You can use a copy field named "text", copy all your searchable fields into this field and specify it as default search field.
<requestHandler name="/select" class="solr.SearchHandler">
<!-- default values for query parameters can be specified, these
will be overridden by parameters in the request
-->
<lst name="defaults">
<str name="echoParams">explicit</str>
<int name="rows">10</int>
<str name="df">text</str>
You can add fields to be copied to a copy field as follows:
<copyField source="field1" dest="text"/>
<copyField source="field2" dest="text"/>
...
<copyField source="fieldn" dest="text"/>
Note that "text" is the copy field here.