cultivate vs strengthen

cultivate

verb
  • To nurture; to foster; to tend. 

  • To turn or stir soil in preparation for planting. 

  • To grow plants, notably crops. 

strengthen

verb
  • To empower; to give moral strength to; to encourage; to enhearten. 

  • To make strong or stronger; to add strength to; to increase the strength of; to fortify. 

  • To reinforce, to add to, to support (someone or something) 

  • To grow strong or stronger. 

  • To substantiate; to corroborate (a belief, argument, etc.) 

  • To augment; to improve; to intensify. 

How often have the words cultivate and strengthen occurred in a corpus of books? (source: Google Ngram Viewer )