With search_api_solr, how can I apply an significant field based boost when using solr 6.x?
None of the API function provided by Search API and Search API Solr Drupal work properly. You can either check solarium docs or switch back to Solr 5 or Solr 4. Boosts work properly with Solr 5.x. Data type indexed is also important. Try indexing it as Fulltext field.