hang_loose,
I have never tried lava rock but people who's opinion i trust say it works fine. There is some lava rock to not use. IIRC the "red" stuff will dissolve into clay over time but the "black" stuff is ok. Also stay away from slag and anything made from metals, obviously. Lava rock is good in that is is relatively cheap, porous and light. Negatives would be the sharp edges which prohibit working in the media with your hands. Its usually a larger media so it's difficult to sow seed directly in the bed and it's difficult to move plants around without damaging the roots. I often sow a lot of seeds in a small area and then space them out when they start getting bigger.
In that indoor system i was using the very expensive expanded clay "hydroton" because i did not know any better. It's porous, light and PH nuetral. Hydroton is relatively large. If you are lazy like me and sow seeds directly into the grow beds you will have a low sprout rate... roughly 20% in my experience. But the main negative is cost because it is very expensive. ~$30-35 per 50L
You will see in future systems where i change to expanded shale which is ~$60 per CY in the dallas area. Expanded shale is probably cheaper than even lava rock in most cases. It's kilned like expanded clay and "popcorns" into a less dense porous rock but it is a little heavier/denser than expanded clay but much lighter than river rock. While they could theortically make expanded shale in any diameter, I have only seen it in smaller "pea sized " diameter. This small diameter can cause it to be susceptible to clogging where fish solids enter the grow bed and are unable to seep into the grow bed to get broken down. It just kind of builds up and puddles on the top.
Supposedly expanded shale is ph nuetral but in my experience it is not. (maybe it's because shale layers are next to limestone layers in the ground and there is leaching? i dont really know). The expanded shale i have worked with buffers my systam at about 7.8 PH. The 7.8 PH is considered "too high" for plants and very good fish but in practice I find it works fine for both if you are mindful to do things to keep micro nutrient levels like iron at good levels.
The smaller diameter has a positive aspect in that I am able to sow seeds directly into the media and get a good (~80%+) sprout rate.
The non-nuetral PH also has a positive aspect because it buffers the system at a specific ph and never budges. The PH of most aquaponic systems go down slowly over time because the nitrification process (process where bacteria break down ammonia [produced by fish] into nitrite and then nitrate.) I also catch rain water [which usually in the low 6's of PH], but my system's PH never changes.
brian
Last edited by bcotton; 05/28/13 05:51 AM.
