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2017 Projects: Phosphorus Index Evaluation in the Northeast

How Can Soil Phosphorus be Better Kept On-farm and out of Waterways?

The New York Phosphorus Index (NY-PI) is a nutrient planning tool that tries to keep P on farmers’ fields and out of water bodies. The NY-PI ranks the relative risk of loss of P from fields based on soil test P levels and landscape factors such as drainage class and distance to water bodies. The NY-PI then gives nutrient management recommendations based on that risk. Manure and fertilizers can then be applied at N-based or P-based rates where risk is not too high. We are currently working to consolidate PIs in the Northeast region to better address water quality. Stakeholder feedback, modeling, and whole-farm P balances (imports minus exports) are being used to evaluate a new PI approach. This approach encompasses a two-step evaluation of fields where first a transport risk score is identified, which is then modified by credits for implementation of best management practices (BMPs). This allows farmers and farm planners to rank fields based on a P loss index score from manure or fertilizer, and then incentivizes the use of BMPs, such as incorporation or injection of manure, setbacks, buffers, split-application of manure, cover crops, etc. Additionally, whole-farm P balance assessments can be used to identify options for P management, including reduction of P sources to avoid build-up.

If you are interested in participating, contact Quirine Ketterings (qmk2@cornell.edu or 607-255-3061). You can also write to: Quirine Ketterings, Nutrient Management Spear Program, Department of Animal Science, Cornell University, 323 Morrison Hall, Ithaca NY 14853.

Goals

Our goal is to evaluate performance of P indices in the Northern US to better address water quality through a combination of stakeholder feedback, modeling, and whole-farm P balances.

Funding Sources

NRCS Conservation Innovation Grants and New York State Department of Environmental Conservation.

