Fact Sheets And Publications

A searchable database organized by category.
Look through a collection of recorded online resources or register for an upcoming class. Topics include gardening, nutrient management, nutrition and more!
Find a complete listing of our Extension staff at our various offices.

Interpreting Soil Phosphorus and Potassium Tests

Introduction and Purpose

The University of Delaware has rated soils in terms of the likelihood of profitable responses to additions of phosphorus (P) and potassium (K) from fertilizers, manures, and other soil amendments for more than 50 years. Our soil test categories and fertility recommendations are based on research conducted in Delaware or in nearby states with similar soils, crops, and climates. Despite the fact that soil testing has been successfully used by farmers and others for many years, there is still often confusion about the interpretation of soil test P and K results. This confusion primarily occurs because soil testing laboratories use different soil test methods and different reporting units to express the results of these tests. The purpose of this document is to describe the method for interpreting soil tests used by the University of Delaware and to illustrate how to compare and convert soil test P and K results from laboratories using different soil testing methods and reporting units to the equivalent University of Delaware reporting units.

Interpreting Soil Tests

Soil test interpretation is conceptually simple (Figure 1). As soil test nutrient concentrations in the soil increase from “low” to “medium” to “optimum” levels, crop yields increase as well. Once soil test nutrients have reached the optimum concentration for crop growth (the “critical level”), further nutrient inputs are rarely needed and are usually unprofitable because yields often do not increase. Over-fertilization of soils with P to “excessive” levels is also undesirable because this increases the likelihood of nonpoint source pollution of surface waters and shallow ground waters by soil P through runoff or leaching of P. While over-fertilization of soils with K to “excessive” levels does not have water quality implications, it is still economically undesirable and could also negatively impact on crop and forage quality.

interpreting soil tests

Figure 1. Interpretation of soil tests.

The University of Delaware uses the Mehlich 3 soil test method for routine fertility testing. Soil test results for the macronutrients P, K, calcium (Ca), and magnesium (Mg) are reported as a Fertility Index Value (FIV). The University of Delaware uses four soil test categories to rate soils in terms of the likelihood of a profitable crop response to nutrient additions in fertilizers, animal manures, or other soil amendments (Table 1).

SOIL TEST CATEGORY SOIL TEST VALUE (FIV)
Low 0-25
Medium 26-50
Optimum 51-100
Excessive > 100


General interpretations are assigned to each soil test category. Specific nutrient recommendations for Delaware crops (i.e., the actual amount of nutrient recommended in pounds per acre (lbs/a) as a function of soil test value) are provided for agronomic, vegetables, and forage and hay crops. The general interpretations for each soil test category are:

LOW (0-25 FIV). The nutrient concentration in the soil is inadequate for the growth of most plants and will very likely limit plant growth and yield. There is a high probability of a favorable economic response to additions of the nutrient.

MEDIUM (26-50 FIV). The nutrient concentration in the soil may be adequate for plant growth, but should be increased into the optimum range to ensure that plant growth and yield are not limited. There is a low to moderate probability of a favorable economic response to additions of the nutrient.

OPTIMUM (50-100 FIV). The nutrient concentration in the soil is in the range recommended for the growth of all plants. Since there is a very low probability of a favorable economic response, nutrient additions are rarely recommended.

EXCESSIVE (>100 FIV). The nutrient concentration in the soil is above the range recommended for the growth of all plants. Additions of the nutrient will be unprofitable and are not recommended. Erosion, runoff, and leaching from soils that are excessive in P can have negative effects on water quality. Applications of P (e.g., fertilizers, manures) are not recommended, with the possible exception of starter fertilizers for some crops.

Comparing and Converting Soil Test Results from Different Laboratories

Several soil tests are used by laboratories analyzing soil samples from Delaware (Table 2). Depending on the method used to extract P and K from a soil and the units used to report test results, numerical values for soil test P and K may differ between laboratories. This does not mean that any of the results are inaccurate, only that they must be converted to the same units, taking into consideration differences in soil test methods, to be compared numerically and interpreted accurately.

