SFM-4 | Updated February 2021
Soil Fertility Management: Converting Among Soil Test Analyses Frequently Used in Maryland
By Frank J. Cole, Joshua McGrath, Gurpal Toor, Brian Kalmbach, Paul Shipley, and David Ruppert
What Do Soil Tests Measure?
Soil tests estimate the fertility status of a soil and help ensure the efficient use of applied nutrients. Soil tests do not directly measure the actual quantity of plant-available nutrients in the soil. Instead, soil tests measure the quantity of a nutrient element that is extractable from a soil by a particular chemical extracting solution. The measured quantity of extractable nutrients in the soil is then used to predict the crop yield response the application of the nutrient. As soil test levels increase for a particular nutrient, the likelihood of a crop yield response to the addition of that nutrient decreases.
Over the years, different soil testing procedures and extracting solutions have been proposed to identify the method that provides the most reliable prediction of crop yield response to the nutrient application. Several different soil testing procedures work well for Maryland soils (e.g., Mehlich-1 and Mehlich-3). Other soil tests, such as Morgan extraction, are not recommended in Maryland.
Who is in the Soil Testing Business for Maryland Producers?
Nutrient recommendations in Maryland were developed when the Maryland Cooperative Extension Soil Testing Laboratory was operating (1954-2003). Since the closing of this laboratory, Maryland producers have relied on private-sector laboratories or laboratories operated by universities from neighboring states. As the testing methods utilized in these laboratories are often different from the methods upon which University of Maryland recommendations were established correlations between other testing laboratories and Maryland Cooperative Extension Laboratory were created before its closing. This allowed results from these other laboratories to be useful to Maryland producers.
How to Create Reliable Nutrient Recommendations Despite Different Testing Labs and Methods?
Before the closure of the Maryland Cooperative Extension Soil Testing Laboratory, an exchange of identical sets of 665 soil samples from throughout the State was provided to different laboratories to determine and consider differences in analyses in soil samples. This resulted in the development of equations to convert laboratory tests into a unitless “fertility index value” (FIV) scale. In the FIV scale, results from any of the laboratories in Table 1 can be placed with the assurance that similar recommendations will result regardless of the testing laboratory. In order to provide comparable values, the highest concentration within the “optimum” range is set equal to an FIV of 100. The numerical value of the soil fertility index is not affected by the method of soil analysis or the units used to express the soil test results.
This publication indicates how to convert the analytical results generated by eight regional soil testing laboratories to the FIV scale used by the Maryland Cooperative Extension Soil Testing Laboratory. Table 1 gives a list of the eight regional soil testing laboratories.
Table 2 provides soil testing conversion factors which permit the direct application of the crop nutrient recommendations developed in Maryland to soil testing data regardless of how, where, or by whom the soil test was performed.
These conversion factors are intended to be simple and easy-to-use. The information was updated in February 2021 to include some laboratories established since the original publication date of this report. Conversion of data from the regional soil testing laboratories generates reliable approximations of Maryland soil test FIV to which the Maryland plant nutrient recommendations may be applied.
Do I Need to Worry about the Different Units Reported in Soil Test Reports?
No, you do not need to worry. Maryland soil test FIV takes that worry away. But to convert the values correctly to Maryland soil test FIV you need to be aware of the laboratory and units utilized for reporting and choose the right conversion factors from Table 2.
Because laboratories typically extract nutrients from a fixed volume (Table 1) of the dried, ground, and sieved soil material, units reported as ‘ppm’ typically mean mg of nutrient per gram (or dm3) of soil.
Units reported as pound (lb) nutrient per acre make an additional assumption about soil density (usually around 1.3 g cm-3) to provide nutrient concentration using those units, and of course sometimes nutrients are reported in their elemental (e.g. P, K) form or oxide form (P2O5, K2O).
