I just germinated some mini sinningia from seed and the seedlings are very small. When I water the soil, the soil mixture just floats on the water because some of them are perlite. The perlite and water crashes into the seedlings and I thought "oops, they are going to die.". I can't separate the soil from the seedling ( without using a toothpick/needle ) and I can't just watch them die.
How do you water in such case? I think that the water will be too much if I use a spray.
Tray watering is the standard way of watering plants so that the soil surface is not damaged. The water is poured in at the bottom - so no damage, and then soaks up through the soil. The seedlings will need to be in a shallow tray for this to work (it won't work with a deep pot unless the plants have a good root structure). For my pepper seedlings, use the standard seed trays, and leave one of the squares "empty" so I can pour water into it - the water goes straight down into the tray without damaging the peat or washing seeds around.
If light surface watering is required, try a spray instead? This will then soak the upper soil surface without washing it around.
The perlite is probably floating due to surface tension effects - is it drying out? Keeping the perlite damp would avoid this (although it might mean the soil gets too wet and lead to damping off / etc of the seedlings).
@winwaed's answer is definitely the right way to do it for conventionally grown seedlings.
But you can sidestep this entire issue (how to best water tender seedlings) by using soil blocks. These are a compressed soil mixture that holds a surprising amount of water. I like to start small seeds in 3/4" blocks, larger seeds in 2" blocks. (When the seedlings in the small blocks get big they can be "transplanted" into a larger block.)
I put the blocks in a tray with low sides and when they need watering I pour a small amount of water into the tray. The blocks absorb moisture from the bottom.
The last time I planted I think I counted 300 of the small blocks and 50 of the larger blocks on a standard sized cookie sheet.
(This assumes that whatever you are starting will germinate and grow properly in the media required to get blocks to form well.)
