"Soil collapse", as I used it, merely refers to plant media getting more compact over time. Preventing this is one of the main reasons for adding chicken grit, course sand, pine bark, etc to soil mixes. As an example, but maybe not the best, I once spent a season adding fine, white play sand as a top layer (for display purposes) to a number of my potted semps. A couple years later, I found out that the sand, over time and with the accumulation of dust, etc, became nearly as hard as concrete due to just settling (or collapsing). When working with my potted plants, I seldom use tools but in this case had to use tools to break the soil layer to get the semps out. Needless to say, the plants did not have the best of growth either which is one of the reasons I was transplanting them.