LABORATORY SOIL TEST METHOD REPORTING UNITS
University of Delaware (UD) Mehlich 3 UD-FIV (P, K, Mg, Ca), ppm (Mn, Zn, S, B)
AgroLab Mehlich 3 ppm, UD-FIV (for P only)
Brookside Laboratories, Inc Mehlich 3
Bray 1 P
Ammonium acetate K
ppm
Penn State University Mehlich 3 ppm
Rutgers University Mehlich 3 lb/ac or ppm
Spectrum Analytical Laboratories Mehlich 3 lb/ac or ppm
Virginia Tech Mehlich 1 lb/ac
Waters Agriculture Laboratories, Inc Mehlich 1 lb/ac
Waypoint Analytical Mehlich 3
Mehlich 1
Bray 1 P
Ammonium acetate K
ppm


While it is preferable to use fertilizer recommendations developed using a specific soil test method, it is sometimes necessary to convert between soil tests and then develop a fertilizer or manure application rate recommendation. Equations for converting soil test results between soil test methods, which are based on research conducted by the University of Delaware Soil Testing Program (Gartley et al., 2002), are available in Appendix-5 of the Delaware Nutrient Management Handbook. Appendix-5 also contains information on converting soil test data for other nutrients, such as Ca and Mg. The following tables provide the estimated numerical equivalent to UD-FIVs for soil test P (Table 3) and soil test K (Table 4) for the most common soil test methods and reporting units used by labs providing services to Delaware growers.

SOIL TEST P MEHLICH 3 MEHLICH 1 BRAY P1
UD-FIV lbs P/acre lbs P2O5/acre lbs P/acre ppm P
5 10 23 5 4
10 20 46 10 8
15 30 69 15 12
20 40 92 20 16
25 50 115 25 20
30 60 138 30 24
35 70 161 35 28
40 80 184 40 32
45 90 207 45 36
50 100 230 50 40
60 120 276 60 48
70 140 322 70 56
80 160 368 80 64
90 180 414 90 72
100 200 460 100 80
125 250 575 125 100
150 300 690 150 120
175 350 805 175 140
200 400 920 200 160
225 450 1035 225 180
250 500 1150 250 200
275 550 1265 275 220
300 600 1380 300 240
325 650 1495 325 260
350 700 1610 350 280
375 750 1725 375 300
400 800 1840 400 320

 

SOIL TEST K MEHLICH 3 MEHLICH 1 AMMONIUM ACETATE
UD-FIV ppm lbs K/acre lbs K2O/acre lbs K/acre ppm K
5 9 19 22 14 9
10 18 37 44 28 18
15 27 56 67 42 27
20 36 74 89 56 36
25 45 93 111 69 45
30 55 111 133 83 55
35 64 130 156 97 64
40 73 148 178 111 73
45 82 167 200 125 82
50 91 185 222 139 91
60 109 222 267 167 109
70 127 259 311 194 127
80 145 296 356 222 145
90 164 333 400 250 164
100 182 370 444 278 182
125 227 463 556 347 227
150 273 556 667 417 273
175 318 648 778 486 318
200 364 741 889 556 364

Summary

Nutrient recommendations are developed based on research that relates the probability of a profitable plant response to application of a nutrient compared to the amount of that nutrient in the soil as measured by a soil test. However, different soil testing laboratories may use different soil test methods or report results in different units, making it difficult to compare results between laboratories. The University of Delaware nutrient recommendations for P, K, Ca, and Mg are based on the Mehlich 3 soil test, with results reported in Fertility Index Values (FIVs). Research based equations are available to convert soil test P and K results from laboratories using different soil testing methods and reporting units to the equivalent UD-FIV. Conversion of soil test data allows for comparison of soil test results across laboratories and for use of University of Delaware nutrient recommendations with soil test results from other laboratories.

It is important to note that even at a given soil test value, recommendations for the amount of nutrient to apply can vary due to differences in nutrient sources selected, the timing and method of application, the crops to be grown, yield goal, tillage practices, and other factors, regardless of the soil test method used. Consult the laboratory conducting the soil test for questions about the test performed and your nutrient management consultant for the most appropriate nutrient management recommendation for your situation.


Author(s):

A.L. Shober, J.T. Sims, and K.L. Gartley
Department of Plant and Soil Sciences
University of Delaware

Revision Date:

March 2017

Original Publication Date: March 2013

Peer Reviewers:

Phillip Sylvester, University of Delaware Cooperative Extension
Jarrod Miller, Plant and Soil Sciences Department, University of Delaware Cooperative Extension

This institution is an equal opportunity provider.


UD Cooperative Extension

This institution is an equal opportunity provider.

In accordance with Federal law and U.S. Department of Agriculture policy, Cooperative Extension is prohibited from discriminating on the basis of race, color, national origin, sex, age, or disability.