To cite the aqp package in publications use:

Beaudette, D.E., Roudier P., and A.T. O'Geen. 2013. Algorithms for Quantitative Pedology: A Toolkit for Soil Scientists. Computers & Geosciences. 52:258 - 268.

Corresponding BibTeX entry:

  @Manual{,
    title = {Algorithms for quantitative pedology: A toolkit for soil
      scientists},
    author = {D.E. debeaudette@ucdavis.edu Beaudette and P. Roudier and
      A.T. O'Geen},
    journal = {Computers & Geosciences},
    year = {2013},
    volume = {52},
    pages = {258 - 268},
    url = {http://ncss-tech.github.io/AQP/},
  }