Additional Resources

Farmer Impact Stories

Fact Sheets

Extension Articles

  • Sadeghpour, A., S. Hetrick, K.J. Czymmek, G. Godwin and Q.M. Ketterings (2016). Managing soil test phosphorus in corn with manure and compost. What’s Cropping Up? 26(5): 86-87.
  • Czymmek, K.J., L. Geohring, Q.M. Ketterings, P. Wright, T. Walter, G. Albrecht, J. Lendrum and A. Eaton (2015). Revised winter and wet weather manure spreading guidelines to reduce water contamination risk. Cornell University Animal Science Publication Series No. 245, Ithaca NY. 8 pages.
  • Soberon, M., S. Cela, Q.M. Ketterings, C. Rasmussen, and K.J. Czymmek (2015). Change in nutrient mass balances over time for 54 New York dairy farms. What’s Cropping Up? 25(3): 22-24.
  • Cela, S., Q.M. Ketterings, K.J. Czymmek. M. Soberon, and C. Rasmussen (2015). Trends in nutrient mass balances on four New York dairy farms. What’s Cropping Up? 25(3): 25-27.
  • Cela, S. Q.M. Ketterings, K.J. Czymmek, M. Soberon, and C. Rasmussen (2015). Feasible whole farm nutrient mass balances for New York dairy farms. What’s Cropping Up? 25(2).
  • Soberon, M. Q.M. Ketterings, K.J. Czymmek, S. Cela, C. Rasmussen (2015). Whole farm nutrient mass balance calculator for New York dairy farms. What’s Cropping Up? 25(2).
  • Czymmek, K.J., and Q.M. Ketterings (2015). Taking care of (manure) business – Strive to do the best possible job every time manure is applied. Eastern DairyBusiness. The Manager. 7(1): 16.
  • Cela, S. Q.M. Ketterings, K.J. Czymmek, M. Soberon, and C. Rasmussen (2014).  Whole farm nutrient balance benchmarks for New York dairies. Cornell Nutrition Conference Proceedings. Syracuse NY, October 21, 2014.
  • Ketterings, Q.M., G. Godwin, W. DeGolyer, and K. J. Czymmek (2013). Manure injection rate study at Table Rock farm. What’s Cropping Up? 23(1): 12-16.
  • Knight, J., Q.M. Ketterings, K.J. Czymmek, and R. Wildman (2012). Phosphorus saturation versus the New York P index? Impact on manure and fertilizer management in New York State. What’s Cropping Up? 22(3): 9-11.
  • Ketterings, Q.M., and K.J. Czymmek (2012). P index survey: what caused impressive improvement in the NYS P Balance? What’s Cropping Up? 22(3):12-15.
  • Ketterings, Q.M., G. Godwin, W. DeGolyer, and K.J. Czymmek (2012). Trial results for manure injection at Table Rock Farm. What’s Cropping Up? 22(2): 17-22.
  • Knight, J., P. Ristow, G. Swanepoel, K. Czymmek and Q.M. Ketterings (2012). Dairy and cash grain farmer perceptions of the value of manure. What’s Cropping Up? 22(1): 3-5.
  • Ristow, P.L., S. Moss, Q.M. Ketterings, K.J. Czymmek (2011). Understanding manure nutrient variability. What’s Cropping Up? 21(4): 19-23.
  • Ketterings, Q.M., K.J. Czymmek, P. Ristow, C. Rasmussen, and S. Swink (2011). State, regional and farm-scale nutrient balances: tools for enhanced efficiency of whole farm nutrient use. 73rd Cornell Nutrition Conference for Feed Manufacturers October 18-20, 2011. Syracuse, NY. Proceedings pages 180-189.
  • Maguire, R., D. Beegle, J. McGrath, and Q.M. Ketterings (2011). Manure injection in no-till and pasture systems. Extension Publication. Mid Atlantic Water Program.
  • Ketterings, Q.M., G. Godwin, S.N. Swink, J. Foster, E. Hong, K.J. Czymmek, A. Wright, M. Hunter, J. Lawrence, S. Canner, C. Albers, A. Gabriel, E. Young, T. Kilcer and P. Barney (2011). Can manure replace the need for starter N fertilizer? What’s Cropping Up? 20(1): 14-17.
