Grow Beds in Home Aquaponics System

Guest Post by Murray Hillam from Practical Aquaponics

Grow Beds in Home Aquaponics System
The GROW BED is the multi function centre of the Home Aquaponics System.

The Grow Bed performs several very important tasks, and if we were to try and describe those functions in a simple way, one suitable description would be, it is a Bio-Filter in which we grow plants.
A very happy and combination of duties. It is extremely convenient for our purposes, because as a Bio-Filter it collects and processes the ammonia and solid waste from the fish and returns the water to the fish tank clean, and by the growing of plants in the Bio-Filter (Grow Bed) we use up the nutrients and nitrates produced in the Bio-Filter (Grow Bed) producing excellent quality fresh veggies for our table.

Because the Grow Bed is such an important part of our total Aquaponics System we need to put some careful thought into it’s design. Materials used, dimensions and location relative to the fish tank are all considerations.
We strongly recommend the use of 300mm (one foot) deep grow beds in your Aquaponics system. Beds with less depth and therefore volume, will also work, but not nearly as well.

The Grow Bed needs to be of such a length and breadth to provide sufficient surface area for the plants and together with depth provide sufficient total volume to be an effective Bio Filter.
Overall volume of a grow bed is an important factor. The more volume the total system has, the more stability in the system , particularly in temperature and pH.

This stability has obvious flow on benefits for the health of the system inhabitants.

Fish, plants, worms, beneficial bacteria and microbes all function better in a stable environment.

In taking the decision to operate an Aquaponics system, we are desirous of producing the very best, healthy, nutrient rich and economical, plant and protein, for our family.
Aquaponics can deliver such produce by making use of natures wonderful interactive systems of worms, microbes and bacterium in a naturally balanced environment. It all works to its optimum when we provide it the best environment possible.

300mm (or more) deep grow beds will deliver optimum plant growth and health. They will provide optimum environment for the processing and delivery of nutrients to the plants, and the processing conversion of the ammonia given off by the fish, to nitrates.

In forming this opinion we have relied, not only on our own actual, very significant practical experience, but also on the experience of dozens of very experienced AP practitioners in Australia and around the world. Many of these people have accumulated a large body of experience in the use of 300mm deep (or deeper) grow beds as part of a well constructed Aquaponics system.

All of these people report exceptional healthy plant and fish growth using systems based on the 300mm deep grow bed principle. There is now a large body of actual evidence that strongly shows that this is a good working principle/method.

We have manufactured and delivered many hundreds of complete AP kits based on the 300mm deep grow bed, and hundreds of 300mm deep grow beds to persons who are constructing DIY Aquaponics systems.

Flood and Drain:
Together with a 300mm (one foot) grow bed depth, we strongly recommend using flood and drain cycle and 20mm drainage gravel or similar in your grow beds.
Flood and drain water movement system ensures the even distribution of water, nutrients and air (oxygen) throughout the system. This provides multiple benefits.
By the use of this method, dry or nutrient and oxygen areas are prevented from forming in the grow bed.

Nothing less than 20mm (3/4”) gravel should be used. This common gravel, by the way it rests together provides easy passage of water, solids, and worms throughout the bed.

Often folk cannot envisage plants growing in such a coarse media and they choose a finer media such as 5 or 10mm. This sized media will impede the action of the worms, the easy movement of solids and nutrients, and the Grow Bed will quickly suffer from partial or complete blockages.
Listed below are some of the reasons why 300mm or deeper grow beds are good….very, very good and highly recommended to deliver excellent results in your Aquaponics System.

Room for plant roots to develop and grow.
Some plant types such as lettuce do not require much depth (or nutrient) to grow successfully, but other garden plants such as tomato and corn, just to name two more common ones, do need depth space to put down good root systems. A given grow bed will have a variety of plants grown in it, so a grow bed depth bed depth that will accommodate a wide range of plant requirements is the way to go.

Depth and volume to process solid waste.

Solids passed by the fish, old roots, and other solid material is processed in the grow beds by those little wonder worms. Without going into detail here about the role of worms in AP, sufficient to say that the worms reduce solid waste by 60% or more and by their work and the action of flood and drain distribute the released minerals and nutrients throughout the grow bed/s. 300mm deep grow beds filled with 20mm drainage gravel with a good population of resident worms deliver an amazing plant growing habitat. (Not to mention excellent filtration for the fish tank.)

Bed Zones are established. (See attached illustration

Surface or dry zone. — # 1. The first 50mm is the light penetration and dry zone. Evaporation from the bed is minimised by the existence of a dry zone. Water waste minimisation is a very important principle in Aquaponics systems.

This dry zone also protects the plant base against collar rot. Additionally, by ensuring that this zone is kept dry, algae is prevented from forming on the surface of the grow bed media. Because this dry zone is present, moisture related plant diseases such as powdery mildew are minimised.

Root zone. — # 2. Most root growth and plant activity will occur in the next zone of approximately 150 – 200mm –in this zone, during the drain part of the flood and drain cycle, the water drains away completely, allowing for excellent and very efficient delivery of oxygen rich air to the roots, beneficial bacteria, soil microbes, and the resident earth/composting worms.

During the flood part of the cycle, the incoming water distributes moisture, nutrients and incoming solid fish waste particles throughout the growing zone. The worm population does most of its very important work in this zone, breaking down and reducing solid matter and thereby releasing nutrients and minerals to the system. ?Worm Tea”, as it is commonly known, will be evenly mixed and distributed during each flood and drain cycle. “Worm Tea” and the fish are entirely compatible, No possible harm can come to the fish by the distribution of this wonderful nutrient material throughout the Aquaponics System.

Solid collection and Mineralisation Zone – # 3.
This is the bottom 50plus mm of the grow bed. In this zone fish waste solids and worm castings are finally collected.

The solid material has been reduced by up to 60% by volume, by the action of the resident garden/composting worms, and microbial action.
During each flood and drain cycle, what is left of the solids perkolates down into this zone
Further and final mineralisation occurs in this area via bacterial and earth worm activity. Due to the excellent action of the flood and drain cycle, this bottom area is kept “fresh” and vital by the excellent delivery of oxygen rich water during the flood cycle.

Some water storage occurs in this bottom zone.
Should the flood and drain cycle stop for any reason, such as a mains power outage, the bed will slowly drain down and leave approx 50mm of water at the bottom of the grow bed.
The stored water provides a safety buffer in the event of power outage or pump failure… This stored water ensures that the plants will survive for very long periods without water flow.

This means that we can simplify our safety backup system to circulate only the water in the fish tank, by the use of low wattage water pumps and/or aerators. We can safely operate the system in backup mode (no main power supply) for very long periods of time–simply using an average size car battery as a power source.

This inbuilt water storage zone will supply the plants with water and nutrient should we need to isolate the fish tank for maintenance purposes, or treatment of the fish.

Happy Aquaponics

Murray Hallam.

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