cultivate vs earth up

cultivate

verb
  • To turn or stir soil in preparation for planting. 

  • To nurture; to foster; to tend. 

  • To grow plants, notably crops. 

earth up

verb
  • To cover the stem or leaves of plants with soil, as to encourage root growth or protect from cold. 

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