The only thing I've read that discussed Mayan agriculture and urbanization at length was a chapter of Alan Weisman's The World Without Us, hardly the most scholarly work on the subject. In it, Weisman mentions the use of charcoal as slow-releasing fertilizer by Mayan farmers. Outside of his book, I've been able to find no mentions of this practice outside of the Amazon. What were Mayan agricultural practices like, were they as productive/sustainable as generally touted, and is what Weisman describes correct?
I think using charcoal is still used today. It also absorbs excess moisture.
I am sure you can find horticultural charcoal at most hardware stores.
There is a documentary on this in the making! http://www.startnext.de/en/undune