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The Organic Way: Soil Science 101

Robert Celt

New Member
Recently a good friend on here has seen what some of us here are growing with organic (water only) soils and is wanting to head in that direction. I started my journey organically from the outset but was flying by the seat of my pants and learning as I went.

I big :thumb: to my friend Kraize for his interest in going organic and asking his questions on my thread. This prompted me to compose my research into a single article on Soil Science for him and others to reference as they follow down the path of organic gardening.

Robert Celt

New Member
Soil Science

The Basics
Soil science is often a mysterious and misunderstood subject to many growers, especially new growers. Many realize the basic requirements of our plants (NPK, calcium, Mg and trace minerals, pH) and there is much information and many recipes available to us for our plants. But without understanding how and when these nutrients are available to the plant, all we have is a hodge-podge of components that have the nutrients needed but no idea if and when they are available. If we are lucky ( I was on my 1st grow), our plants grow and we are rewarded for our efforts and all is good. On the other hand, if trouble arises, how do we determine where the problem lies?

A properly prepared Living Organic Soil is the most forgiving and least maintenance of grow mediums. Nutrient burn is highly unlikely and deficiencies are rare. Nutrients are available as the plant needs them and the plant will only take up what it needs when it needs.

THIS is where a better understanding of soil science helps us.

Many growers have heard of SubCool and his supersoil. On the surface, it seems well rounded and contains everything our plants need, but if we break it down, we find that a simpler and cheaper soil can be obtained that will perform as well and maybe even better.

When we are building a Super Soil, or more accurately a Living Soil which we will get into after the basics, the most important things we need to know are NOT NPK ratios and pH, but rather we need to understand "release rates" and CN ratios. These 2 things go hand in hand, and if we understand this, we can pick our components much more intelligently and also save ourselves money.

The CN ratio refers to carbon-nitrogen ratio in any given component of our soil, and this in turn determines our "release rate", or the time frame over which the nutrients of the component become available to our plants. For all organic amendments we want to keep the CN ratio to 20:1 or less. Higher ratios take longer to break down, but even more importantly to us as growers, the microbes that break the organic matter down making it usable to our plants, use nitrogen during the breakdown process. When the CN ratio is high, these microbes use all available nitrogen and rob its availability to our plants.

For us as organic growers, whether it be vegetables or cannabis (same rules apply), we are looking at a grow season of 6-8 months. Therefore, we want amendments that will be available to our plants over that range.
Now that we have set down the basic requirements, lets look at some of SubCool's ingredients and whether they are good/bad or even needed.

Bat Guano: NPK varies and release rates from weeks to months, depends on the source. This can make it a good choice but is expensive and there are better, environmentally friendly, cheaper alternatives.

Worm Castings: NPK varies according to what they have been giving to eat. Release rates are essentially immediate. If fed a varied diet, worm castings are essentially Mother Nature's perfect fertilizer.
Worm casting are neither cheap nor easily available to many, and in the quantities listed in SubCool's recipe gets quite costly.

Rock Phosphate (granite dust): source of P but extremely slow release, 5 yrs and more. For an annual plant, this is a waste of effort, the plant is long harvested before the P is available.

Azomite and Humic Acid: source of micro nutrients, can easily be replaced by many plant based amendments: Kelp meal, Alfalfa meal etc.

Overall, this recipe is not bad but can get expensive and has some un-needed components.

Now let us look at some components of a cheaper Living soil:

We need a base, typically our base will be peat based. ProMix BX is a good choice, it is peat, perlite and lime inoculated with mycorrhizae (a beneficial fungus). If you want to save even more money you can make your own base with the same ingredients as the ProMix, you will just have to test your pH and adjust the lime accordingly.
Now you need amendments to supply nutrients for your plants. I am going to list amendments with their NPK ratios and release rates. Afterwards I will explain how to choose your amendments. I have not bothered with CN ratios for these as they are all in the range we are looking for.

Alfalfa meal: NPK of 2.5-0.5-2.0, this will vary depending on the source, and it is also a good source of many micro-nutrients. Release rates from 2-6 months.

Blood meal: NPK of 12.5-.25-0.6, again varies, fast release of 6-8 weeks.

Bone meal: typically high in P (14+) may also have N. Release rate of 2-4 months.

