Only going by what answers have been on here on previous similar questions it may be the slow release plant food.

Are they your own seedlings which you have grown, if so did you mix into the soil the plant food? Or did the soil which you bought already have them in?

Nothing in the soil in this country has orange or yellow eggs - slug eggs are greyish white, so the most likely explanation is, indeed, spent fertilizer pellets.

I'd already guessed slow release pellets when I saw your answers, Bamboo and Marion - I got taken by these myself once, and was convinced they were some exotic mollusc's eggs.

If in doubt squish - slow release fertilizer will be yellowish inside and they do sound like slow release plant feed. As others have said, eggs of slugs tend to be smaller, whiter and translucent and found in quantity together.