Most succulent plants come from arid, sub-desert conditions, where the soil is loose and well drained. To simulate such natural conditions we can use an ideal soil with several components mixed in. Certainly the soft and rich universal soil is very useful; but on its own this soil tends to be too compact and moist for a succulent, so you should lighten it. To lighten the soil the material that it is easiest to find is river sand, which is added to the soil and mixed well in order to obtain a more porous and permeable substrate, which retains less moisture. Remember that the river sand to be used in gardening should be well washed, as this will eliminate the fine dust that would otherwise harden the ground and make it excessively compact. There are other materials that can simulate the soil of arid areas. Finely chopped pumice stone is widely used, but you can also use volcanic lapilli or pozzolan, which are small stones that create areas in the substrate where the water passes through very quickly and drains away downwards. The ground thus prepared, in addition to having very good drainage, also allows correct development of the root systems of your plants, allowing air to circulate between the small stones.