For all my plants I'm using unglazed terracotta pots, mostly just because that's the pots my father was using and for the "authentic look". Then I found out that really nice property of those pots to absorb extra water and show water marks (pic below). I was wondering if there were any kind of "science" that would allow to "read" this information and optimize watering in consequence. For instance, should I wait to water my plant until the mark has completely disappeared? This is kind of a funny question, but I'd like to know if anybody found some tips related to this property of terracotta pots!
It will depend on the plant and the size of the pot. In general though, if the mark is gone the soil is pretty dry. As a side note, this same property of unglazed pottery has been used to water plants for centuries. Here is a link that explains: http://permaculturenews.org/2010/09/16/ollas-unglazed-clay-pots-for-garden-irrigation/ and is also used to water violets as explained at this link: http://homeguides.sfgate.com/use-ceramic-selfwatering-pot-38934.html