Adding node into search api solr programatically
Two things first. These two items are purely a suggestion and have no impact on the code samples bellow:


You should post the code that saves your nodes. The Solr API module should automatically pick up nodes when you save and update them (the search_api module code runs when you call "node_save"). It is odd that it doesn't for you.
You should think about running your solr re-indexing operation after you import all of your data. This will speed up your bulk imports and you can schedule the solr re-index via cron or run it via drush.


The following information comes straight from the search_api module. 

These are the pre-conditions for the code examples bellow:


You need to run these after you call the node_save function.
The $type variable refers to the entity type (node, user, term, etc..). In your case it sounds like $type='node'
the $entity variable refers to the node object you're saving. You would pass this object to the node_save($entity) function during the bulk import operation.


If you're inserting nodes:

  $type='node';
  list($id) = entity_extract_ids($type, $entity);
  if (isset($id)) {
    search_api_track_item_insert($type, array($id));
    $combined_id = $type . '/' . $id;
    search_api_track_item_insert('multiple', array($combined_id));
  }


If you're updating nodes:

$type='node';
list($id) = entity_extract_ids($type, $entity);
if (isset($id)) {
    search_api_track_item_change($type, array($id));
    $combined_id = $type . '/' . $id;
    search_api_track_item_change('multiple', array($combined_id));
}