Avoid organics in the soil mix. Organics lead to problems such as what you are describing and others such as proliferation of fungus gnats.
You can use straight Turface as suggested above (John Deere sells it, don't get too fine stuff). Another substitute is PondCare Aquatic Planting Media (APS) which tends to be cleaner, has less TDS when wetted. Some wallmarts have it and it can be used as is.
I have experimented with a variety of mixes that included APS, Sand (never use fine stuff from home depot, find industrial quartz, where each grain is about 2-3mm), Small lava rocks and Perlite in equal ratios or slightly favoring APS. Perlite tends to occasionally induce algal growth so at the end the mixes that had the lest amount of it remained algae and moss free the longest.
I strongly suggest washing your soil components individually with tap water first to remove fine particles and then several times with DI water to lower TDS. Also dont mix stuff too vigorously as sand will grind turface and perlite and produce a lot of fine particles.
Pings benefit from a bit of Red Iron oxide in the soil. And it is also good idea to cover the surface after planting with 1/4" of washed sand to keep moss at bay.
If you are concerned with soil washing out from the pots use a bit of grass shield fabric on the bottom (Home Depot/Wallmart)
Hope it helps.