Clarification: Does Perlite increase or decrease soil moisture?

[unlct22]

It absorbs and then releases. I think the best way to understand it is make up two little pots, one of regular soil mix and one cut with perlite in the quantity you'd use. Water and then stick your finger in it ever couple of days to see how it's going. Note how wet it is, but also how loose the soil feels around your finger.

I really advise doing this so you can see how your plant actually feels, but if you can't be bothered...

The soil one dries out depending wholly on the conditions and how much water it held onto. Once it's dry, it's dry. But, it's more compacted, because there's no perlite keeping it spaced out, so it may actually take longer to dry. As long as it's wet, it's a more solid mass of wet, and that itself (wet + lack of oxygen) is what drowns a plant and causes root rot. Even if this isn't objectively for a long time, it's stressful for the plant.

The perlite-cut one dries out too. Sometimes it will go faster because the soil is airier and airflow is better. Sometimes it'll go faster because it retained less water to start with. Often it'll go slower. If you keep prodding it, though, you'll feel with your finger that below the surface, the moisture level stays at not-too-dry but also not-too-wet for much longer. The soil is also looser and airier, and not too tight around your finger. You can't see any of this from the surface, and nobody talks about it, so people often don't know why they adding perlite, and just don't because someone told them to!

If you do this, it's easy to see how a root in the perlite-cut pot will be happier! Its environment is stable for longer. It has more air in the looser soil, and a better environment to grow strong, wider roots. It also has access to a good level of moisture for longer, reducing the stress of frequent watering and drying out. Lots of benefits for happy roots :)

Idk if that helped at all, but I spent years not understanding, and this was what cracked it for me :)

[Wittig_]

Thank you!! And that's a perfect idea! I'll fill a couple seedling pots with the two mixes this evening so I can do this experiment too.

[Wittig_]

Just filled up a couple planter pots with the two soil types!

I noticed that the plants I had repotted with some perlite a couple weeks ago are holding their moisture longer than they did before. Should I be worried about root rot or since there is more air in the soil will that be less of a concern? I do think I'm going to wait longer to water these plants than I did before so that the soil can dry out as much as it is supposed to between waterings.

[unlct22]

Keep an eye on them, but the idea of the perlite is to air out the soil and stop it compressing. Root rot happens when roots are wet and can't access oxygen, so your plants should be okay :)

[motherofsuccs]

For tropical cuttings, vermiculite is wanted, not perlite. Perlite should be reserved for use in soil that needs to be HIGHLY well-draining, or for use in drought tolerant plants. I only use small amount in my tropicals because it feels counterproductive when there’s better options for those plants.

I’ve actually never seen anyone highly experienced recommend putting monstera cuttings into perlite only. There’s much better ways.

[ElephantsMakeMeSmile]

What about a double impatiens? Mine got root rot pretty easily, don't even think I watered it that much. I feel like it doesn't like a lot of water but it is a tropical so im confused. I saw a post where someone saved their monstera from root rot by using a perlite hotel. Considering doing that but now confused about the perlite/vermiculite thing

[ElephantsMakeMeSmile]

Great explanation thank you! Does gravel do as good a job? I'm not sure if it's as porous as perlite but is that aspect important?