Water retention in pumice vs. crushed granite

[BarryRice]

Hey guys

I grow my plants in an inorganic medium of pure aggregate, fertilizing lightly with acidified water with every watering.

I have easy access to pumice, but I also value crushed granite because it is denser, and discourages pots from tipping over due to top heavy plants. Also, after I water my plants (which are in plastic pots), if they are in pure pumice, the pots and the planting medium can flex a lot as I move the plants--I am afraid this might cause some abrasive damage to the roots? In any event, granite doesn’t shift as readily like this.

I was curious about the relative water retention of pumice vs. granite, and also how well the media retain this moisture over time. So, I set up an experiment.

Materials: I dried, in my oven at 175 C (350 F) for 30 minutes, planting media for three pots: pure pumice, pure crushed granite, and my general planting medium which is 4:3 pumice: granite.

After waiting for the media to cool (heh heh), I filled three 12.5 cm (5”) pots (747 ml each) with the media. I then watered all three pots thoroughly, then let them drain--tilting the pots a few times to facilitate the draining. The pots were weighed at each step of this process. My scale is accurate to +- 1 g, and so with masses of 480-1500 g my measurements are reliable. For brevity, I will not post my data, but if anyone wants to see it, I can.

Over the next ten days, I kept the pots with my growing plants in bright shade, at normal early fall growing conditions. Each day I’d weigh the pots so I could track the decreasing water content in the three media. Here are my basic results. I start off with the % volume in the pot of medium, water, and air, immediately after the pots were drained on day 0.

Pumice: 46-22-32%
Granite: 58-14-28%
4:3 mix: 56-17-27%



Nothing too surprising here. Pumice retains significantly more water throughout all stages of the experiment. Does anyone know if there is a lower threshold to % water content in the soil, below which cactus roots are unable to uptake moisture from their surroundings?