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

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 I g soil in 3 ml of 1.7 M NaCl/0.01 M EDTA solution + 0.2 g Na-TPB and incubated at 20degreesC for between I 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 (I 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 (K-c-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. The linearised functions for a number of soils suggested that K release was biphasic and their description could be better approximated by two line segments, corresponding to initial rapid K release with a second phase of more slowly-available K. This release pattern equates approximately to K held predominantly in "wedge zones" and clay mineral interlayers, respectively, and suggests that the kinetics of reserve-K release often involves more than a single mechanism and thus, more than one associated bond energy. Implications for New Zealand reserve-K soil testing methodology are that three measures of K status are required to characterise K release and potential pasture response; exchangeable K (initial), readily-available K (short-term incubation; 1-16 h) and long-term available K (168 h incubation). Currently, insufficient field trial data is available to fully optimise soil test conditions for the readily-available K measure.