Seedlings stopped growing.

[BenCSS]

I have grown a lot of different plants from seed but there is a challenge I encounter with many cactus and succulent seeds. At around the size of a dry pea they stop growing. Eventually some bounce back and start growing again but some never do and eventually die. (currently I have some frailea I started in 2021 the largest is about the size of a dry pea the rest about the size of peppercorn.) So I'm looking for any advice to improve seedling growth. I will provide as much info about my current growing conditions as I can below.

I live in Chicago and so I grow my seeds indoor under grow lights until it is safe to move them outdoors the next summer. I started a variety of different seeds over summer all have stopped growing. I started them in a humidity tray in washed builders sand. (In the past I used coconut coir but stopped do to mold and plants damping off.) I left them in the humidity trays until haworthiopsis, psudolithos, &, Whitesloanea crassa seedlings started dying about 5 months later. After that I took them out and cut back on watering to twice a week. PH of water is about 6.3 Temperature average around 72°F/ 22°C Fertilizer I started with was Bonide Liquid Cactus Plant Food, but I switched to Liqui-Dirt a few weeks ago. (I haven't noticed any change)
Light - Cresmar P1200 LED Grow Light. The lighting varies depending on their position:

120 µmol/m²/s - Psudolithos cubiformis, pseudolithos migiurtinus , Whitesloanea crassa, & Pilosocereus leucocephalus
The Psudolithos & Pilosocereus appear a heathy green color. The Whitesloanea looks kind of yellow/brown looking, not sure if that's normal for it.

70µmol/m²/s - aloe hybrids
They look sun stressed, but I am not very worried about these since I grow a lot of aloe and they always start growing again in Summer.

40 µmol/m²/s - aloe hybrids, & haworthiopsis
These aloe hybrids look healthier.

[esp]

I use mineral only mix for raising seeds and use sealed plastic containers, sterilised and soaked with fertilised water prior to sealing. I find I get a growth check after a few months if I don't open up each container and start fertilising after germination and some growth has occured.

As you are using a nutrient-free mix, I wonder if you are having a problem with the roots absorbing nutrients, and / or aeration of the roots as well?
Builders sand is not at all porous, so may lend itself to nutrients being washed away from the roots. It may also lend itself to compaction, with very few air pockets remaining in the medium as any mineral content in your water becomes deposited as solid.
If you add fired molar clay granules, perlite etc it may help open up your mix, and perhaps assist in retaining nutrients closer to the roots. Nutrients are supposed to bind to molar clay, as well as having both water retaining and draining properties, so may be a useful addition to your root medium.

[BenCSS]

As you are using a nutrient-free mix, I wonder if you are having a problem with the roots absorbing nutrients, and / or aeration of the roots as well?
Builders sand is not at all porous, so may lend itself to nutrients being washed away from the roots. It may also lend itself to compaction, with very few air pockets remaining in the medium as any mineral content in your water becomes deposited as solid.
If you add fired molar clay granules, perlite etc it may help open up your mix, and perhaps assist in retaining nutrients closer to the roots. Nutrients are supposed to bind to molar clay, as well as having both water retaining and draining properties, so may be a useful addition to your root medium.

I should have mentioned the builders sand is washed out with a sieve so its fine but not compact. I will try adding molar clay to my sand next seed batch and see if it makes a difference.

[BenCSS]

Took some suggestions from different forums unfortunately nothing made a noticeable positive difference, except for increasing the brightness of the grow lights a little bit.
Tried repotting some in the recommended ratio 1/3 potting soil,1/3 builders sand, and 1/3 perlite (But I substituted red lava rock because I couldn't find my perlite at the time. These seedling did the worst of the bunch.
Tried increasing watering 3 to 4 days a week on the aloe and Pilosocereus.
Tried increasing the hours of lighting from 12 to 16 hours a day.
Tried changing the fertilizer to Schultz 20:20:20 at 1/4 tsp per gallon
Photos on the left at the beginning of the month/ right at the end of the month.

Still open to any subjections.
I might try watering some daily. Also thinking about trying plant growth regulators.

[habanerocat]

Have you checked the PH of the water after the fertilizer is added?

[esp]

How much are you watering? It's hard to tell much from the photos, except that the Aloes / Agaves at the end look very dehydrated.

If you are using containers with no drainage holes, it's tricky to get the amount of moisture correct. Too wet, they get waterlogged and rot / damp off. Too dry and they don't grow / die.
I find transparent containers easier to use than opaque ones for this reason, the colour of the potting mix at the bottom of the pot changes colour when it's completely dry. The aim is to keep the bottom of the mix slightly damp. Once seedlings have big enough roots to penetrate a decent dept down into the container, this normally gives decent results.

[SimonT]

I'd swap out the builders sand as it might contain lime/limestone which would cause problems for some plants.
Washing would get rid of salt but lime/limestone is not very soluble and so could remain in the sand.

You should consider adding Vermiculite - it tends to hold water and this stops rapid drying out. It also buffers nutrient release.

But I agree with the last post, the plants look very dry in the photo. You could always dig up a few and look at the roots
and see how dry the compost is under the surface? I've found doing this quite informative.

You don't say if you are using a heating mat or another source of bottom heat? Mats can really bake compost dry- you have to increase
watering quite a bit to compensate.

[BenCSS]

@ habanerocat, Yes I make sure the PH is about 6 before I water.

@ esp, Those were the ones I repotted based on a suggestion from another forum. I had started watering twice a week but increased to 3-4 times a week. I will be trying daily watering to see if they improve. The trays dry out completely within a day.

@ SimonT, I am reluctant to use vermiculite because it could be contaminated with asbestos. I don't use a heat mat. The air in my home is very dry during the winter so the trays dry out very fast on their own. That's why I'm going to try watering daily.

[SimonT]

You have to decide for yourself whether or not you think it is safe to use vermiculite. In the UK we can download the safety data sheets and you can read the health and safety assessment that accompanies the product. This is very helpful. It then makes it easy to take the appropriate precautions to use the product safely.

The bottom line is that your plants look very dry and you don't have the most common components of the
compost that are used to retain the most moisture for seedlings eg organic, loam or clay.
So in a dry environment your compost dries out very quickly.

You might consider partial/fully covering the seedlings to retain some moisture. You can also top-dress with something like decomposed granite grit as this can help slow the drying of the compost. You could water every day as you suggest but this will be quite tricky to get just right.

If you can post a higher res photo that would be good.

[BenCSS]

@ SimonT They were in a humidity tray but the psudolithos & Whitesloanea started to die in there so I took them out. Its hard to find ways to retain moisture without the environment getting to humid. I will try using molar clay in the future if I can find it at garden supply stores near me.
The forum page shrank my photos. I posted them here, you should be able to open them in full size.
https://bonsaiexperiments.blogspot.com/ ... lings.html