I think magnesium alone should work too. It will bind to the coconut, and the more active sodium will pass into the solution. According to the concentration, probably 1-2 tablespoons of Epsom salt should be enough for 40 liters. Leave it overnight or for a day. Maybe someone will tell you more precisely.
Nice, should be cheap and easy enough.
Most of the sand where I live is sugar sand very small. Some larger particles but mostly small. It works well in a mix but over waterings it settles downward until.the roots are dense enough to hold it in place. I sprinkle some on the top of the dirt and it makes it's way down. But we also have a lot of small shells that are larger than the sand particles.
From what I read the downside of small sand can be that it fills the air pockets in the soil and clogs everything up. So it still helps in the sense that the particles themselves don't soak up moisture, but it can downgrade the amount of air in the soil and also how quickly it drains.
Citrus don't like mulch generally. But it also depends how hot it is and how much you water. If you have good drainage you will need to water more often. That's ideal, really good drainage let it dry between watering but in the hot summer probably water almost every day. If you are going to be gone for a long time you might need to add clay or something that will hold water. Otherwise perlite with a small amount of sand and compost is pretty good. The perlite floats upward and the sand downward.and the perlite is bigger. If it's not hot perlite can get soggy but in hot weather dries really fast. In Europe with cooler weather I would go heavier on sand and only a small amount of perlite perhaps. Though sand will make it heavy and perlite is light. Citrus grows in Florida in almost pure sand so it won't hurt it just moving the pots is more difficult with more weight. Even small rocks at the bottom of the pot can be good. There is no perfect soil because you have to consider the temperature how frequently watered etc. Just keep the dirt from staying wet for prolonged period because citrus doesn't like wet roots though they like a lot of water as long as they can dry between watering
I don't want to have to water everyday, but I think the coco coir and potting soil can take care of that. I also want to influence this by using fairly large pots, then they hold more water, but using enough perlite and sand so that it can't get oversaturated or have a lack of air in the soil.
My plants are small though, so pots will be 20 - 35 cm I think.
In summer it should still be possible to have it (almost) dry out once a week. In winter it will rarely or never dry out, even if using small pots, so I don't think using larger pots would change anything in winter. I think in that case it would actually be better to have larger pots with lower saturation point than small pots.
I don't like using rocks or anything at the bottom. I tried that at the beginning using some gravel, mostly to make the pots a bit heavier and also thinking it might help water to flow out of the holes at the bottom of the pots. But what happened was that the citrus liked this thin layer of gravel and grew all it's roots there instead of in the soil. So when the weather started to become hotter, the plants had a lack of water because most of the roots had grown inside the layer of gravel, which didn't hold much water. And even with daily watering it would just cause the soil above it to stay wet and risk root rot. Because of this I prefer to make the substrate as homogenous as possible. Which also means I don't want sand to move to the lower half of the pot and perlite to move to the top half.
@Everyone
Some good information here in the topic, comments are still welcome of course, but I think I will have to experiment to really get further now. Buy a bag of sand and see how coarse it is, see if I can easily sift out the finest particles / dust. Will have to wait a few weeks before I have time for that unfortunately.
Maybe I'll start with something like this:
25% perlite
25% light potting soil
25% coco coir
25% sand
And then start changing the percentages and see how the soil behaves.
Like @pagnr says it will be a bit like cooking. I suspect that the ideal mix will also depend on the exact type of sand, potting soil and coco coir. I imagine the type of sand and soil / coco coir can influence how they cling to each other, same for the perlite. Which may influence how the amounts of each ingrediënt will have an effect on if the mix stays homogenous