In the end all units can be translated from one to another, sometimes with no assumptions and sometimes with an assumed density either for dried, ground and sieved soil material or for soil intact and in place in the field. Your agricultural extension agent or nutrient management consultant can assist. But because of (unitless) Maryland soil test FIVs the worry associated with different units is taken away.
| Laboratory | Address | Nutrient | Extraction Method (extract solution:soil) | Instrumentation |
|---|---|---|---|---|
| AgroLab | AgroLab, Inc., 101 Clukey Dr., Harrington, DE 19952 | P, K, Ca, and Mg | Mehlich-3 (10 ml:1 cm3) | ICP |
| AgroLab | AgroLab, Inc., 101 Clukey Dr., Harrington, DE 19952 | pH | Water (10 ml:10 cm3) | Glass Electrode |
| Brookside | Brookside Laboratories, Inc., 308 East Main St., New Knoxville, OH | P, K, Ca, and Mg | Mehlich-3 (20 ml:1.7 cm3) | ICP |
| Brookside | Brookside Laboratories, Inc., 308 East Main St., New Knoxville, OH | pH | Water (7 ml:7 cm3) | Glass Electrode |
| Penn State University | Agricultural Analytical Services Laboratory, Penn State University, University Park, PA 16802 | P, K, Ca, and Mg | Mehlich-3 (25 ml:2.13 cm3) | ICP |
| Penn State University | Agricultural Analytical Services Laboratory, Penn State University, University Park, PA 16802 | pH | Water (5 ml:5 g) | Glass Electrode |
| Spectrum Analytic | Spectrum Analytic Inc., P.O. Box 639, 1087 Jamison Rd., Washington Court House, OH 43160 | P, K, Ca, and Mg | Mehlich-3 (10 ml:1 cm3) | ICP |
| Spectrum Analytic | Spectrum Analytic Inc., P.O. Box 639, 1087 Jamison Rd., Washington Court House, OH 43160 | pH | Water (5 ml:5 cm3) | Glass Electrode |
| University of Delaware1 | University of Delaware, Soil Testing Program, 149 Townsend Hall, University of Delaware, Newark, DE 19717-1303 | P, K, Ca, and Mg | Mehlich-3 (25 ml:2.5 cm3) | ICP |
| University of Delaware1 | University of Delaware, Soil Testing Program, 149 Townsend Hall, University of Delaware, Newark, DE 19717-1303 | pH | Water (10 ml:10 cm3) | Glass Electrode |
| University of Maryland (closed 6/30/03) | P | Mehlich-1 (25 ml:5 cm3) | Colorimeter 420nm | |
| University of Maryland (closed 6/30/03) | K | Mehlich-1 (25 ml:5 cm3) | Flame Photometer 768nm | |
| University of Maryland (closed 6/30/03) | Ca | Mehlich-1 (25 ml:5 cm3) | Flame Photometer 623nm | |
| University of Maryland (closed 6/30/03) | Mg | Mehlich-1 (25 ml:5 cm3) | Colorimeter 630nm | |
| University of Maryland (closed 6/30/03) | pH | Water (20 ml:20 cm3) | Glass Electrode | |
| Waters | Waters Agricultural Laboratories Inc., 257 Newton Highway, P.O. Box 382, Camille, GA 31730-0382 | P, K, Ca, and Mg | Mehlich-1 (20 ml:5 cm3) | ICP |
| Waters | Waters Agricultural Laboratories Inc., 257 Newton Highway, P.O. Box 382, Camille, GA 31730-0382 | pH | Water (20 ml:20 cm3) | Glass Electrode |
| Waypoint VA2 | Waypoint Analytical, 7261 Whitepine Rd., Richmond, VA 23237 | P, K, Ca, and Mg | Mehlich-3 (20 ml:1.7 cm3) | ICP |
| Waypoint VA2 | Waypoint Analytical, 7261 Whitepine Rd., Richmond, VA 23237 | pH | Water (10 ml:8.5 cm3) | Glass Electrode |
| Waypoint PA | Waypoint Analytical, 280 Newport Rd., Leola, PA 17540 | P, K, Ca, and Mg | Mehlich-3 (20 ml:1.7 cm3) | ICP |
| Waypoint PA | Waypoint Analytical, 280 Newport Rd., Leola, PA 17540 | pH | Water (10 ml:8.5 cm3) | Glass Electrode |
1University of Delaware also uses a 10 ml:1 cm3 ratio, depending upon the analyses requested.
2The use of Bray P1 for P and ammonium acetate for Ca, Mg, and K was discontinued as the standard extraction suite in 9/05.
