Best soil mix for seedlings kept 3 months in plastic

[PedalWench]

I'm having pretty good success getting germination and growth by using standard C&S soil and microwaving it and using boiling water and setting the whole thing in plastic containers and ziploc bags.
However, after a while, the soil becomes quite hard and when it's time to transplant, I'm causing root damage.

What is the best recommended medium for starting seedlings using this method? When my plants are bigger, I use a blend of pumice, perlite, grit and an oil-stall-dry product, but I think it's too chunky for little baby seedlings.

[7george]

I don't know what is the content of your "standard C&S soil " but it shouldn't be very hard when moist. Not much water will be needed to ad for 3 months if baggies are sealed all that time so salt content would not be so high at next transplanting. You can also try to crush manually some of your normal (for bigger plants) mix if the needed volume is not huge. I also use quartz sand in my mix for seedlings that is sold in bags and has small particles (~ 1 mm), retains moisture on and never hardens.

[greenknight]

I use 3 parts crushed pumice to one part potting soil, for really fine seeds I add a surface layer of sand. Pumice crushes easily - I just put it in an aluminum cake pan on a hard surface and crush it with a heavy object. You can also buy propagation grade pumice - google it.

[PedalWench]

Thanks, folks! The standard mix I was referring to is a store-bought blend. I never use it for my grown-up plants, just the babies and seedlings. It starts out fine, just after a few months and when I'm trying to transition out of the bags that the soil cakes up. I'll definitely try crushing some pumice and adding that. And, I'll look into some quartz sand. I live on the beach, so it seems funny to buy sand, but I will!