Dirt and Grass

I'm trying to make a procedure that turns dirt into grass if y+1 has air, and grass into dirt if y+1 has a block.

Event trigger - triggered by external call or when (global trigger): No additional trigger
  if ( (Get NBT number tag "Soil" of block at x: (x) y: (y) z: (z) if it has tile entity) > 0)
    do
      (Set NBT number tag "Soil" of block at x: (x) y: (y) z: (z) to ( (Get NBT Number Tag "Soil" of block at x: (x) y: (y) z: (z) if it has tile entity) - 100) if it has tile entity)
      if (Is air at x: (x) y: (y + 1) z: (z)
        do
          Place (Grass Block) at x: (x) y: (y) z: (z)
        else
          Place (Soil Block) at x: (x) y: (y) z: (z)

This is in update tick of both grass and soil blocks.