The 2012 ASHS Annual Conference

Estimating plant-available copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) in horticultural substrates is important in developing management practices for producing crops that are free of nutrient disorders (toxicity or deficiency) related to these micronutrients.  The chelating agent diethylenetriaminepetaacetic acid (DTPA) is commonly used in extraction methods to estimate soluble Cu, Fe, Mn, and Zn in soil and soilless substrates.  There are numerous variations of the DTPA extraction method, and the objectives of the study were to assess the experimental variables associated with (1) DTPA concentration, (2) pH/buffering of extractant solution, and (3) equilibration time, on extraction of micronutrients from peat-based substrate.  Treatments included unbuffered 2, 4, and 6 mM DTPA extractant solutions at 15, 30, 45, and 60 min equilibration times (Expt. 1); and 4 mM DTPA extractant solutions buffered to pH 5.5, 6.0, 6.5, 7.0, 7.5, and 8.0 at 45 min equilibration time (Expt. 2).  A 1:2 dilution by volume substrate:extractant solution method was used.  Controls were deionized (DI) water extractions.  In Expt. 1, extractant DTPA concentration and equilibration time were significant for Cu and Fe at P < 0.05, but there was no interaction between these variables.  DTPA concentration but not equilibration time was significant at P < 0.05 for Mn and Zn.  For Cu and Fe, as DTPA concentration increased or equilibration time increased, soluble levels of these metals also increased.  For Mn and Zn, DTPA concentration was not different but was significantly greater than the DI water extraction.  In Expt. 2, buffered DTPA extractant treatments were significant for Cu, Fe, Mn, and Zn (P < 0.01).  From pH 5.5 to 8.0, Cu (0.05 to 0.02 mg·L^-1), Fe (3.61 to 0.01 mg·L^-1), and Zn (0.25 to 0.15 mg·L^-1) concentrations in extracts steadily decreased linearly (r² > 0.93).  Concentrations of Mn decreased from pH 5.5 to 6.0 [3.14 mg·L^-1 Mn (mean for pH range)], leveled-off from pH 6.5 to 7.0 (2.84 mg·L^-1 Mn), and decreased again from pH 7.5 to 8.0 (2.32 mg·L^-1 Mn) with data fitting a 3^rd order polynomial (r² > 0.99).  The results of the study demonstrate the importance of considering chelating agent concentration, pH, and equilibration time for DTPA extractions when interpreting results of this soil/soilless test method when estimating micronutrients from peat-based substrate.