http://esdac.jrc.ec.europa.eu/public_path/JRC_global_soilbio_atlas_online.pdf
http://www.isric.org/isric/webdocs/docs/ISRIC_TechPap09_2002.pdf
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1244466.pdf
http://www.nrcs.usda.gov/wps/PA_NRCSConsumption/download?cid=nrcs142p2_052523&ext=pdf
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1253871.pdf
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_050993.pdf
http://www.nrcs.usda.gov/wps/PA_NRCSConsumption/download?cid=stelprdb1247203&ext=pdf
http://www.nrcs.usda.gov/wps/PA_NRCSConsumption/download?cid=stelprdb1252094&ext=pdf