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Simple substrate test.

Hi all,
It is reasonably straight forward.

Conductivity
All the metallic cations (Ca++, Mg++, Al+++, K+) will conduct electricity. These ions (charged particles) are formed from the complete "disassociation" (it just means they dissolve completely) of any salt (they are called salts after "real" salt (NaCl), because they all make the water taste salty).

Water is "covalent" and the OH- and H+ "ions", don't conduct electricity. As we add (dissolved) salts we add ions, which conduct electricity, more ions = more conductivity in a linear (straight line) relationship.

CEC
Some compounds, like clay and humus, have negatively charged sites (-) on their surfaces which can exchange cations with the water. You can think of it like velcro, the cations are held, but not permanently stuck. If a clay mineral has lots of Na+ ions on its surface it will exchange them for nearly all other cations, Ca++, Mg++, K+ etc in the water column. This won't have much effect on conductivity, if the exchanges cations are all metals, but if the exchanged cations are non-metallic e.g. NH4+ or H+ ions, conductivity will rise as the Na+ ions will conduct electricity, whereas the H+ ion doesn't and the ammonium ion (NH4+) only conducts weakly.

Substrate
Akadama, for example, has a low "base saturation percentage", this just means that it comes from a wet climate, where over time the rain-water has washed all of the other cations from the exchange surfaces, just leaving the tightly bound aluminium (Al+++) and iron ions (Fe+++), and the less tightly bound H+ ions.

If we put Akadama into alkaline "water" with lots of calcium ions (like tap water from limestone aquifers we get in the SE of the UK), Ca++ ions in solution will be exchanged for H+ ions from the Akadama, the base saturation percentage of the Akadama will rise (more calcium ions bound) and the conductivity of the water will fall (less calcium ions in solution, and replaced by hydrogen ions that don't conduct electricity).

Cat litter
In the case of the cat litter it initially has either:
  • 1. A very high base saturation percentage, which means H+ ions in the water (which don't conduct electricity) are being replaced by metallic cations (which conduct electricity), or
    2. Dry compounds present that will dissolve and go into solution as ions, raising the conductivity
In either case more ions = more conductivity.

cheers Darrel
 
Thanks Darrel for the clear explanation, I really don't think anyone may come with a better one. 🙂
Do you know why some soils harm some shrimp species and others don't. I can see some clues in your post above but I don't want to guess.

Cheers,
Mike
 
Hi all,
by adding RO water in a water change, doesnt neccessarily lower TDS as the clay will release what it has collected?
Click to expand...
Quite possibly at first the TDS won't go down, but if you carry on adding RO, over time the exchange sites on the clays would become filled with H+ ions. This might take quite a long time to remove the more tightly bound multivalent ions (Ca++ etc) as they are higher up the lyotropic series than H+, and will only be exchanged when there is a very large concentration gradient between the clays exchange surface and the water.

By the look of "Aqua sobriquet's" figures you would need to wash the "Tesco's Cat Litter" in quite a large volume of salts poor water, before use, if you wanted to maintain soft, low conductivity water in your tank. My suggestion would probably be to leave it out-side for 6 months with the rain washing through it, which is going to be impractical for most people, or to use it with plants and fish that don't mind harder, higher conductivity water. I like it as a substrate, and probably for most people it isn't going to matter that it initially may make your water harder.

I recorded the conductivity of the 3 tanks in the lab this morning, they all have water changes with rain-water and they have all been set up for at least a year.

The rain-water was 100 microS, Dicrossus tank was 80 microS (100% silica sand substrate), the nano with Tesco Cat litter and a lid was 170 microS and the 3rd tank (another nano) open top and 100% silica sand substrate was 125 microS.

cheers Darrel
 
plantbrain said:
The test will illustrate the leaching effects over the short term(weeks).

It does not show nutrients etc or bioavailability/growth rate differences for plants.

There is a rather easy way to do that however.

Use the same amount of water and soil types in flask and place a soft but relatively air tight sponge at the flask neck and wrap around the stem gently.

Use something like an emergent aquatic plant, say Ludwigia repens.

Place in the window or a hot house etc.

Measure the leaf no# and the stem length over time, take pics for visual differences.
Use 3-4 plant/soil flask per treatment.

So say you have 5 soil types:

You will need 25-30 flask and stems of the plant.
3-4 treatments then a control(say hoagland's solution(upper non limiting control) and then DI pure water(no nutrients, lower control). All other results should fall in between these 2 controls.

Measure the leaf and length weekly over say 8-12 weeks.

This is not hard.

CO2 will be non limiting as well as light.
So only the nutrients from the soil will be at play.

While not entirely natural like aquariums........it's a better test without other confounding factors.
This way you can conclude something growth wise about the sediments.
Click to expand...

I'd say that if the objective is to test for differecnes in substrates alone, there is no need for positive or negative controls. Growth being different (or not) among substrates suffices as a result, all else being equal. Just control variability in initial plant measures (e.g. length) and measure things before and after the experiment (and saving some flasks here).

I would also stress the need to label each flask and display them at the window sill at random (or haphazardly at least) so that replicate flask belonging to different treatments (substrates) are all interspersed. This will ensure position of the flask is not a confounding factor (e.g. substrate x was closer to the window and hence more light,...).

anyways, for most of us, I think this would be overkill for a home experiment...

GM
 
TEST OVER. Unfortunately I've not had the time to take readings or keep an eye on things and I've inadvertently let some of the contents evaporate so it's pointless to continue with the samples. 🙁

The other half will be pleased to get the Kitchen table back ... :lol:
 
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