Help with melocactus seedlings

[Benyellow]

So 5 months earlier I've planted a few melocactus seed in a container, the soil is poor in nutrients (I wasn't really good in soil stuff back then.) They are about half a cm tall now, but I noticed they have been staying the same size for 2 months. Is this a sign of no nutrients in the soil?
I've bought a fertilizer of NPK 4-16-16, is it safe to use the fertilizer on the seedlings?

[adetheproducer]

I started some melocactus just over a year ago and they do seem to stall the growth at that size for a while then pick back up again, my discocactus are doing the same thing. Just watch the moisture levels they won't need quite as much water for a while then when the grown spurt kicks in again carry on as normal.

[Benyellow]

Alright thanks a lot

[7george]

Some people commend re-potting or changing the soil of seedlings every 6 months or so. I don't do this thing even a year or two very often and don't think the depletion of nutrients is the problem here. But I haven't been doing Melocactus seedlings so far as well.