Converting phosphorus P soil test results based on Mehlich l, Mehlich ll, Mehlich lll Bray l AB-DTPA and NaHCO3 extractions methods - Colorado State University Extension Tri River Area Mesa Delta Montrose and Ouray Counties

Converting Phosphorus Soil Tests Results.

Curtis E. Swift, Ph.D., Colorado State University, Area Extension Agent, Horticulture

Placed on the Internet October 1, 2006

Soil testing facilities use different extraction techniques to determine the amount of phosphorus in the sample. This web page provides guidance on how these extraction methods compare. If you have fertilizer recommendations based on the ammonium bicarbonate DPTA extraction method (AB-DPTA), the results recieved from the soil testing facility may need to be converted to ensure proper application of phosphorus.

The following web pages provide recommendations based on the AB-DPTA extraction method. To use these recommendations you may need to utilize the conversion factor provided in Table 1 below. Table 2 lists labs most commonly use in Colorado and the extraction method they use.

The following web pages provide fertilizer recommendations based on the AB-DPTA extraction method.

for cool-season turfgrass
for vegetables and flowers
for trees and shrubs

Table 1. Multiply the results provided in the soil test report (left column) by the number under the AB-DPTA column (far right column) to determine the phosphorus level resulting from the AB-DPTA extraction method.

Results given as	Mehlich-l	Mechlich-lll	Bray-1; Mehlich-II	NaHCO₃	AB-DPTA
Mehlich-1	1	2	2	0.67	0.33
Mehlich-3	0.5	1	1	0.33	0.17
Bray-1; Mehlich-2	0.5	1	1	0.33	0.15
NaHCO₃	1.5	3	3	1	0.5
AB-DPTA	3	6	6.7	2	1

Table 2 Extraction methods used by various soil testing facilities. Additional soil testing facilities can be added to this list if requested.

Soil testing laboratory	Phosphorus analysis	Web Address
A & L Analytical Laboratories	Mehlich 3	http://www.allabs.com
Colorado State University	AB-DPTA	http://www.colostate.edu/Depts/SoilCrop/soillab.html
MDS Harris	Bray 1	www.mdsharris.com/ag
ServiTech	Mehlich 3	http://www.servitechlabs.com/
Ward Laboratories, Inc.	Bray 1; Mehlich 2	http://www.wardlab.com/
Warren Analytical Laboratory	NaHCO₃	http://www.warrenlab.com/

References for further information:

Hanlon, E.A., and G.V. Johnson. 1984. Bray/Kurtz, Mehlich III, AB/D and ammonium acetate extractions of P, K and Mg in four Okalahoma soils. Commun. in Soil Sci. Plant Anal. 15:277-294.
Labhsetwar, V.K., and P.N. Soltanpour. 1985. A comparison of NH₄HCO₃-DPTA, NaHCO₃, CaCl2 and Na₂-EDTA soil tests for phosphorus. Soil Sci. Soc. of Am. J. 49:1437-1440.
Michaelson, G.J., C.L.Ping, and G.A. Mitchell. 1987. Correlation of Mehlich 3, Bray 1, and ammonium acetate extractable P, K, Ca, and Mg for Alaska agricultural soils. Commun. in Soil Sci. Plant Anal. 18:1003-1015.
Olsen, S.R., and L.E. Sommers. 1982. Phosphorus. pp. 403-430 In A.L. Page (ed.) Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. American Society of Agronomy; Madison, WI.
Soltanpur, P.N., and A.P. Schwab. 1977. A new soil test for simultaneous extraction of macro and micro nutrients in alkaline soils. Commun. in Soil Sci. Plant Anal. 8:195-207.
Wolf, A.M., and D.E. Baker. 1985. Comparisons of soil test phosphorus by Olsen, Bray P1, Mehlich I and Mehlich III methods. Commun. in Soil Sci. Plant Anal. 16:467-484.

WebMaster
Dr. Curtis E. Swift, Area Extension Agent, Horticulture
Colorado State University Extension
2775 US Hwy 50, Grand Junction, CO. 81503
voice: 970-244-1840
fax: 970-244-1700