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This article in SSSAJ

  1. Vol. 63 No. 4, p. 902-911
     
    Received: Jan 16, 1998


    * Corresponding author(s): bjoern@purdue.edu
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doi:10.2136/sssaj1999.634902x

Plant-Available Potassium Assessment with a Modified Sodium Tetraphenylboron Method

  1. A. E. Coxa,
  2. B. C. Joern *a,
  3. S. M. Broudera and
  4. D. Gaoa
  1.  aCH2M HILL, 2525 Airpark Drive, Redding, CA 96001 USA

Abstract

The conventional 1 M NH4OAc-exchangeable potassium (K+) soil test is inadequate in soils where nonexchangeable K+ contributes significantly to crop nutrition. Studies were conducted (i) to compare the abilities of the 1 M NH4OAc method with a modified NaBPh4 method to estimate critical soil K+ levels, (ii) to estimate the contribution of nonexchangeable K+ to plant-available K+, (iii) to compare the abilities of the 1 M NH4OAc method and the modified NaBPh4 method to estimate plant dry matter yield and plant-available K+, and (iv) to compare the abilities of both methods to measure soil K+ balance. Winter wheat (Triticum aestivum L. `Abe') was grown in eleven Midwestern soils in a greenhouse using consecutive 28-d defoliation and regrowth cycles. Soils also were incubated for 6 mo with five K+ rates (0–809 mg K+ kg−1). Ammonium acetate- and NaBPh4-extractable K+ (5-min extraction period) were determined in soil samples taken after every three defoliation cycles and after incubation. Critical soil K+ levels could not be determined by either method alone but could be predicted by including cation-exchange capacity (CEC) and illitic K+ content in regression models. Nonexchangeable K+ represented a significant portion of plant-available K+ Plant-available K+ and dry matter (DM) yield were well related to NH4OAc-extractable K+ only in soils with low nonexchangeable K contribution (r 2 = 0.889 and 0.915, respectively), but they were well related to NaBPh4-extractable K+ in all soils (r 2 = 0.984 and 0.874, respectively). Slopes for NH4OAc-extractable K+ vs. soil K+ balance varied widely among soils (0.16–0.68) depending on NH4OAc-extractable K+, illitic K+, and clay content, but for NaBPh4-extractable K+ slopes were near unity. These studies suggest that the modified NaBPh4 method may be a superior K+ soil test compared to the NH4OAc method. Illite content and CEC data may help in developing better soil K+ management guidelines.

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