Solr search page and custom filters
Just a regular space seems to be working for me.  Add the following to the end of your search URL: "&amp;solrsort=ds_created desc".

Just to be sure though, you can always go to the following reports page, to make sure the ds_created field has been indexed.  You can see all the fields apachesolr has in its index there: /admin/reports/apachesolr

Let me know if that helps! 

============== EDIT ===================

Another option I've used is to create a small module that modifies my solr query and/or solr results... I actually set my default sorting in that module.  Here's a snippet of my code:

function YOURMODULE_apachesolr_query_alter(&amp;$query) {
  //Set the sort default options.  These are different options.  Just choose one!
  $query-&gt;setSolrsort('sort_label', 'asc');  // by title
  $query-&gt;setSolrsort('ds_created', 'desc');  // by created date
  $query-&gt;setSolrsort('score', 'desc');     // by best match
}


:-)  Hope that helps, since the above wasn't quite what you needed...