Experiment: Sand vs Grit for seedlings

[ElieEstephane]

I had an excess of seeds this summer so i thought i'd experiment with soils.
I primarily use hematite grit to grow my seedlings (and and mature plants) as it's hard to rot seedlings this way. It's not porous or anything special but i can get it in large loads for free. The grit i get has a variable size distribution so i use a window screen to remove dust and sand particles which i later discard. This time i though i can use the nutritious moisture holding sand to start seedlings thinking they would benefit from the much higher humidity. Boy was i wrong.
I had a very bad case of algae that eventually formed a layer 1-2mm thick and rotted off a couple of seedlings. After scooping it off this what was left:

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With repeated hydrogen peroxide treatment, most of it is now gone.
Now back to the main point, i think they are around 5 months old now and there's a huge difference on health, vigour and size:

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Grown in grit: 0 cases of algae

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Picked off the smallest seedling to show roots. They like to grip their roots around the grit particles and wrap around it. It's hard to remove grit without ripping roots.

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Grown in sand: brown spots are burns.

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Even though a week or so older, seedlings are half thr size of grit grown seedlings.

[AnalogDog]

Very interesting. The grit and sand seem to have a noticible iron content. Is there any magnetic properties to either. But seriously a fascinating study.

[stefan m.]

Are these trichocereus seedlings?

[ElieEstephane]

Very interesting. The grit and sand seem to have a noticible iron content. Is there any magnetic properties to either. But seriously a fascinating study.

Yes this a low content iron ore which is a pretty common rock formation around here. I never tested for magnetic properties but i often find iron chunks.

[ElieEstephane]

Are these trichocereus seedlings?

E. Subdenudata

[stefan m.]

So close... but not surprising cus they are related.

[toson]

so they are stone eaters

[ElieEstephane]

so they are stone eaters

After a while i start applying very dilute fertilizer so i cannot confirm that.

[toson]

so they are stone eaters

After a while i start applying very dilute fertilizer so i cannot confirm that.

that doesn't matter they'll completely become stone eaters when mature.

[DaveW]

"that doesn't matter they'll completely become stone eaters when mature."

It depends whether they have the right symbiotic bacteria, since it's the bacteria that dissolve the rock, not the cacti. It is said the bacteria are passed on in the seeds, but I don't know if tests have been done to show if seed from commercial sources still contain them, or also whether all species of cacti possess them?

http://news.bbc.co.uk/earth/hi/earth_ne ... 209687.stm

Young seedlings do not need a lot of nutrients since it makes them form a better root system to find them, so there are probably enough initial free nutrients in the grit to start with before they need fertilisation.