1. Size
What is optimal particle size of lava rock for a small coffe mug sized plants. A uniform particle size of .100 (1/10 inch) ensures the medium cannot hold excess/ perched water in the spaces between soil particles. "Optimal" would be no particles smaller than .100 (2.5mm) and all particles as close to .100" as possible. It's not a perfect world, so you'll need to decide how close to perfect is practical for you, but if you're to err, it should be on the side of particles being slightly >.100 as opposed to smaller.
I actually sifted my red lava rock by few sizes because I could not find any smaller than 1-5mm grade:
below 4,5mm (arround 1/6 inch)
between 4,5 to 7mm (slighly less than 2/7 inch)
and above 7mm (which im sure is too big, it really looks like that big chunky dog food some people have for their pets) Unless the pot/plant combo is very large, the upper particle size should be limited to particles <.200 (5mm) for best results.
Should I use the smallest one or can I combine it with the medium one, since there is very little of those finer particles and I really dont feel like crushing that lava. That would be up to you. I would try to find another material closer in size to the ideal ..... perhaps some mixture of crushed granite, pumice, Turface, pine/fir bark ...... Using materials with different water holding characteristics provides you with the ability to 'adjust' water retention by varying the ratio of ingredients.
2. Washing
I've heard it contains a lot of usefull minerals for plants, but at the same time if I wash all this dust it is kinda like washing all those minerals away, is that dust really that bad for drainage and if anything blocking those holes in lava rocks that it needs to be washed? Yes. Even a 1/16" layer of sludge covering the bottom of the pot can provide an environment which favors any of several fungal pathogens gathered under the umbrella of damping-off diseases. Rocks are not candy and minerals already have really hard time disolving from them, that dust would be literally the only source of those minerals if any. Even if the dust was as fine as talc, the particles would still be hundreds of times too large to be beneficial to the plant, and very little mineralization will occur in containers where the environment is more hostile than welcoming to soil life, especially if the medium is comprised of all or almost all inorganic materials. That isn't a problem, at all, as that's what fertilizers are for. The grower would do well to stay focused on how the grow medium is structured and not its ability to provide nutrients to the plant. Growing in containers is much closer to hydroponics than growing in the earth/garden.
Also I assume the dust, especially the finest one might increase CEC (cation exchange capacity) since it is like clay and clay has high CEC, I plan to combine it with coco coir, which has good CEC but not the greatest tho and since I'm watering with hydroponic nutriens, I want to avoid fertiliser burn, which high CEC helps avoid I urge you to avoid sacrificing drainage on the altar of nutrient retention, and thinking mineral dust in your medium will have potential to be anything but limiting. You can use a calcined clay or DE product which gives you added CEC w/o having to worry about the sludge.
Are you sure you want to use coir as the only organic fraction of the medium? Why go through all the trouble of worrying about particle size and building an 'ideal' medium when you intend to undo all your well intentioned efforts by filling up the spaces between particles with coir?
Coir has many issues that would need workarounds. It's high pH precludes use of dolomite as a liming agent, it is very high in K, very low in Ca, often very high in soluble salts, can be so high in Mn that Fe uptake becomes an antagonistic issue. Fresh coir also contains allelochemicals (toxic to plants susceptible to the chemicals) that affect a large enough % of species that it shouldn't be ignored. Most growers think they'll simply replace peat with coir and the plant won't notice. It will.
Also the dust kinda stops those sharp edges from being sharp What sharp edges do you refer to? You could grow a perfectly healthy plant in a tub of broken glass if you had willingness to keep up with the watering it would require (think hydroponics).
3. Porosity
While the red lava rock has a lot of nice holes, the grey one literally has almost none, I was kinda disappointed when I opened the bag, because it really looks like any random gravel from an unpaved side of a road, also after washing small amount to see how it looks like, unlike red lava rock and pumice it looked like the water stayed on its surface... Did I just buy a bag of kinda expensive grit or there is more to it? There is more to it.
4. Desinfection
Can I bake my lava rock in the oven, and if so on what temperature... I'm quite sure its ok but it will never hurt to ask... No need.
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