To cite package ‘usfertilizer’ in publications use:

Liu W (2018). usfertilizer: County-Level Estimates of Fertilizer Application in USA. R package version 0.1.5, https://github.com/wenlong-liu/usfertilizer.

Corresponding BibTeX entry:

  @Manual{,
    title = {usfertilizer: County-Level Estimates of Fertilizer
      Application in USA},
    author = {Wenlong Liu},
    year = {2018},
    note = {R package version 0.1.5},
    url = {https://github.com/wenlong-liu/usfertilizer},
  }