Plus or minus sign using Apache Solr
You need to be using Apache Solr 3.1 and have the edismax plugin enabled to allow for these boolean operator queries. You should see this in your Solr logs as 'an unknown query type' error. This should be an option in the ApacheSolr Drupal module since this patch but if not (or if you're running Apache Solr 1.4.x) you can check out this ticket on the solr forum for instructions on how to patch your solr and solrconfig.xml to allow for edismax queries.