To cite the aqp package in publications use:

Beaudette, D.E., Roudier P., and A.T. O'Geen. Algorithms for Quantitative Pedology: A Toolkit for Soil Scientists. 2012

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},
    year = {2013},
    url = {http://r-forge.r-project.org/projects/aqp/},
  }