Many houseplant books recommend manure for fertilizing. Why is that?

First, many books were written before balanced commercial fertilizers were widely available (or the books were written by people who were brought up using such fertilizers). Second, manure is a good fertilizer and adds a lot of humus to the soil (many commercial fertilizers do not). However, manure is not easy to come by for everyone and even dried kinds sold by garden stores smell. Furthermore, the amount of chemical elements provided varies among manures. In other words, manure is fine to use if you can get it. But for most growers of houseplants, the balanced, commercial fertilizers work better.