  • Czymmek, K.J., and Q.M. Ketterings (2011). Shallow manure incorporation works. Eastern DairyBusiness 3(3): 20.
  • Czymmek, K.J., L. Geohring, J. Lendrum, P. Wright, G. Albrecht, B. Brower, and Q.M. Ketterings (2011). Manure management guidelines for limestone bedrock/karst areas of Genesee County, New York: Practices for risk reduction. Animal Science Publication Series No. 240.
  • Czymmek, K.J, Q.M. Ketterings, L.E. Chase, and L. Goehring (2010). NY Phosphorus Index may not be perfect, but has served state well. Chesapeake Bay Journal; Forum. November 2010 issue.
  • Grandt, S., Q.M. Ketterings, A. Lembo, F. Vermeylen, and K. Czymmek (2010). In-field spatial variability of soil test phosphorus and implications for agronomic and environmental phosphorus management. What’s Cropping Up? 20(3): 8-9.
  • Place, A., Q.M. Ketterings, G. Godwin, J. Lawrence, B. Aldrich, P. Barney, and T. Kilcer (2010). Shallow incorporation of manure minimizes soil disturbance and conserves nitrogen. What’s Cropping Up? 20(3): 3-5.
  • Swink, S.N. Q.M. Ketterings, K.J. Czymmek, L.E. Chase, and J. Mekken (2009). Upper Susquehanna River Watershed cropland P balances. What’s Cropping Up 19(2): 1-3.
  • Place, A., Q.M. Ketterings, G. Godwin, K.J. Czymmek, S. Bossard, P. Barney, J. Lawrence, B. Aldrich, T. Kilcer (2009). Comparing manure incorporation methods in reduced till systems. What’s Cropping Up 19(2): 4-5.
  • Ketterings, Q.M., and K.J. Czymmek (2009). Getting a good read on P. Eastern DairyBusiness 1(2): 21 (+23).
  • Swink, S., Q.M. Ketterings, K. Czymmek and L. Chase (2009). Dairies improve P management. The Manager. Eastern Dairybusiness 1(1): 20-21.
  • Bruulsema, T. and Q.M. Ketterings (2008). Best management for fertilizers on Northeastern dairy farms. IPNI. 7 pages.
  • Swink, S., Q.M. Ketterings, K.J. Czymmek, and L.E. Chase (2008). Proactive agricultural and environmental management by New York dairy farmers greatly reduced cropland P balances. What’s Cropping Up? 18(5): 1-4.
  • Ketterings, Q.M., J.H. Cherney, K.J. Czymmek, E. Frenay, S.D. Klausner, L.E. Chase, and Y.H. Schukken (2008). Manure use for alfalfa-grass production. Department of Animal Science Mimeo 231/Department of Crop and Soil Sciences Extension Series E08-3. Cornell University. 43 pages.
  • Lawrence, J. Q.M. Ketterings, S. Bossard, K. Czymmek (2007). Manure N conservation with chisel plow versus Aerway incorporation. What’s Cropping Up? 17(4): 1-3.
  • Haden, R., Q.M. Ketterings, J. Kahabka, and K.J. Czymmek (2007). How quickly will soil test P levels increase? What’s Cropping Up? 17(3): 3-4.
  • Ketterings, Q.M., and K.J. Czymmek (2007). New York corn producers make a difference! Phosphorus fertilizer imports 2003-2005 reduced by 26.7 million lbs of P2O5. What’s Cropping Up? 17(2): 4.
  • Ketterings, Q.M., K.J. Czymmek, S. Swink (2007). The state of New York’s phosphorus balance. The Manager. Northeast DairyBusiness 9(2): 20-21.
  • Ketterings, Q.M., E. Frenay, J.H. Cherney, K.J. Czymmek, S. Klausner, L.E. Chase, and Y.H. Schukken (2006). Application of manure to established stands of alfalfa and alfalfa-grass (2006). In: Field Crop Dealer Meetings, Department of Crop and Soil Sciences Extension Series No E06-4, October 24-27, 2006. p 515.
  • Mekken, J.C., S.N. Swink, and Q.M Ketterings (2006). Statewide and country-based phosphorus balances for New York State. Department of Crop and Soil Sciences Extension Series E06-03. Cornell University, Ithaca, NY. 126 pages.
