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Growbed media

There are various types of grow media and all of them have their pro’s and con’s. This is a brief guide to give you an idea of what the grow bed media does and what is important to consider when selecting your media.

The grow bed media is only in the Flood and Drain system, in both the raft and NFT system the plants sit directly into the water (however, the media can be used in the biofilters of these systems and so some of the selection criteria is still important). The grow bed media has a number of different functions. Without the soil, the growbed media gives the plant a structure in which to anchor their roots. The media helps moderate the temperature of the roots and also allow air to circulate. The bacteria that are needed to convert the fish waste into plant nutrients (nitrifying bacteria) will also colonise the media – the higher the surface area the better. The growbed media will also act like a giant sieve to filter out the solid waste so that worms can break it down further.

There are a several different criteria that must be considered when selecting your media as some media will have a negative impact on your system.

It is very important to select a media that is pH neutral or as close to neutral as possible and is clear of any other contaminants that can cause harm to your fish or plants. Different types of gravels will have a varying effect on your water quality. If you are not sure what effects the gravel will have on your water then it is important to do a test before adding it to your system – fill 2 buckets or containers with water from your system and add the gravel to one of the buckets, test the water over a 3 week period to see if there is any difference or effect on the pH. For example if you add limestone or any gravel with Calcium carbonate present in it then this will push up your pH. You will be unable to lower it over time and will be fighting against this all the time. It is simply not worth it. Make sure you know what gravel you are using and what effect it will have.

The size of the grow bed media is also important. If the media is too small then it will drain very slowly and also can become blocked with debris and roots. The debris will give rise to aerobic bacteria which will use up oxygen from the water and also create “dead spots” in your growbed (areas that are blocked and nutrient rich water will not reach it, stopping anything from growing). If the roots of a plant dont drain well they will get waterlogged which will lead to more complications. The larger the media, the more oxygen will be able to get to the roots (plant roots need oxygen in order to be healthy).

The media with a higher surface area can host more bacteria which will convert the fish waste into plant nutrients. Media that are porous are better than solid pieces for this.

So what are your options when it comes to what to use?

 Hydroton – a type of Lightweight Expanded Clay Aggregate

Hydroton made from clay, essentially the same stuff that makes up clay plant pots. They are little pebbles of clay that are fired in an oven. They are chemically neutral so will not effect your pH. It has a high surface area which encourages large colonies of bacteria. The pebbles themselves are smooth and easy to work with. They are much lighter than gravel and so the grow beds themselves don’t need as much reinforcement or support. They come in various sizes that can be suitable for your system.


Lava Rock

Lava Rock is used a lot in different areas due to its light weight (only a third of ordinary gravel) and that it is relatively cheap. However, in Ireland it is much more expensive due to shipping costs. It has a lot of the same characteristics of the clay pebbles – high surface area, light weight, porous etc. It is normally quite rough and so gloves may be needed when working with it. It is important to clean it thoroughly before use and to make sure that there are no other contaminants or other types of rocks present.


Pea Gravel

River bed or pea gravel has a lot of advantages and disadvantages. It is always going to be much cheaper than the other alternatives. It is very difficult to be sure that all the gravel you have doesn’t contain any limestone (especially in Ireland as it makes up a large percentage of our bed rock, especially around the Dublin area), and as above, it will raise your pH and effect your water quality – it is important to test it to be sure. It has a much lower surface area than the hydroton or the lava rock so will not host as many bacteria but depending on the size of your grow bed and system this might not be noticeable. It is a lot heavier than lava rock or hydroton which means it will need more reinforcement of your grow bed. However, this also helps larger plants such as tomatoes and cucumbers as they are anchored better. Without support they will topple in hydroton or lava rock.

Other materials?

There are a number of other growbed materials that have been tried and tested however, you will just need to follow the basic rules above on what the media is supposed to do – support plants, pH neutral, drain easily, be the right size and have a large surface area.

Washing media?

There are arguments both for and against washing the media. Some people prefer to clean the media so as not to add any “unknowns” into your system like other organisms etc. Other people say that by washing it you get rid of any extra nutrients and minerals on the media (this is the case for pea gravel and lava rock but not for hydroton). It also will take a large volume of water to clean it so it may not be economical. Washing the gravel will help keep your system clean for the first while. However, in a few days unwashed media will settle down and the water will clear. Its down to a personal preference


We recommend using the hydroton clay pebbles as they are light weight and easy to work with. However, they are the most expensive. It really depends on how much money you have and what you have access to.


Do you have any more questions you would like to ask us? Please feel free to leave us a comment and we will answer it as soon as possible.

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