The envelope specified the encoding for the body as http://www.wwww.org/soap/encoding/. This is a method for structuring the request that is suggested within the SOAP spec itself, known as the SOAP serialization.
It's worth noting that one of the biggest technical complaints against SOAP is that it mixes a specification for message transport with a specification for message structure. You needn't feel constrained to use the SOAP serialization encoding style.
Other possible encoding styles include Web Distributed Data Exchange (WDDX), XML Remote Procedure Call (XML-RPC), Resource Description Framework (RDF), or just a custom XML structure. The latter is probably good enough if you're not worried about whether an attribute value of "1" is supposed to be interpreted as a string or an integer, for example. See Resources for information on these encoding styles.