  • Ketterings, Q.M., G. Godwin, T.F. Kilcer, P. Barney, M. Hunter, J.C. Cherney, and S. Beer (2006). Nitrogen, phosphorus, and potassium removal by brown midrib sorghum sudangrass. What’s Cropping Up? 16(1): 4-6.
  • Rasmussen, C.N., Q.M. Ketterings, G. Albrecht, L. Chase, and K.J. Czymmek (2006). Mass nutrient balances – a management tool for New York dairy and livestock farms. In Silage for Dairy Farms; Growing, Harvesting, Storing, and Feeding. NRAES Conference. Harrisburg, PA, January 23-25. Pages 396- 414.
  • Rasmussen, C.N., Q.M. Ketterings, J. Mekken, K.J. Czymmek and L.E. Chase (2005). Statewide and whole farm phosphorus balances- tools to help with long-term nutrient planning on dairy and livestock farms. What’s Cropping Up? 15(6): 7-9.
  • Ketterings, Q.M., C. Rasmussen, J. Mekken, and K. Czymmek (2005). Statewide, County-based and whole farm nutrient balances. Tools to help with long-term nutrient planning. In: Field Crop Dealer Meetings, Department of Crop and Soil Sciences Extension Series No E05-1, October 25-28, 2005. pages 19-21.
  • Ketterings, Q.M., and K.J. Czymmek (2005). How much P is enough? Nutrient Management. April 2005. The Manager. Northeast Dairy Business 7(4): 37 and 39.
  • Czymmek, K.J., Q.M. Ketterings, J. Ten Eyck, and T. Horvath (2005). The New York CAFO Program: successfully connecting science, policy, regulation, and implementation. New York Water Environmental Association Inc. Clearwaters Magazine 35(1): 27-28.
  • Czymmek, K.J., Q.M. Ketterings, and G.L. Albrecht (2005). Dairy producers: is it time to take a look at your phosphorus fertilizer rates for corn? Small Farms Quarterly Spring 2005 (April 11, 2005). Page 8.
  • Czymmek, K.J., D. Beegle, and Q.M. Ketterings (2005). Implications of agronomic and environmental considerations in land application of manure in dairy farm systems of the Northeast and Mid-Atlantic Regions of the United States. NRAES Conference. March 15-17, 2005. Dairy Manure Management: Treatment, Handling and Community Relations. NRAES-176. pp 1-10. Also appeared in Ag Nutrient Management 2(2)(2006): 6-12.
  • Czymmek, K.J., L.D. Geohring, Q.M. Ketterings, P. Wright, and A. Eaton (2005). Supplemental manure spreading guidelines to reduce water contamination risk during adverse weather conditions. What’s Cropping Up? 15(3): 1-3.
  • Ketterings, Q.M., J.H. Cherney, T.F. Kilcer, and P. Cerosaletti (2005). Phosphorus removal by brown midrib sorghum x sudangrass. What’s Cropping Up? 15(1): 5-7.
  • Ketterings, Q.M., G. Albrecht, M. Hunter, P. Carey, S.N. Swink, and K.J. Czymmek (2004). Whole farm corn starter phosphorus fertilizer imports. What’s Cropping Up? 14(6): 3-5.
  • Ketterings, Q.M., J.E. Kahabka, and W.S. Reid (2004). The phosphorus fertility status of New York agricultural land. What’s Cropping Up? 14(5): 3-6. (Also appeared as abstract in Field Crop Dealer Meetings, October 26-29, 2004. CSS Extension Series No. E04-33, October 2004).
  • Ketterings, Q.M., S. Swink, G. Godwin, K.J. Czymmek, and G. Albrecht (2004). New York Starter Phosphorus Project: Does starter P fertilizer impact silage quality? What’s Cropping Up? 14(5): 1-2.
  • Ketterings, Q.M., S. Swink, G. Godwin, K.J. Czymmek, A. Durow, and G.L Albrecht (2004). New York Starter Phosphorus Project – Results of the 2003 growing season. What’s Cropping Up? 14(1): 1-3.