Crab shell meal: NPK of 10-0.25-0.05 plus Ca and micros with a release rate of 4-6 months.
Crab shell is high in chitin, which promotes the growth of chitin eating bacteria. The exoskeletons of fungus and harmful varieties of nematodes eggs are high in chitin. When added to the soil, crab shell helps to create a hostile environment for the fungus and root destroying nematodes by feeding the biological life that eats chitin and chitin based organisms. The chitin in the crabshell stimulates soil organisms to secrete enzymes called chitinases. These enzymes degrade chitin, which is a component of flea egg shells. The presence of chitin in the fertilizer makes it a natural biopesticide that is non-toxic to birds, animals, fish, and plants.

Feather meal: 15-0-0 with a release rate of 4-6 months

Fish meal: 10-(4-6)-0 with a release rate of 4-6 months

Kelp meal: 1-0.5-(4-13) and a release rate of 4-6 months, also an excellent source of micro-nutrients

Wood ashes: 0-5-(3-7) and releases 1-4 months. Be careful, wood ash is alkaline and may raise pH

Finely ground egg shells are an excellent source of calcium

Epsom salt for Mg and sulphur

Gypsum for calcium and sulphur

Stone/crusher dust, it goes by many names but is essentially the left overs from quarries after they have crushed and screened stone for gravel. It is a mix of stone chips (usually ¼" and under) and stone dust. You want the dust of granite (as opposed to limestone). It is a LONG term amendment containing phosphorous and trace minerals that will slowly release over years. I prefer to use it as a top dressing, about ½" thick to prevent nasties, like fungus gnats, from having a place to lay eggs.

Lastly the often overlooked silicates: Silicates are used by plants to strengthen cell walls giving us sturdier, disease resistant plants. Potassium Silicate is commonly sold for this purpose but there is a cheaper alternative that is more readily available for uptake by our plants. Diatomaceous earth.

"Fortunately, many growers are now discovering the benefits of flowable silicon as an alternative to potassium silicate. Flowable silicon is pH neutral and it is derived from natural sources of silicon dioxide. The best source of flowable silicon comes from clean, finely-powdered diatomaceous earth."


"DE is approximately 3% magnesium, 33% silicon, 19% calcium, 5% sodium, 2% iron and many other trace minerals such as titanium, boron, manganese, copper and zirconium."

Worm castings as mentioned above are an excellent all round amendment

Lastly the skins of fruits and vegetables (cucumber, bananas, potatoes, etc) in their ash form (burnt) are high in both P and K. Rather than burn them to get the ashes, compost them separately from your other food wastes and use the compost or if you have your own worm bins, feed them to the worms.

Now as can be seen from the list above, many of these amendments have release rates between 2 and 6 months. What those numbers actually refer to is the time over which micro organisms in the soil break down the organic matter making the nutrients available to our plants. Once broken down, the nutrients remain in the soil and available until the plant uses them up.

For our purposes, building a soil, the NPK refers to how MUCH of a given nutrient is available (% weight) and the 1st number in the rate determines WHEN those nutrients become widely available. The ratios of amendments we add really doesn't matter so long as we err on the side of too much. We really can't over fertilize with organic amendments, plants will only take what they want (need), but we can under fertilize in which case our plants will use up all available nutrients before they are done growing.

After choosing your base and amendments, mix thoroughly, water well and let "cook" for a few weeks. The length of time will depend on a number of things:

First being the amount of organic matter to used, large quantities of organic matter, like alfalfa, will generate heat as it breaks down. In the first few weeks it can generate enough heat to literally cook your plants (over 100 degrees F) so make sure you have left it long enough to cool before using.

Secondly, depending on the size and nutrient requirements of your plants, you want it to cook long enough to have nutrients available to your plants. 2 to 4 weeks is usually long enough for smaller plants with lower requirements, as they grow, more and more nutrients become available.

If you are transplanting larger plants, you will want to let it cook longer, 2 months or more, to ensure that there are enough available nutrients to keep your plants from starving.

