Backyard Aquaponics
A place to discuss aquaponics

Ok, a mate has been asking me this question for a while, and I haven't been able to provide a very good answer.

NPK - main ingredients in most fertiliser, right?

Are they considered trace elements, or are they fundamental nutritional components for plant growth.

If it's the latter, where does the Potassium and Phosphorus come from? Are they present in large enough quantities in fish food?


Cheers!


Mark.

Hi Curly
Nitrogen Phosphorus and potassium are considered the Macronutrients essential for all plant growth

Nitrogen and Phosphorus are major excretements from fish both through faeces and gill exchange. Potassium can be a problem and is probably a limiting factor for some plants.

Have a look here http://www.thekrib.com/Plants/Fertilizer/
lots of links to good discussions on elements for plant growth in Aquaria and a lot will be directly applicable to Aquaponics as well.

Although it is not mentioned much in aquaponics there is a great range of bacteria other than the nitrogen specific ones that play an important role in converting all sorts of nutrients to forms suitable for plant uptake

Some have said that rock dust is a good addition. Over east, the guys use a product called Minplus.
http://www.minplus.com.au/ingredients/index.html

But over in WA, we don't have Minplus (they don't ship here), but I have tracked down Men of the Trees WA (Hazelmere)
and City Farm (East Perth) that appears to carry rock dust. A lady from Men of the Trees WA emailed advising that it is a small and heavy bag (not sure how many kg) for $5.

http://www.menofthetrees.com.au/

http://www.cityfarmperth.org.au/new_lite/about.html

Have not tried it myself but do a search in the forum for "rock dust" and read comments from the other APers.

Rock dust primarily supplies the necessary micronutrients necessary for plant growth... the trace element minerals ...

Whereas, as JohnH pointed out nitrogen, phosphorus and potassium are considered essential macronutrients... along with possibly calcium, magnesium and sulpher as well...

Seasol/Maxicrop will safely the macronutrient needs in an aquaponic system... with supplementation of iron and potassium products when/if required....

As a note... saltwater/seawater is the highest known source of all the essential trace elements...

Ok, cheers for that. So basically, the fish do excrete sufficient quantities of Phosphorous, but Potassium might be an issue.

Thanks.

Phosphorus doesn't seem to get talked about much here for probably two reasons.
1-the freshwater master test kit doesn't come with a test for phosphorus and that is the test kit most of us use. You can get aquarium phosphorus tests but for reason number two here, most of use don't bother.
2-if there is enough Nitrate (Nitrate test comes with the master test kit) for the plants to grow, there doesn't seem to be any problem with phosphorus deficiency so most of use don't bother even testing for it.

Potassium and/or trace minerals often need supplementation which is easily done. And if your source water is low in Iron, you might need to supplement that too.

Phosphorus is easily taken up by the plants in the grow bed. Nitrogen is a harder nut to crack. The plants take up some, but you still need a place for anaerobic bacteria to convert nitrites to N2 gas, without forming other gases you don't want.

Not quite Sparkchaser... yes the production of N2 is an anaerobic process... part of the "de-ntrification" part of the total "nitrogen cycle"...

Which is why we avoid it... we want an aerobic conversion to nitrates... a plant friendly and highly uptakable form...

Happily, aquaponics utilising flood and drain media filled growbeds does exactly that...

I know the real solution is to add more growbeds and have the plants remove all available nitrates, But what do you do in a smaller saltwater system growing sea bass or cod?

Borrowed from:
http://www.answers.com/topic/denitrification
Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen, a more energetically favourable electron acceptor, is depleted, and bacteria respire nitrate as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in environments where oxygen consumption exceeds the rate of oxygen supply, such as in some soils and groundwater, wetlands, poorly ventilated corners of the ocean, and in seafloor sediments.

Denitrification generally proceeds through some combination of the following intermediate forms:

NO3− → NO2− → NO + N2O → N2 (g)

The complete denitrification process can be expressed as a redox reaction:

2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O

You either grow ulva or other seaweeds/kelps.... or you use a pure RAS based system of bio-filtration....


Absolutely correct..... and you'll note that the process... "de-nitrification".... is the reduction of nitrates - NO3.... not "nitrites - N02"....

A process that requires oxygen... resulting in oxygen depletion... an anerobic process....

Nitrates are not toxic... unless in huge concentrations.... or unless you leave them, in which case the whole tank turns into a "soupy" anaerobic mess through the oxygen depletion, the anaerobic de-nitrification process...

By which time your fish would be well and truely long dead due to lack of oxygen... and continued ammonia load up to that point...

The same would occur even in freshwater systems.... if you don't oxygenate and bio-filtrate...

You end up with dead fish... and a rotten aquarium.... either way...

And nature kicks in the de-ntrification process to de-toxify" clean up the mess... and restore a "balance"....

Crushed basalt (bluestone) and clay stone / mud stone / slate (Lilydale toppings) combined have a really good balance of macro nutrients. Bluestone needs to be crushed to a powder for it to work but mudstone does not. I found mudstone grows better veggies than soil alone. I also use it as a mulch and it makes an instant difference.

I would imagine placing the media somewhere in your tank or growbed would allow mosses, bacteria and lichens to break it down (acid does it).

Mudstone is ancient swamp so it contains everything good.