  • Whole farm nutrient management on dairy farms to improve profitability and reduce environmental impacts (2004). Department of Crop and Soil Sciences Research Series R04-1 and University of Wisconsin Extension Publication A3794. Report on collaboration among Cornell University, University of Wisconsin-Madison, and USDA-ARS Dairy Forage Research Center.  April 2004.
  • Ketterings, Q.M., G.L. Albrecht, and K.J. Czymmek (2003). Whole farm crop and manure nutrient management. Help! In: Proceedings 2003 Cornell Nutrition Conference. Department of Animal Science, Cornell University. October 21-23, 2003. Pages 245-250.
  • Ketterings, Q.M., K.J. Czymmek, S. Swink, G. Albrecht, G. Godwin, and A. Durow (2003). New York State Starter Phosphorus Project. In: Field Crop Dealer Meetings. Department of Crop and Soil Sciences Extension Series E03-25. Pages 11-12.
  • Czymmek, K.J., Q.M. Ketterings, L.D. Geohring, and G.L. Albrecht (2003). New release: New York phosphorus index: user’s manual and documentation. What’s Cropping Up? 13(4): 7.
  • Ketterings, Q.M., K.J. Czymmek, and S.D. Klausner (2003). Phosphorus guidelines for field crops in New York. Second Release. Department of Crop and Soil Sciences Extension Series E03-15. Cornell University, Ithaca, NY. 35 pages.
  • Czymmek, K.J., Q.M. Ketterings, L.D. Geohring and G.L. Albrecht (2003). New York State Phosphorus Runoff Index. CSS Extension Bulletin E03-13. 72 pages.
  • Ketterings, Q.M., T. Byron, G. Godwin and K.J. Czymmek (2003). Phosphorus Starter Project – Results of the 2002 growing season. What’s Cropping Up? 13(1): 4-6.
  • Ketterings, Q.M. and K.J. Czymmek (2002). Phosphorus soil testing and nutrient management planning in New York. What’s Cropping Up? 12 (4): 1-3.
  • Byron, T.M., Q.M. Ketterings, and K.J. Czymmek (2002). Phosphorus starter demonstration project. Results of the 2001 growing season. What’s Cropping Up? 12 (2): 4-5.
  • Ketterings, Q.M., T. Byron, and K.J. Czymmek (2001). P starter demonstration/research project – 2001. In: Field Crop Dealer Meetings. Department of Crop and Soil Sciences Extension Series E01-7. Pages 3-6.
  • Ketterings, Q.M., B. Bellows, K.J. Czymmek, W.S. Reid, and R.F. Wildman (2001). Do Modified Morgan and Mehlich-III P have a Morgan P equivalent? In: Field Crop Dealer Meetings. Department of Crop and Soil Sciences Extension Series E01-7. Pages 15-19.
  • Ketterings, Q.M., K.J. Czymmek, and S.D. Klausner (2001). Phosphorus recommendations for field crops in New York. Department of Crop and Soil Sciences Extension Series E01-5. Cornell University, Ithaca, NY. 32 pages.
  • Czymmek, K.J., Q.M. Ketterings, and L.D. Geohring (2001). Phosphorus and Agriculture VIII: The New Phosphorus Index for New York State. What’s Cropping Up? 11(4): 1-3.
  • Czymmek, K.J., L.D. Geohring, and Q.M. Ketterings (2001). What’s the risk of phosphorus runoff from your fields? Northeast DairyBusiness 3(8): 26-28.
  • Ketterings, Q.M., B.C. Bellows, K.J. Czymmek, and W.S. Reid (2001). Conversion Equations Part 2: Do Mehlich-III K, Ca, and Mg have Morgan equivalents? What’s Cropping Up? 11(4): 4-5.
  • Ketterings, Q.M., B. Bellows, K.J. Czymmek, W.S. Reid, and R.F. Wildman (2001). Do modified Morgan and Mehlich-III P have a Morgan equivalent? What’s Cropping Up? 11(3): 2-3.
  • Czymmek, K., J. Degni and Q.M. Ketterings (2001). Phosphorus starter demonstration project. Results of the 2000 Growing Season. What’s Cropping Up? 11(3): 4-6.

