The Organic Way: Soil Science 101

Robert Celt;2611644 said:

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."

and:

"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

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