Composting and Worm Castings
Firstly, what you DON'T want in your compost: meats, bones and fatty substances, pet feces, these things break down slowly, attract all kinds of unwanted pests, generally smell foul and are not needed by your plants.
Now that we know what not to put in, what DO we add? Composting is fairly simple if you understand what is taking place in your bin. You want to aim for a certain ratio of greens and browns, which is a simple way to describe C-N ratios  Greens are high nitrogen/low carbon and browns are low nitrogen/high carbon. We want to aim for a C-N ratio of around 30:1 for best results. Lower ratios generate a lot of heat and higher ratios take longer to break down.

So how do we distinguish between greens and browns? As a general rule: any fresh vegetable/fruit waste (skins etc) are greens and if it's dry and brown (dark) like dead leaves in your yard or straw it's a brown. Mix 30 parts brown to 1 part green and you are good to go . For those who would like to be more scientific about it, do a google search for C-N Ratios, there is lots of information on the web concerning these ratios with lists of C-N ratios for various things.

If you want to kick start your compost into high gear, add a shovel full of soil from your garden to give it a microbe boost. Well rotted horse or cow manure is also a good addition. Avoid fresh manure and wood shavings/sawdust in you composter.

For those of you wanting to do worm composting as well, feed them your compost or the same things that you would put in your compost bin including any of the soil amendments listed early on. Just avoid things like onions and citrus unless it has been composted first As a rule of thumb, if it is something that would sting your eyes, don't feed it to your worms.

True Living Organic Soil
Now after building your soil, you have 2 options. First, after you have harvested your plants, you can recycle the soil, add new amendments and reuse OR you can maintain it for "no till" use with ACTs (aerated compost teas) and have a True Organic Living Soil. I am not going to delve into "no till" and ACTs as there are many on here (Doc Bud, SweetSue, SoilGirl just to name a few) who have much more experience than I in this area and are great sources of information on this subject.

So in conclusion, organic soil is not a mystical subject and is really quite easy once you know what to do. There is no muss and fussing with fertilizers, no hard fast rules, and nutrient burn, deficiencies and lockout are virtually non-existent. You will have healthier, better tasting bud (and vegetables) with no need to flush the soil prior to harvest.

Once you've build your soil, added the things your plants require, just add water and grow 


Well-Known Member
A big :thumb: to you too mate this was an outstanding read that even I could understand. So much so that I do have a question regarding the C-N ratios :)

I'm not sure if I have misunderstood something but at the top of your post you mention that the C-N ratio should be 20:1 or less which led me to assume that if it was a higher ratio (30:1 as later in your post) the Carbon would eat through the Nitrogen leaving a Nitrogen deficiency in the soil. But later as I stated you go on to say that the ratio should be around 30:1 of C-N.
Should that have read 20:1 or more in which case is it at risk of being Nitrogen deficient or am I missing something? Was it a test? I love challenges lol or am I missing a point of information here?

Also I was always wondering whether I would need to dilute my compost with soil so that my seeds wouldn't burn but from reading this I get the idea that if I leave the compost alone for a month or so without adding anything extra to break down this will in effect 'cool' the soil and the plant will be able to take nutrients as and when needed at whichever stage of it's life it is at. Would this be correct? Sorry if I sound dumb but admittedly I am when it comes to helping a soil to live as you know.

Once again thanks for the excellent read and I really appreciate that you took the time to write this wealth of information in terms that the layman like me can understand. +reps for this and well deserved I say. Have a fantastic day and sending some good karma your way :thumb: :thumb: :thumb: There we go, you got the big thumb :)

Robert Celt

New Member
Morning Kriaze, the first C-N ratio was really in regards to soil amendments. Going over this would starve your plants of Nitrogen.

The second C-N ratio was what you want to shoot for in your compost for optimum results :) Once your compost has cooled (no longer generating heat) and looks like black earth, the C-N will be fine for your plants :) As the compost decomposes the carbon levels drop and the nitrogen rise.

Your compost will be very nutrient rich and you could grow directly in it, but I would suggest using it as an amendment into a peat and perlite mix for better aeration and a looser soil :)


Well-Known Member
Ah I see TheCelt, thanks for that explanation :thumb:

I will be adding Perlite to it for sure and I will be on the lookout for peat (silly thoughts intruding reminding me of the old 'Where's Wally?' for a moment there, surprising how reading and writing does that lol. If Pete is hard to come by (possibly still not found...sorry :19: ) is it an absolute necessity or would extra Perlite cover it?

