The type of soil used for extracts must be found out by trial and error. One german recipe calls for a specific swine pasture at a given (secrete ?) location. I have found a sandy (not too rich), seemingly old cow pasture up in the Hudson heights close to our home that gives very good extracts. The soil was collected at the entry where cows tend to run around and drop their goodies.
While some animal fertilizer seems to be essential, very rich garden soils that have received lots of organic or inorganic fertilizers do not seem to work too well at all.