plow under vs work over

plow under

verb
  • To destory (crops) by turning them into the ground with a plow 

work over

verb
  • To physically attack in order to cause injury. 

  • To subject (a person) to a severe scolding, interrogation, etc. 

  • To improve a prototype, or first draft. 

How often have the words plow under and work over occurred in a corpus of books? (source: Google Ngram Viewer )