Heat retention in grow bed media

[txdurk]

I live in Houston, Texas and it gets a bit warm down here in summer. We call them 95-95 days for 95F and 95% humidity but it isn't unheard of to go over 100F. I am planning a system and was concerned with the grow bed media retaining solar heat and the negative affects it would have on my plants and fish. My fish tanks can be shaded from the sun and half buried in the ground to capture some cooling effect but the grow beds will be in the SE texas sun all day. The media types I am considering using are shale, hydroton, or pea gravel. I have shale and pea gravel readily available locally. I'd have to ship in hydroton I believe. Ideally, I think the one that retains the least heat wins out. In worst case I could also place shade cloth over the grow beds or slatted wood but I may be limiting sunshine and it adds cost.

Maybe the evaporative effect of the water in the grow beds will cool everything down a bit. I just don't have first hand experience with it yet. Any advice or experiences from other hot weather APers would be appreciated.

[Carey Ma]

Hi Dave, I worked with a Dr Sing about a decade or so ago, trying to design a hydroponic system, suitable for growing in the tropics. Two factors that may help in your situation are: Insulating heat from the top layer via wood or Styrofoam. Thin plastic mulch or evaporation does not work. We bored out holes where the plants would grow and lay the styorfoam over any media, then planted seeds or placed plugs in the appropriate holes. 

The second factor is root zone temp. We used block ice to cool the water in grow beds. This can be done passively by running water through a piped heat exchange or by direct melt if there are evaporation issues. Block ice is more cost effective than ice cubes or any other method of cooling in high temp conditions. What I did was to use an old soup pot for ice making and an old 5 gal, insulated igloo drink dispenser, found in most construction sites. Grow bed temp can be controlled by the flow rate which also has a direct effect on how fast the ice melts.

I hope this helps.

Cheers

[Jason Yahner]

I was wondering this same thing. Does granite retain more or less heat than the pea gravel I wonder? I have container of both I'll use lamps to test the idea.

[txdurk]

Thanks Carey.
Let us know what your findings are Jason.

[TCLynx]

Make sure your system is big enough to help mitigate extreme temp swings.

Some aluminet shade cloth for the extreme summer mid day sun may be beneficial. (I've used 40% direct over head for my 300 gallon system during may-Sept here in central FL)

If things are really extreme you might add more water and let the system run constant flood to reduce media heating.

Lots of plants to shade the media and the sun hitting the media will be less of an issue.

Or a sturdy arbor over the system and grow lufa for the summer (just don't let it at the trees or power lines.)

I use river rock (be sure the pea gravel won't affect pH do a fizz test)

Expanded shale will probably also work fine.

[txdurk]

TC: what exactly is a fizz test please?

[txdurk]

Disregard. I found out what it is. I have some pea gravel outside that I'll rinse and test.

[txdurk]

I would estimate 10% of my sample of pea gravel fizzed using white vinegar.

[Chip Pilkington]

Hi Dave,

I live in the Tropics - currently in Thailand and previously on the Equator in Indonesia.

You do not need anything additional, outside of shadecloth during the hotter periods. A sliding system can be really nice to allow morning sun when ideal.

The critical issue is raising appropriate species of both fish and plants for the climate. If you want to raise trout in the summer months, you'll need to worry about more than just heat transfer in the GB's.

There are many AP and Hydro systems all over the Tropics, Africa and Middle East and most I've seen require nothing more than shadecloth for Tilapa. Seeds should be appropriate for similar climate. I grow beautiful (US seed) heirloom tomatoes here, as they are zoned for Key West, Houston, San Diego, etc.

Also, river rock and crushed brick are the media I use successfully, though any acceptable AP media will work in your climate with appropriate species.

[txdurk]

Thanks Chip. That's good to know. I'll plan for shade cloth or lattice for shade. Due to the heat i have resigned myself to native fish (Coppernose bluegill, crappie, or chanel cats) since they can better tolerate the temps here. I'm jealous of anyone able to raise trout or yellow perch though.

[TCLynx]

Catfish and bluegill are both good fish to raise

[Jason Yahner]

Dave, I started out with all pea gravel. On the advice of the person that told me about this site I changed my mix to 70% 1/2 inch crushed granite and 30% pea gravel. Our PH out of the tap is around 8.2 to 8.4. The granite I'm told has acidic property that are causing the PH to drop. Where the pea gravel alone cause the PH to go off the charts. So much to learn just to create mother nature's handy work. Now if I can just figure out the lighting thing. 

Dave Durkin said:

I would estimate 10% of my sample of pea gravel fizzed using white vinegar.

[TCLynx]

It may not be the granite that drops the pH but actually the bacteria working the nitrogen cycle.  Most systems will require adding some form of buffer on occasion or hard well water may take care of that too.

And "pea gravel" describes the size/shape of gravel not what it is made out of so you have to find out what type of rock it is before you assume pea gravel would be appropriate to use or not.  Around here the cheap pea gravel will be limestone which will keep the pH too high for aquaponics to work well for the plants.  Hence why I recommend the "fizz test" with vinegar to make sure you are not using limestone.