Journal Articles

  • Ketterings, Q.M., S. Cela, A. Collick, S. Crittenden, and K.J. Czymmek (2017). Restructuring the P index to better address P management in New York. Journal of Environmental Quality. doi:10.2134/jeq2016.05.0185.
  • Crittenden, S., Q.M. Ketterings, and K.J. Czymmek (2017). Soil phosphorus saturation ratio sets comparable manure application cutoffs across states differing in agronomic soil test. Soil Science 182:00-00. doi: 10.1097/SS.0000000000000192.
  • Cela, S., Q.M. Ketterings, M., Soberon, C. Rasmussen, and K.J. Czymmek (2017). Upper Susquehanna watershed and New York State improvements in nitrogen and phosphorus mass balances of dairy farms. Journal of Soil and Water Conservation 27: 1-11. doi: 10.2489/jswc.72.1.1.
  • Cela, S., Q.M. Ketterings, K.J. Czymmek, J.L. Weld, D.B. Beegle, and P.J.A. Kleinman (2016). Nutrient management planners' feedback on New York and Pennsylvania phosphorus indices. Journal of Soil and Water Conservation 71: 281-288. DOI: 10.2489/jswc.71.4.281.
  • Cela, S., Q.M. Ketterings, K.J. Czymmek, M. Soberon, and C.N. Rasmussen (2015). Long-term trends of nitrogen and phosphorus mass balances on New York dairy farms. Journal of Dairy Science 98: 7052-7070. http://dx.doi.org/10.3168/jds.2015-9776
  • Soberon, M., S. Cela, Q.M. Ketterings, K.J. Czymmek, and C.N. Rasmussen (2015). Changes in nutrient mass balances over time and related drivers for 54 New York dairy farms (2015). Journal of Dairy Science 98: 5313–5329. http://dx.doi.org/10.3168/jds.2014-9236.
  • Cela, S., Q.M. Ketterings, K.J. Czymmek, M. Soberon, and C.N. Rasmussen (2014). Characterization of N, P, and K mass balances of dairy farms in New York State. Journal of Dairy Science 97:7614–7632. http://dx.doi.org/10.3168/jds.2014-8467.
  • Ketterings, Q.M. (2014). Extension and knowledge transfer; adaptive management approaches for timely impact. Journal of Agricultural Science, Cambridge. DOI:10.1017/S002185961300066X.
  • Soberon, M.A., Q.M. Ketterings, C.N. Rasmussen, and K.J. Czymmek (2013). Whole farm nutrient balance calculator for New York dairy farms. Journal of Natural Resources and Life Science Education (renamed Natural Sciences Education) 42: 57-67.
  • Ketterings, Q.M., and K.J. Czymmek (2012). Phosphorus index as a phosphorus awareness tool: documented phosphorus use reduction in New York State. Journal of Environmental Quality. 41(6): 1767-1773 (doi:10.2135/jeq2012.0050).
  • Sharpley, A., D. Beegle, C. Bolster, L. Good, B. Joern, Q.M. Ketterings, J. Lory, R. Mikkelsen, D. Osmond, and P. Vadas (2012). Phosphorus Indices: Why we need to take stock of how we are doing. Journal of Environmental Quality 41(6): 1711-1719. (doi:10.2135/jeq2012.0040).
  • Kleinman, P, K. Saacke Blunk, R. Bryant, L. Saporito, D. Beegle, K. Czymmek, Q.M. Ketterings, T. Sims, J. Shortle, J. McGrath, F. Coale, M. Dubin, D. Dostie, R. Maguire, R. Meinen, A. Allen, K. O’Neill, L. Garber, M. Davis, B. Clark, K. Sellner, and M. Smith (2012). Managing manure for sustainable livestock production in the Chesapeake Bay Watershed. Journal of Soil and Water Conservation. 67(2): 54-61. (doi:10.2489/jswc.67.2.54A).
  • Ketterings, Q.M., K.J. Czymmek, and S.N. Swink (2011). Evaluation methods for a combined research and extension program used to address starter phosphorus fertilizer use for corn in New York. Canadian Journal of Soil Science 91(3): 467-477.
  • Maguire, R.O., P.J.A. Kleinman, C. Dell, D.B. Beegle, R.C. Brandt, J.M. McGrath, and Q.M. Ketterings (2011). Manure application technology in reduced tillage and forage systems; a review. Journal of Environmental Quality 40(2): 292-301.
  • Chunyu, S., and Q.M. Ketterings (2010). Impact of soil temperature and moisture on Mehlich-3 and Morgan soil test phosphorus. Soil Science 175(10): 511-518.
  • Grandt, S., Q.M. Ketterings, A.J. Lembo Jr., and F. Vermeylen (2010). In-field spatial variability of soil test phosphorus and implications for agronomic and environmental phosphorus management. Soil Science Society of America Journal 74:1800-1807.
  • Swink, S.N., Q.M. Ketterings, L.E. Chase, and K.J. Czymmek, and J.C. Mekken (2009). Past and future phosphorus balances for agricultural cropland in New York State. Journal of Soil and Water Conservation 64(2):120-133.
  • Haden, V.R., Q.M. Ketterings, and J.E. Kahabka (2007). Factors affecting the change in soil test P levels following manure and fertilizer application. Soil Science Society of American Journal 71: 1225-1232.
  • Brock, E.H., Q.M. Ketterings, and P.J.A. Kleinman (2007). Measuring and predicting the phosphorus sorption capacity of manure amended soils. Soil Science 172: 266-278.
  • Brock, E.H., Q.M. Ketterings, and P.J.A. Kleinman (2007). Phosphorus leaching through intact soil cores as influenced by type and duration of manure application. Nutrient Cycling and Agroecosystems 77: 269-281. DOI: 10.1007/s10705-006-9065-3.
  • Sato, S., D. Solomon, C. Hyland, Q.M. Ketterings, and J. Lehmann (2005). Phosphorus speciation in manure and manure-amended soils using XANES spectroscopy. Environmental Science and Technology 39: 7485-7491.
  • Lehmann, J., Z. Lan, C. Hyland, S. Sato, D. Solomon, and Q.M. Ketterings. Long-term dynamics of phosphorus forms and retention in manure-amended soils (2005). Environmental Science and Technology 39: 6672-6680.
  • Ketterings, Q.M., S.N. Swink, G. Godwin, K.J. Czymmek, and G.L. Albrecht (2005). Maize silage yield and quality response to starter phosphorus fertilizer in high phosphorus soils in New York. J. Food, Agriculture and Environment 3: 360-365.
  • Ketterings, Q.M. and M. Flock (2005). Comparison of Bray-1 and Mehlich-3 tests in high phosphorus soils. Soil Science 170: 212-219.
  • Ketterings, Q.M., J. Kahabka, and W.S. Reid (2005). Trends in phosphorus fertility of New York agricultural land. Journal of Soil and Water Conservation 59: 10-20.
  • Ketterings, Q.M., J.H. Cherney, T.F. Kilcer, and P. Cerosaletti (2004). Phosphorus removal by sorghum sudangrass in Northeastern USA. Online. Forage and Grazinglands DOI:10.1094/FG-2004-1015-01-RS.
  • Ketterings, Q.M., K.J. Czymmek, W.S. Reid and R.F. Wildman (2002). Conversion of modified Morgan and Mehlich-III soil tests to Morgan soil test values. Soil Science 167: 830-837.