Thanks again :thumb:

Robert Celt

New Member
Peat is not needed, just a suggestion LOL some of the best pumpkins and squash I have ever seen grown were from seeds thrown in a horse manure pile LOL which is for all intents and purposes just compost ;)


Well-Known Member
Excellent TheCelt, thanks for the clarification. Now to see how far I can push the wife before she threatens to leave me, going to get the tents filled with horse sh*t and just throw my seeds in :)


New Member
Good info here Celt. Kudos for taking the time to do a write up on this subject.

I tried to give you a rep but it appears that I need to spread reps around elsewhere before giving you another. Go figure...

Robert Celt

New Member
That's funny Kraize LOL I am on my 2nd wife. The 1st and I were 2 people that should never been together LOL The 2nd is great :)


Grow Journal of the Year: 2017 - Grow Journal of the Month: Sept 2017
It's good info for beginners, I just want to add some important info. First, ProMix is not 100% organic, cause the producer uses synthetic wetting agent for the humidity, but if someone's not a purist it's as good as it gets, and works for High Brix grows, but I never used it. Then, much more important thing is that you also need proper microlife as without it you're NPK + micronutrients ratio is useless. You shoot for 50% funghi and 50% bacteria roughly, you want to even it out. But compost tea + incoulation with mycorrhizae will fix it. Good luck :thumb:


New Member
Worm castings or a shovel full of cow shit added to the mix during composting will provide ample amounts of microlife as well.

I like to manipulate my watering schedule to allow my soil go through wet / dry cycles to help promote the bacterial/fungal balance.

Do you have a preferred method for maintaining a healthy bacterial/fungal balance? If so, please share.


Grow Journal of the Year: 2017 - Grow Journal of the Month: Sept 2017
You have to use microscope, but I've never done it. However there are course online which teach you how to do it!

Robert Celt

New Member
Hi Conrad

You are right about the Pro-Mix (BX or HP), I included it for anybody starting out because it saves having to mix and adjust for initial pH and has mycorrhizae in it :)


New Member
You have to use microscope, but I've never done it. However there are course online which teach you how to do it!
As a master grower I'm sure that you are very knowledgeable about this subject but for the benefit of those who may not be so experienced I'll try to give a short explanation, if that's even possible.

Assuming that beneficial bacteria and fungi are already present, maintaining healthy amounts of organic matter and minerals in the soil, maintaining healthy soil ph levels, maintaining good soil aeration, maintaining an adequate bacterial/fungal food source, and not allowing the soil medium to remain either too wet and soggy or too dry for long periods are all good ways to maintain a healthy bacteria/fungi balance.

By allowing your soil medium to go through slight dry and wet cycles this will create somewhat of a yo-yo type effect with the bacteria/fungi balance swinging it back and forth which is good for creating a healthy balance.
This also has a similar yo-yo effect with the soil ph as well allowing for optimum nutrient availability across the range of the ph swing.

A quick note for those new to organic growing or maybe just curious about going organic.
Do not worry so much about all of the science behind organics that you allow yourself to become intimidated by it all and think that this organic stuff is way too difficult.

Any organic soil mix that will grow a decent vegetable crop can also be used to grow a decent MJ crop. This would be a good place to start while one learns more about organics and the whats and why's of it all. No microscope needed.

Let's not forget that mankind has been growing food crops with 100% organic methods since the dawn of time without knowing much if anything of the science behind it all.

Robert Celt

New Member
Well put UncleC :420:

That's why I didn't go into 'recipes" as and left it more as general info for others to use to make their own recipe. There are numerous members here including you and I that all have different methods and recipes yet the end result is much the same :)


New Member
I totally agree buddy. There are many paths to the mountaintop.

I was just trying to pry some potentially hidden secrets from Conradino but he didn't take the bait. Darn it!! LOL!!


Grow Journal of the Year: 2017 - Grow Journal of the Month: Sept 2017
The secret is simple ;) Add hyperaccumulators and leaves from the trees to your compost pile. Inoculate your plants with mycorrhizae in early stage. Use beneficial bacteria like lactobacillus acidophilus or bacillus subtilis, and never use any strong fertilizers, especially nothing high in potassium or phosphorus.
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