Basically, both plants succumbed to rapid, overwhelming rot. One sec they seemed fine and the next completely caved in and collapsed over a gooey rotten core. As a post-mortem I unpotted the corpses and it was the plant themselves that rotted. They both had great looking root systems.
I don't know what killed them, but considering both my plants from this species died within a week of each other I'm suspicious that it was something I was doing wrong. The plants were bought from two different nurseries, so it's not like I got them from a bad grower. They were even kept on opposite sides of my cactus shelf with no other plants around them showing any sign of distress/disease after a thorough inspection.
Anyone have any advice about how to take care of this plant more successfully next time? The cultivation advice in my cactus books is rather limited, consisting mostly of "they're rot-prone, grow them in an extra-fast draining mix." I take what Llife says with a grain of salt, but they did have one specific bit of cultivation notes that I'm wondering about:
Any thoughts on the veracity of this claim? It goes against a lot of what I've learned here on the forums regarding cactus nutrition, but each species is different."During the growing season enrich the soil using a fertilizer rich in potassium and phosphorous, but poor in nitrogen, because this chemical element doesn’t help the development of succulent plants, making them too soft and full of water."
Any other bits of advice from growers who have successfully taken care of this species?