Rates of release of non‐exchangeable potassium in New Zealand soils measured by a modified sodium tetraphenyl‐boron method

Abstract

A modified sodium tetraphenyl-boron method (Na-TPB), which uses an incubation period and boiling with a copper solution to destroy the tetraphenyl-boron-K precipitate (TBK) instead of shaking and resolubilisation with acetone, is proposed as a replacement for Jackson's TBK method. The method as recommended is 1 g soil in 3 ml of 1.7 M NaCl/0.01 M EDTA solution + 0.2 g Na-TPB and incubated at 20°C for between 1 and 168 h. Testing of the method showed it to be a more rapid, accurate, and cost-effective means of routinely testing soil K reserves than Jackson's test. Correlation of TBK values against Jackson's method were good (r 2 = 0.88) for short extraction times (1-4 h) but decreased with increasing extraction time. Regressions of TBK values with K c values, the current method for differentiating K supplying ability between soils, were only fair (r 2 < 0.75). This was due to mechanistic differences between dissolution (Kc-acid extraction) and solubility based (TBK-precipitation) methods.Potassium release curves were established for a range of 24 New Zealand soils for 1-672 h. The bulk of K release (c. 80%) occurred for most soils within the first 48 h with release rates in significant decline after 168 h. Release curves were modelled using a range of simple kinetic models: empirical Elovich, power, exponential, simplified-Elovich, and parabolic diffusion equations were all examined. The quality of fit for each model tended to decrease in that order, although this varied between soil classes.

Monitoring long‐term changes in reserve potassium in some New Zealand soils using a modified sodium tetraphenyl‐boron method

“…These incubationextraction times were chosen as representing "shortterm" (i.e., readily), "medium-term", and "long-term" available K, respectively. Initially, selection of extraction times involved optimally reducing K release curves obtained from Carey & Metherell (2003) to several linear segments, but pragmatic considerations were also necessary to establish a routine suitable for a commercial soiltesting laboratory operation. The release of nonexchangeable or reserve-K was estimated by subtracting exchangeable-K values from TBK values measured over the three extraction times (1,20, and 168 h).…”

“…Measurements of non-exchangeable-K using the Na-TPB method of Cox et al (1996) were made according to the modifications outlined in Carey & Metherell (2003). Briefly these consist of 1 g soil incubated with 3 ml of a 1.7 MNaCl/0.01 MEDTA buffer solution and 0.2 M Na-TPB (added as a solid) for 1, 20, and 168 h at 20°C.…”

“…The quantity measure in this work was arbitrarily chosen from Carey & Metherell (2003) as the K released after 168 h, but the intensity measure is estimated from shorter incubation-extractions. Complicating the K demand picture is the crop or pasture species grown, its stage of development, and soil moisture conditions.…”

“…This method uses an incubation technique, and uses neither acetone nor mercury but employs copper to destroy the potassium tetraphenyl-boron (TBK) precipitate and re-solubilise the extracted K. This method was tested on a range of New Zealand soils in a preceding paper (Carey & Metherell 2003) and offered the potential to perform a large number of analyses more rapidly, less laboriously, more safely, and more accurately. With modest adaptation, the method would be suitable to routinely measure non-exchangeable-K significantly more cheaply.…”

Potassium fixation in soil size fractions of arid soils

“…Similar relationships were also found for HNO 3 -extractable K. Nonexchangeable K is calculated from differences between HNO 3 -and NH 4 OAcextractable K. Some salt precipitants, such as sodium tetraphenyl-boron, are also able to extract nonexchangeable K from micas, but are less able to extract K from the structures of feldspars. However, much information about nonexchangeable K has been gained using destructive HNO 3 methods, although they have been less favoured because of their severe extraction conditions (Song & Huang 1988;Carey & Metherell 2003). According to the threshold level of K for calcareous soils reported by Tafarroji and Haghparast Tanha (2005) as 148 mg/kg, nearly 60% of the surface soil samples had NH 4 OAc-extractable K well above this limit.…”