Answer: Sounds like the former tenant had a passive compost pile, as opposed to a working compost pile. Either is acceptable; they both produce organic matter, but a working compost pile, where things are turned regularly, exposing them to air, will produce small particles of organic matter. This is called aerobic decomposition. Passive composting often produces layers of decomposed organic matter. What you've discovered is a layer of material that is decomposing without air (anaerobic fermentation). If you dig the layer and break it up, it will finish decomposing. Hope this solves the mystery!
Q&A Library Searching Tips