium and trace minerals. Fish bone meal is nearly identical to classic steamed bone meal in terms of the nutrients it can provide, but it smells worse and generally breaks down faster. Since a super soil needs to last around 10 weeks we recommend using a product like Down To Earth Bone Meal.
When this recipe refers to “bloom” bat guano, it means a guano high in phosphorus, as opposed to more commonly available forms which are higher in nitrogen. Guanos have many of the same plant benefits as earthworm castings such as improved soil structure and the addition of trace minerals and microbes. One downside to guano is the environmental harm caused by its production. Many developing countries use bat guano as a source of income, bulldozing caves and damaging or permanently destroying bat habitats in the process. If you’re set on using a guano, we prefer Peruvian Seabird Guano, which is harvested using more sustainable practices.
Like bone meal, blood meal is a byproduct of the meat industry. Blood meal is another readily available source of organic nitrogen, and its acidity helps buffer the soil against pH shifts. An added benefit of blood meal comes in the deterrence of certain garden pests. Small mammals like rabbits, squirrels, and raccoons will associate the smell of dried blood with the presence of a predator. Keep in mind that larger animals like deer won’t be so easily fooled.
Rock phosphate, sometimes called rock dust, is exactly what it sounds like: a mined mineral containing high levels of phosphorus and trace minerals. Like many of the inputs on this list, rock phosphate can help improve soil structure and aeration. It can also prevent some types of insect larvae from burrowing into the soil. Since rock phosphate is a mined clay, the exact consistency and nutrient ratios will depend on the source. Down to Earth Rock Phosphate is a great cost-effective version we always keep in stock.
You’re probably used to seeing epsom salt in the personal care aisle of the pharmacy. Chemically known as magnesium sulfate, epsom salt is a simple ionic compound made of magnesium, sulfur, and oxygen, all of which are essential to your plant’s overall health. Unlike organic compounds, ionic compounds like magnesium sulfate are immediately plant-available. Epsom salt also dissolves easily and makes a great addition to a foliar spray. While epsom salt is easy to find in many forms, we prefer the high purity and fine grain size of Monterey Epsom Salts.
Lime is made from pulverized limestone and has been a popular agricultural amendment for centuries. The high pH of lime makes it another great soil buffer, and it can help to balance out low pH inputs like peat and blood meal. Dolomitic lime, sometimes known as sweet lime, contains the mineral dolomite which is comprised of calcium and magnesium. It can also contain other important elements like iron, manganese, zinc, and cobalt.
Azomite is a mined mineral which comes from a single source, an ancient volcanic ash deposit in the Utah desert. While it can technically be defined as a silica ore, Azomite’s complex mineralized structure contains a wide array of plant-available elements. The name “Azomite” actually stands for “A to Z Of Minerals Including Trace Elements”. Since there’s only one source of Azomite, it’s hard to go wrong.
The final item on the list might just be the most complex and least understood ingredient in this recipe. Humic acids are the end result of the breakdown of organic matter over millions of years, forming complex carbon-based molecules. They enhance the soil physically by improving water retention and soil tilth, and by preventing surface cracking. Humic acids also enhance the soil chemically by buffering pH and chelating minerals and metal ions, allowing them to be more easily taken up by plant roots. Entire books have been written about the roles of humic and fulvic acids in soil biology, and we may focus more on them in a future post.
Now that you have a basic idea of what each ingredient does let’s talk about mixing it together. The obvious and most basic method would be to simply pour everything into a large bin or kiddie pool and mix it up. Although that’s an effective enough way to go, the labor can be back-breaking, especially if you are doing a large batch for outdoor plants.
One less labor-intensive method involves a large tarp and, ideally, two people. Open the tarp up and spread it out over a large, flat surface. Pour around 25% of your base soil onto the center of the tarp. Pour your first amendment in a circle around the edge of the base soil you just poured. Grab any corner of the tarp and fold it over toward its opposite corner, then fold it back out flat. Repeat this process at each corner, folding the mixture over on itself each time. Add the next amendment and a little more base soil, and continue folding.
Whichever mixing method you choose, make sure everything is evenly blended before moving on to the next step.
Pour everything into large trash cans. This particular recipe should fit nicely into two 44 gallon Brute trash cans or equivalent containers. The mixture should be moist, but not soppy wet. Add 2-3 gallons of water to each trash can if it seems too dry. The goal is to have a homogenous blend of soil, water, and organic inputs that the microbes can break down into plant available nutrients. Place the trash cans in a warm area for at least 30 days. If cooking the mix outside make sure to place the cans where they can get maximum sunlight. In cooler months you might have to cook the mix for up to 90 days to get everything to break down.
Once your concentrated organic mixture has finished cooking you’ll need to mix it with more base soil before transplanting, otherwise you run the risk of burning young seedlings or clones. The most basic method would be to cut the cooked concentrate 1:1 with more base soil and transplant into containers as usual. Subcool’s instructions were to fill the bottom half of each container with concentrate and the top half with base soil; this way the plant has more time to get established before the roots reach the heavily fertilized layer. Either method will work, but we consider the 1:1 mix to be easier and less risky.
[Do not skip the cooking step. Rapidly dissolving organic fertilizer can contain high concentrations of harmful chemicals like ammonia, and microbial activity can spike soil temperatures to over 130 °F.]
While super soil methods can yield some really fantastic results, they can be a roll of the dice. If you stay 100% true to the method you won’t have many options for correcting any deficiencies that may arise. Using a larger pot will help provide a buffer against swings in pH and nutrient deficiencies, and drought; if you normally use a #5 container try upsizing to a #7 or #10.