https://sustainablescientist.net/2010/09/04/potatoes-by-olla-or-not-here-is-the-answer/
 
https://kobra.bibliothek.uni-kassel.de/bitstream/urn:nbn:de:hebis:34-2011101139325/1/JARTSVol112No1S11.pdf
 
http://permaculturenews.org/2010/09/16/ollas-unglazed-clay-pots-for-garden-irrigation/

https://www.youtube.com/watch?v=qkNxACJ9vPI