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Using the Nutrient Management Fertility Recommendations



Converting from Soil Test Results to University of Delaware Fertility Index Value (FIV)

Nutrient recommendations are based upon soil test calibration studies that relate the probability of a profitable plant response to nutrient addition to the plant nutrient concentration in the soil as measured by a soil test. However, the concentration of a soil nutrient measured by a specific soil test extracting solution is a function of the chemical composition of extracting agent and the forms of the nutrient present in the soil. Consequently, the concentration of a plant nutrient measured on a soil sample will vary with the soil test extracting solution used.

University of Delaware Nutrient Recommendations are based on the Mehlich 3 soil test. The University of Delaware Soil Testing Program reports results for phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) as a Fertility Index Value (FIV). Results for manganese (Mn), zinc (Zn), sulfur (S), and boron (B) are reported in pounds per acre (lb/ac). Other regional soil testing laboratories may use different soil test extractions or report results in different units (Table 1).

Table 1. The most common soil tests used by laboratories that analyze samples from Delaware and the Delmarva Peninsula.

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
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

If available soil test results were obtained from a soil testing laboratory that uses a soil extract other than Mehlich 3 or that reports Mehlich 3 results in units other than UD-FIV (P, K, Ca, Mg ) or lb/ac (Mn, Zn, S, B), it is necessary to first convert those soil test values to the equivalent UD-FIV or Mehlich 3 equivalent in lb/ac before making a nutrient recommendation.

NOTE: Nutrient recommendations prepared using the University of Delaware Nutrient Recommendations from soil test results that were not properly converted to UD-FIV or Mehlich 3 (lb/ac) may be completely inaccurate. Inaccurate recommendations could result in under- or over-application of the required nutrient and subsequent loss of yield or unnecessary expense.

Conversion factors are available based on correlation studies between Mehlich 3 and the other extracts using 300 Delaware soil samples) in the following tables. NOTE: Some laboratories offer multiple extractants (e.g., Brookside, Waypoint). Confirm which soil test extractant was used with the soil testing laboratory before selecting a conversion table.

Writing a Lime Recommendation

Step-by-step instructions for writing a lime recommendation is available in Calculating the Lime Recommendation Using the Adams-Evans Soil Buffer If the lime requirement test was performed by laboratory using a different buffer pH method (Table 2), refer to Measurement and Management of Soil pH for Crop Production in Delaware.

Table 2. Methods for water and buffer pH tests used by regional soil testing laboratories.

Laboratory Soil pH Method Lime Requirement Method
University of Delaware Water (1:1) Adams-Evans
AgroLab Water (1:1) SMP
Brookside Laboratories, Inc. Water (1:1) SMP/Sikora
Penn State University Water (1:1) Mehlich
Rutgers University Water (1:1) Adams-Evans
Spectrum Analytical Laboratories Water (1:1) Sikora
Virginia Tech Water (1:1) Mehlich
Waters Agricultural Laboratories, Inc. Water (1:1) Adams-Evans (KY location); Mehlich (NC location)
Waypoint Analytical Water (1:1) Mehlich; SMP (by request only)

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