Greenhouse Aquaponics

[Ivor]

Hello all,

I started a small 200 gallon tank aquaponics project about 2 months ago. I took a 255 gallon tote that in the past held liquid iron, cut the top half off and flipped it over and set it back on top of the base. There is still approximately 200 gallons in the bottom tank which I stocked with 75 Tilapia fingerlings.

The top bay I filled with clay pebbles to serve as a bio filter as well planting area. I know most people use an ebb and flood system, but I decided to just try a constant flow system with a small pump continuously bringing water up from the fish tank, to the growing bay. The water then leaks back into the fish tank through multiple small holes I drilled.

I recently read about adding chelated iron to the water to assist plant growth. Is there any basic rubric of how many lbs or oz per gallon of water per week?

Also, my ec is staying at about .2 which is quite low. The water I started with was reverse osmosis filtered and had an ec of .03 so there has been some increase but I am looking for closer to 1.5-2. Is this simply an issue of not enough fish in the tank?

Also if anyone knows of a good source for tilapia fingerlings around California I would be interested to know. I bought mine from a place in San Diego but I am pretty sure his price is overinflated.

Thanks!

[Johnh]

I have had the EC discussion on other aquaponics forums and the overwhelming opinion seems to be that in an aquaponic setting EC is not a unit of measurement that has any relevance to nutrient levels.

After some trials of my own I have to concur that those of us with a Hydroponics background will have to accept the fact that because the nutrients are not supplied as chemical salts the results of EC readings are pretty meaningless for this purpose.

EC readings will have some relevance if running sea salt in the system as one can keep an eye on the levels and not kill too many plants if evaporation and transpiration rates go up and salinity increases, or to keep the levels at prefered rates as the salts are made available to the plants and the EC falls.
EC reading in this case are pretty frightening from a hydroponics point of veiw as many people run up to 3PPT or 3000PPM if they are having fish health problems which can give an EC of around 4.7 or so. (thats a fairly vague figure as it will depend on the souce of the salt and a lot of other factors)
John

[GaryD]

Hi JohnH,

I get the impression that most of the stuff about EC is premised on a "need to know and control" (not a bad thing for most farming purposes) but that it has no real place in aquaponics as things currently stand.

For most practical purposes, it seems that the presence of nitrates, and the addition of those elements known to be unavailable at higher pH (like iron, potassium and calcium), suffice for the production of many plants.

Running an aquaponic system at pH 6.0 (or thereabouts) unlocks the availability of these elements so that even their addition becomes unnecessary.

As always, the operational determinant of whether there are adequate nutrients in the system is the condition of the plants and their rate of growth.

Having said that, the nutritional adequacy of the plants is another thing entirely. Modern agriculture has taught us that plants can look great but still be nutritionally lacking.

For aquaponics, we may be better off to go by the appearance and growth rate of the plants and test for nutrition levels..... rather than worrying about EC units.

Gary

[Johnh]

Yep absolutely agree Garry,

John