Limit the search to only one content type
These instructions by selvam on KnackForge worked for me. Basically, you can use the MODULE_search_apachesolr_query_prepare hook to add a filter to the query before it's run. 

&lt;?php
function MODULE_search_apachesolr_query_prepare($query) {
  if (/* not correct page */)
    return;
  $compiled_filter = new SolrFilterSubQuery('AND');
  $compiled_filter-&gt;addFilter('bundle', 'TARGET_CONTENT_TYPE');
  $query-&gt;addFilterSubQuery($compiled_filter);
}


I haven't actually tried the hook, but I did use the filter and apply it to a query in a different context (API) and it worked as expected.