Table 2. Factors used for converting regional soil-testing laboratory report data to Maryland soil fertility index values (FIV) scale.
To determine an equivalent Maryland FIV value for each soil-test nutrient, multiply the regional laboratory reported value, expressed in the units shown, by the value in column A and then add the value in column B.
| Regional Soil-Testing Laboratory | Units | Phosphorus A (P) | Phosphorus B (P) | Potassium A (K) | Potassium B (K) | Calcium A (Ca) | Calcium B (Ca) | Magnesium A (Mg) | Magnesium B (Mg) |
|---|---|---|---|---|---|---|---|---|---|
| AgroLab | ppm | 1.086 | 2.5 | 0.648 | (-1.8) | 0.126 | (-26.6) | 0.757 | 3.1 |
| Brookside | Ppm* | 1.1964 | 3.5 | 0.7182 | (-2.8) | 0.1354 | (-22.8) | 0.7866 | 11.5 |
| Brookside | lb/at | 0.2596 | 3.5 | 0.3591 | (-2.8) | 0.0677 | (-22.8) | 0.392 | 11.6 |
| Penn State | Ppm* | 1.1138 | 6.9 | 0.5973 | 0.1 | 0.1171 | (-21.3) | 0.7555 | (-1.1) |
| Penn State | lb/at | 0.245 | 6.9 | 0.2509 | 0.1 | 0.0422 | (-21.3) | 0.2267 | (-1.1) |
| Spectrum | lbs/a | 0.7544 | 9.1 | 0.3318 | (-1.5) | 0.0763 | (-15.9) | 0.4273 | 8.5 |
| Spectrum | ppm | 1.056 | 9.1 | 0.558 | (-1.5) | 0.114 | (-15.9) | 0.752 | 8.5 |
| U. Delaware | index | 1.0092 | 6.9 | 1.0964 | 0.9 | 1.0547 | (-8.6) | 0.9675 | 9.6 |
| Waters (Mehlich 1) | lbs/a | 1.1785 | 4.1 | 0.03795 | (-1.5) | 0.0582 | (-12.1) | 0.4255 | 3.6 |
| Waters (Mehlich 3) | lbs/a | 0.5569 | 6.9 | 0.29865 | 0.1 | 0.05855 | (-21.3) | 0.3778 | (-1.1) |
| Waypoint VA; PA (Mehlich 3) | ppm | 1.086 | 2.5 | 0.648 | (-1.8) | 0.126 | (-26.6) | 0.757 | 3.1 |
| Waypoint VA (Bray P & ammonium acetate bases ) | ppm | 1.6854 | 6.3 | 0.6263 | 0.8 | 0.1335 | (-18.4) | 0.6691 | 21.4 |
* Use ppm phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca ) values.
t Use lb/a phosphate (P2O5), potash (K2O), magnesium oxide (MgO), and calcium oxide (CaO) values.
Example: A soil-test report from AgroLab contains the following data:
Phosphorus, P 29 ppm
Potassium, K 93 ppm
Calcium, Ca 1210 ppm
Magnesium, Mg 114 ppm
To determine an equivalent Maryland FIV for each soil-test nutrient:
P, Maryland FIV = (29 x 1.086) + 2.5 = 34
K, Maryland FIV = (93 x 0.648) – 1.8 = 58
Ca, Maryland FIV = (1210 x 0.126) - 26.6 = 126
Mg, Maryland FIV = (114 x 0.757) + 3.1 = 89
Originally prepared and revised by:
FRANK J. COLE
JOSHUA MCGRATH
Revised by (2021):
GURPAL TOOR
gstoor@umd.edu
BRIAN KALMBACH
PAUL SHIPLEY
prs@umd.edu
DAVID RUPPERT
This publication, Soil Fertility Management: Converting Among Soil Test Analyses Frequently Used in Maryland (SFM-4), is a part of a collection produced by the University of Maryland Extension within the College of Agriculture and Natural Resources.
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When citing this publication, please use the suggested format:
Cole, F. J., McGrath, J., Toor, G., Kalmbach, B., Shipley, P., & Ruppert, D. (2021). Soil Fertility Management-Converting Among Soil Test Analyses Frequently Used in Maryland (SFM-4). University of Maryland Extension. go.umd.edu/SFM-4