Sphagnum moss at bottom of pot for seedlings?

[eulaspiegel]

First of all I am not sure if this topic fits better into the "Cultivation" or the "Grown from seed" section, please let me know if this is the wrong forum for this question. I have searched the forum with the keywords "sphagnum" and "bottom" but didn't find anything directly about this method (described below). However, mixing sphagnum moss into the seeding soil seems to be more common, and maybe it has the same effect.

Now to the story:
I have managed to obtain a relatively cheap copy of the book "The Encyclopedia of Cacti" by Cullmann, Götz and Gröner from 1986, and I have already learnt A LOT I didn't know before (despite spending hours on different forums) from reading through all the introductory chapters about cactus biology, care, and propagation.
I am currently on the propagation from seed chapter, and was surprised to see them recommend using a layer of sphagnum moss at the bottom of pots for seedlings (see attached photo), both in the sowing stage and the first times they are pricked out. The reason they give for this (as far as I understand) is twofold, 1) to keep the soil moisture at a more even level, and 2) to buffer the pH to stay at a consistently low level, the reasoning here being that bottom watering otherwise leads to salt deposits in the upper layers of the soil, which will increase the pH to alkaline levels and thus stall the growth, and the sphagnum peat in the bottom will prevent this effect to a certain degree due to its low pH value. Now I was wondering if anyone maybe has tried this? Intuitively it doesn't seem like a good idea to put such a moisture retaining material at the bottom of the pots?

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[jerrytheplater]

I have used Long Fibered Sphagnum (LFS) on the bottom of my pots mainly to keep the potting mix in the pot. It is only this year that I began using window screen at the bottom of my pots. I have started seedlings in 2.5" deep trays with LFS in the bottom. I watered from the top. I transplanted the seedlings after a year without issue.

I own the same book, but can't say that is the reason why I started to use LFS.

[Steve Johnson]

First of all I am not sure if this topic fits better into the "Cultivation" or the "Grown from seed" section, please let me know if this is the wrong forum for this question.

Not the worst thing in the world if you post here, but you'll get more page views (and hopefully more responses!) if you post again in the "Grown from seed" section.

[MikeInOz]

There's no need to do this. I have the same book as well and I did it once. I would advise against it because after a while it leaves your mind and you end up with a mix that stays too wet for too long. I now use course perlite at the bottom of the pot (you will be glad you did later) for aeration, and use a mix at about pH 5.5 for everything. It is important to have a source of Calcium (best via Gypsum) and you can regulate the pH by using a small amount of sphagnum PEAT in the mix. This will also raise the buffer capacity of the mix (resistance to pH change) If you start with an acid mix you won't need to worry about ''alkaline salts'' precipitating at the surface and causing problems during and after germination. I have raised several thousand cacti of many species over the last 3 years this way. If you are still concerned with a high pH, use some acidified (or boiled rain water) to water in the seeds with a fine spray and make sure a good amount comes out the drain holes before you bag them.

[eulaspiegel]

Thanks for your answers Jerry and Mike, really useful to read other people's experiences.

I will probably not put a layer of sphagnum moss at the bottom of my pots, but I do indeed experience the problem they are talking about in the book. I have noticed white deposits on the surface of my pots (for the once I bottom water), and a pH test i did after dissolving some of it in water indicated that it is indeed alkaline. I already use acidified water, but it seems like I will have to acidify it even more, and be sure to leach out the salts from the pots from time to time. I have already sown all the seeds I ordered this year, otherwise I might have tried the sphagnum moss bottom layer on a few duplicates.. Maybe next time.
I am using some sphagnum peat in the mix for South American species, but that doesn't seem to be enough with the water I am using- it's not hard water, but it does have a pH of around 8 when it comes out of the tap. I use rainwater when I can but it's not accessible for me most of the time, living in an apartment.