we just used the soil we found around the plant nursery which has a reddish and claylike soil texture. Not ideal for the seeds to germinate, we found out. It makes them hard to thrive, because of the slow drainage rate and therefore the high water-holding capacity. Then we tried an other mixture with sand, which we bought by the river bank nearby and an almost black soil which we discovered near our workers house. A nice and smooth soil. Who thrived pretty well and now, after several times, throwing pots away and start from scratch, we had our first success! So, best mixture is 80% Sand and 20% Compost or black soil.