You need to focus on developing a silo on your website... Your long tail keyword articles should link to related content i.e. root keywords which are on other articles...
I recommend that you read up on Google's Hummingbird as then you will better understand how G crawls and understand your site... In sum G looks at all of your content and assesses the semantic relationship between your posts/pages...
Here is a quick example:
1) You want to rank for the word "red apples"... So you write a massive post on that...
2) You create a bunch of articles which are related e.g. "where to buy the best apples ...", "best apple recipes" .. Where appropriate I would add link in these posts which point back to the red apples... This basically tells G that the "red apples" post is highly relevant for that keyword and further you implement the semantic relationship between those keywords and topics....
Hope that helps...