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I made a custom cacti soil mix for succulent, which is 1:2:1:1 of potting soil, pumice/perlite, coir and fine vermiculite.

I think it should be quite loose, should I add gravel?

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Soil mixes are a topic that has many opinions. Some common tips are not as good for a plant as you might think.

  • Putting gravel in the bottom of a pot to improve drainage is a long time practice that does not benefit the plant. Gravel reduces the amount of soil in the pot and acts as a barrier to drainage until the soil is saturated.
  • A clay pot to provide some weight for potentially top heavy plants is a good idea. A clay shard over the drain hole prevents fine soil particles from washing out but does not improve drainage either. You could put a piece of landscape fabric or nothing.

For the soil mix you mention the components that degrade are the potting soil, which is probably peat based, the coir which slowly breaks down and the vermiculite. If you do not intend to repot the plants on a regular basis a soil mix that would last longer could incorporate some portion of a clay based soil. One recipe calls for:

1 part garden soil 1 part coarse builder’s sand 1 part peat moss

Notice the use of sand, usually clean sharp washed sand to improve drainage. Your mix uses perlite and vermiculite to improve drainage.

Try your mix out and see which plants do better in it but don't bother with gravel or clay shards.

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  • That's new. I've never heard that gravel doesn't work at the bottom. Good one. – lamwaiman1988 Mar 29 '13 at 4:18
  • Gravel do work. She said "Soil Acts Like a Sponge: Water won’t run out into the gravel, or out of the pot, or anywhere, until the soil is saturated." But what if you put to much water? Water falls downwards, pass quickly through the gravel and leaves the pot. Ad soil doesn't act like sponge, unless the mixture of soil is wrong. Clear that, if the soil does not contain sand but only peat, it will be spongy and absorbent. The sand helps drainage until the bottom of the pot, and the gravel helps excess water to drain away. That means that it avoids the stagnation at the bottom of the vessel. – violadaprile Mar 29 '13 at 11:37
  • She says too: "Not Enough Soil: Just like with a sponge, water naturally settles toward the bottom of the soil. But because you’ve partially filled your pot with gravel, that soggy soil bottom is now higher (and closer to your plant’s roots). Basically it’s like having a smaller pot!" - Yes it's true, but, of course, depends on how much gravel you put, and you must take this into account in the choice of the jar and prefer a slightly larger one. In addition, I add ground, always in the spring and a couple of times a year. The soil is not missing! – violadaprile Mar 29 '13 at 11:44
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You really want to match the potting mix to the natural habitat of the plant you are growing and to your watering habits. Some growers go with completely inorganic pumice or fired clay mixes for things like haworthia and gasterias and water more frequently and with special fertilizers.

If you use sand make sure it is not Builder's sand which is a type of sand that is supposed to compact down. You need sharp sand. These are two different things. Builder's sand will pack hard and eventually result in root death because roots need oxygen and the sand density will suffocate them.

An excellent and cheap additive for succulent potting mixes is Napa Auto Parts Oil Dry #8822. This is a porous fired clay that doesn't break down or clump.

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Yes, please. The ground already prepared is generally good. In my opinion, I would add some clay to the mixture.

But, for excellent drainage, you have to put on the bottom of the vessel a shard of broken pot for each hole of drainage. And, in addition, about half a centimeter/1 cm of gravel on the bottom. Depending on how big the pot is (and the plant, of course). Don't mix the gravel with the soil.

Use a new pot, or an old one, thoroughly disinfected.

For disinfecting clay or gravel, or pot if you are not sure, use hot steam, by a machine steam or an iron steam.

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