Someone here called it "gardening for engineers". Probably apt.




High thermal mass=GOOD! If you have a large thermal mass of water (big sump and fishtank), everything is more stable. Mine tends to vary by no more than 1degC per day and sits right near average daily temp, even though we often get 20degC daily fluctuations in air temp. Chemistry is far more stable, too.

Thermal mass of rock is typically about .2 to .25 of that of water, so the rock mass is not nearly as important for temperature stability as having a large volume of water. Also, the once or more per hour flood/drain should keep growbed temperature fairly close to that of the fishtank.

The surface area of hydroton is roughly similar to other media. The trapped air will allow it to warm or cool faster (less thermal mass than solid media), so it should not lose or gain as much actual heat as solid media with the same color in the same circumstances. The hydroton will actually insulate the growbed, but how much? And will the surface get too hot and burn plants since it cannot conduct heat well to lower gravel?

That said, dark, unshaded growbed material will absorb far more heat which will warm the whole system. Shaded or white media will keep it far cooler. I like light media as it tends not to burn plants in direct sunshine, but cheap trumps white.

Any water heating above average temperatures just seems to evaporate water and bring temps back to average. 2degC above average is OK, sometimes. Cooling water is far easier with a fan and shading 5degC might be reasonable, but I haven't tried it.

Wow, I didn't realise that water has such a high specific heat value! I thought, with the density of rock, it'd be much better for storing heat, but you're right, water does it much better! Thanks for pointing that out.

Does anyone know if hydroton is high in iron? The red exterior and black interior give a strong hint that the answer is "yes" and my indoor system (the only one where I have sprung for that expensive stuff) is the only one that has no nutrient deficiencies. Of course, the ph is down around 6.8, even with lots of shells for the catfish to hide under, so that is probably even more of a factor.
viewtopic.php?f=18&t=6870

I use hydroton on top of red scoria. I had to supplement iron via Greensand when the ph was 7.4. It dropped to 6.6 on its own and I haven't had to supplement since.

Looking through here I couldn't find anyone mentioning quartz. We have an abundance of the stuff and have been building gabion fences out of it. Just up the road is a quarry that specialises in the stuff http://www.creswickquartz.com.au/product.html. Yet to check prices but I have heard it is really cheap.

Anyone have experience with this??

Quartz is very very stable, so will not change your system ph or nutrient balance. Should be fine, but check ph occasionally in case it tries to plunge off the scale and kills off your bacteria.

I know that it is not all that shell grit is not all that effective once ph is off the scale, but I wonder if adding some after the system drops below 7.0 might be reasonable to prevent ph from plunging. It needs numbers, though...

If you can get scoria from the garden supplies for around the same price i would go fir that [20 mm] scoria is pour is[holds water ]

I haven't assayed it of course, but there's no reason hydroton should be high in iron - it's clay based, and no clays are very high in iron compared to other media discussed here.

Hmmm.

I usually think of iron-containing minerals either being reddish (like hydroton) or black (like the rust from an anchor buried in the sea bottom). I think one is ferrous oxide and the other ferric oxide, but the details are lost in the distant past of high-school chemistry. Anyway, hydroton certainly looks as though it contains both of these.....

Ferrous oxide is black. Ferric oxide is the red-brown oxide colour you are thinking of.

Could be possible.... the clay used in Hydroton is a shale...

That's interesting. Also interesting is the "clay shale" mine in Berrima: http://majorprojects.planning.nsw.gov.a ... ob_id=2713

Any chance we'll get a local clay ball maker? (the "food miles" on the stuff rubs me the wrong way- mined who knows where, made in germany, washed in holland, shipped to australia).

This one might be off the wall a bit, but has anyone considered glass as a grow media? Kind of thinking old beer/wine/soda bottles etc could be broken and tossed in a tumbler with some water and sand to smooth them up. I know it would not have the micro pores, but might still work and look pretty too! Perhaps some etching acid could be used to make some pitting. Anyone?

I would have thought that broken Glass particles would be too small to allow efficient solids removal (not enough air spaces as compared to 20mm gravel or Canna clay etc.) and that it would allow sunlight to penetrate the media so that algae growth would become a problem?
If plant growth disturbed the media, I would not like small glass chips/particles being allowed to recycle into the Fish Tank!
Just some thoughts.

Cheers IanK

ahh ya - did not think about the sunlight=algea thing. I'm not to sure I would worry about small particals though. Some of the exanded shale I have not is not all that big. 10mm x 5-7mm seems to be the average. About the same as a thick piece of glass. The glass thing would also be a TON of work, but could be cheap.