First off the title of this blog post is a bit misleading since hydroponics is generally associated with water culture where all of the essential nutrients are dissolved and then fed to plants directly. Organic regulatory bodies generally don't recognize these types of methods since soil health is one of the tenants of their philosophy. Using a soilless media in a container and feeding regular doses of a nutrient solution would be a fair compromise however, and this post has this method in mind.
Preparation of Soilless Media
Start out with the base material, either peat moss or coco. Since they are mineral based, perlite and/or vermiculite can be added to aid in aeration and water retention. From there a compost is added at around 10%, worm castings are likely the best option here. A mineral component should also added at around 5 - 10%. Sand or clay are a cheap and easy option. Additionally insoluble plant nutrients such as lime or rock phosphate can also be added at this point (more on those later).
Priming the medium before planting with a high quality humate such as Royal Black is also beneficial as it will allow more time for the break down of the humic acid into fulvic acid, as well as chelate any nutrients present in the medium.
Generally when growing with conventional nutrients in a soilless medium we recommend to feed the plants and then on subsequent days to give 2 rounds of straight water and then repeat. This is playing it pretty safe and is a very general guideline and can be modified to fit the individual situation.
We'll go through each of the nutrients one by one and they can all be added together in the same feeding. If compost tea is part of the regiment then prepare it as normal and once the steeping is complete add the additional nutrients and feed via the roots. Additional humates can also be added in order to aid in nutrient solubility and absorption.
The most important thing to understand is that solubility is the key to how well the crop will grow. Insoluble nutrients need time to break down and in many cases can take years. This is the biggest obstacle for organic growers but with a bit of knowledge and planning it can be overcome.
- Nitrogen: Unfortunately animal waste is the most common high and easily available source of organic nitrogen. Of these, fish emulsion is the most readily broken down and is the easiest to use. Plant based amino acids are another great option, but they can be difficult to find.
Sodium Nitrate is a mineral salt permitted by some but not all organic regulatory bodies. It is very high in nitrogen and is readily available to the plant, but the sodium component can stay in the soil increasing salinity. Long term usage can seriously damage the soil.
- Phosphorus: As far as nutrients go, this is the biggest headache. There are no soluble forms of phosphate in nature, therefore there is no such thing as an organic phosphate fertilizer which will dissolve in water. Growers will have to use an insoluble source and do their best to try and free it up. Beneficial microbes, either bacteria or fungi will free it up over time so keeping a healthy population is essential for proper phosphorus nutrition.
As previously mentioned blending in a phosphorus source into the media is a good way to get off to a head start with solubilization. Bone meal is a little more easily broken down than rock phosphate, but if keeping the grow vegan is a priority then this is not an option.
The source of organic phosphorus most easily broken down is fruit bat guano and this should be used as a dressing on top of the media. It is a very limited resource and so the price of it is a lot higher than the other types.
- Potassium: Finally an easy one! Potassium Sulfate is the most obvious choice here as it it highly soluble, fairly inexpensive, and has no restrictions on its usage by regulatory bodies. Another good option is adding a kelp product to the regiment as it contains decent amounts as well, but may need a bit of extra time for the potassium to become available.
- Calcium: When dealing with plant nutrition in general terms (not exclusively for organic) calcium is a problem as it tends to react with other nutrients locking up both of them.
The most common and cheapest form is Calcium Carbonate, this is the major ingredient in such things as lime, egg, and oyster shells. It tends to push up the pH, but the calcium can be freed up in an acidic environment.
For a full soluble source of liquid organic calcium check out our Organical Magic, which has been formulated based on the traditional cal mag products used in the hydroponic industry.
- Sulfur and Magnesium: More nutrients which shouldn't be an issue as they are found in soluble minerals, most importantly Epsom salt (Magnesium Sulfate). The sulfur component can also be found together with several of the other nutrients and are soluble, with the exception of Calcium Sulfate (gypsum).
- Micronutrients: All included together as you may or may not want to fertilizer with them, but rolling the dice can be risky. The good news is that they are soluble, the bad is that they can be difficult to source.
If feeding these is something that you would like to do, look for the sulfate forms of iron, manganese, copper and zinc and consider feeding them separately from the other nutrients. While they're soluble together, it doesn't take much for them to react with other nutrients and fall out of solution. Of these pay the closest attention to iron as it's needed in the highest amounts and iron sulfate is very unstable. If brown particulate is seen on the bottom of the nutrient mixing container it is likely iron that has fallen out.
Boron in the form of borax is another fairly easy option to include in the feeding, as it's cheap and readily available. Molybdenum on the other hand can probably be left out since the amount required is so minute, it's expensive and hard to source.
This blog post is written with the small hobby grower in mind, but if you are a larger producer I strongly recommend that you check out the local rules and regulations if the intention is to produce an organically certified product. What can and can not be used or done will vary by jurisdiction and it is up to the grower to understand the regulations pertaining to them.
For the small hobby grower who doesn't want to delve into the rules and regulations, my best advice is to only use ingredients which have been certified by third parties such as OMRI or Eco Cert. Fertilizer manufacturers may say organic or natural on their label, but without verification by a third party it is impossible to know what is actually in the bottle.
If the goal of the crop is to reuse the media and try to achieve a long term balanced environment, then lighter doses of nutrient might be considered as there would be less risk of any kind of a build up. On the other hand if reusing media is deemed too risky because of concerns over transferring pathogens from one crop to the next, then do your best to recycle the media in a garden or lawn and then start fresh. Keep in mind that any of the nutrients freed up in the first round (especially phosphorus) will be discarded.