Julie wrote:
potatoes... sugar... air...

Standard sourdough starter ingredients:
1. Wheat or rye flour
2. cold to room-temperature water (55-100% as much water as flour by
weight)
3. sourdough starter if you have it (up to 50% as much starter as flour
by weight)

Most recipes are written for a standard starter. A very few are
written for a starter containing milk or potatoes. Generally a
standard starter is the most reliable. Additional ingredients such
as sugar are unnecessary and may even lead to dangerous contamination.

Note: the amount of water in the starter (its /hydration/) and the
temperature of the starter will have a big impact on its growth
rate. A low-hydration starter will be slower. A low-temperature
starter will be slower. Generally, if you use a high-hydration
(liquid) starter you must refrigerate it and feed it at least once
a week [will someone who has a lot of experience with these please
correct my figure?]. If you use a low-hydration (solid/dough-like)
starter you may keep it at room temperature if you feed it 2x per day.
The amount of old starter used to make a new starter (the innoculum)
also affects the amount of time needed for the starter to mature, but
the innoculum must be changed a lot to make a significant difference.

David