i had the same problem as you and this is what i was told from anouther member on here

(I think there is too much of a big deal made over soil. These plants are literally weeds, they will grow through cracks in tarmac if you let them. I have always used bog standard seedling compost to start my plants off and it has worked fine every time. The only thing I would suggest is not to use soil from the garden as it will have hundreds of insect eggs and bugs in it, which will end up living off your plants.