Rotting is a drainage issue. An overabundance of any of the various critters that thrive in such areas is a symptom. Until the soil/drainage/tilth in this area is improved, most plants put there are likely to suffer the same fate, although putting the crown under the soil is not something one should do with Hosta. Few plants would appreciate that, which would cause the plants to rot and be a perfect meal for any critters that ingest dead plant material, and could have contributed to the demise of these plants. I'm not sure correct placement would save future Hostas or other plants, so am here babbling about soil again.
Although they are usually quite sensitive to drying out, shade plants are also especially sensitive to poor drainage, most of which come from the understory of forests, if not epiphytic. The natural layer of fertile, moist-yet-airy, loose duff created by decomposing OM on the surface is the perfect environment for them to thrive. The only way to begin to replicate/repair that is to add OM to the soil. Adding it to the surface is how it is done naturally. Disturbing the soil is devastating to both the drainage and moisture-retention properties of soil, and to most of the beneficial life forms, although can be repaired over time, and is necessary to add plants to a garden, sometimes desired to incorporate OM into the soil. I wouldn't attempt that under a tree, any kind of tilling.
http://permaculturenews.org/20...
Potting soil is not meant for use in ground beds, whether primarily peat (addressed above) or not. Potted plants notoriously have issues with rotting roots from excess moisture retention/lack of air pockets. Such added to the ground would exacerbate a drainage problem in times of too much rain, and become hydrophobic in times of too little.
Ammonia has a very high PH, I would not put it on plants, or the ground, where the effects are likely to linger. It is used for the purpose of altering soil PH.
https://www.google.com/#q=ammo...
