Placed on the Internet October 1, 2006
Soil testing facilities use different extraction techniques to determine the amount of phosphorus in the sample. Different tests provide different results based on the pH of the sample. This web page provides guidance on how these extraction methods compare in high pH soils.. If you use the fertilizer recommendations from the links below, they are based on the ammonium bicarbonate DPTA extraction method (AB-DPTA). If the results received from the soil testing facility used a different extraction method you will need to convert those results in order to use the AB-DPTA-based recommendations 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; Olsen -P |
Bray-1; Mehlich-II |
NaHCO3 |
AB-DPTA |
| Mehlich-1 | 1 | 2 | 2 | 0.67 | 0.33 |
| Mehlich-3; Olsen-P |
0.5 | 1 | 1 | 0.33 | 0.17 |
Bray-1; |
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 | http://harris.agsource.com/ |
| 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/ |
| Colorado Analytical | NaHCO3 | http://www.coloradolab.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.
Mallarino, A.P. 1995. Comparison of Mehlich-3, Osen, and Bray-P1 Procedures for Phosphorus in Calcareous soils.
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