Gas buildup in my substrate

Hello all.  I've had a fish tank for most of my life.  Recently I've
decided to create a tank that is mostly plants with some sup****tive fish.

I've created a substrate that is composed of vermiculite and soil (50/50
and about 2-3 inches deep) topped by about 1/2 inch of general purpose
sand.  My plants are growing well and showing good color and my fish
look healthy too.  HOWEVER, my substrate has swelled about 1/2 to 1
inch.  If I push on it a ton of gas bubbles come out.  Is this a bad
thing?