New Zealand Journal of Agricultural Research abstracts
Monitoring long-term changes in reserve potassium in some New Zealand
soils using a modified sodium tetraphenyl-boron method
P. L. Carey
A. K. Metherell
AgResearch Lincoln
c/o Soil and Physical Sciences
P.O. Box 84, Lincoln University
Canterbury, New Zealand
Abstract Changes in reserve-K status were measured
on a number of historical and current pasture trials using a modified sodium
tetraphenyl-boron incubation method (TBK incubation). Trials included four
P, K, and S trials from North Otago, a long-term (>40 years) P fertiliser
trial from Canterbury (Winchmore), a long-term (>20 years) nutrient depletion
trial from Manawatu, and a 2-year N × K trial from Southland. While only
two trials showed a clear DM response to K, testing by both the original
(Jackson’s) test and the TBK-incubation method showed changes in soil K status
for all soils. The long-term fertiliser trial showed that K reserves declined
with normal annual superphosphate additions when no K fertilisers were applied.
Failure of net changes in K reserve status to adequately account for K removal
in nutrient budgets appeared to be best explained by significant plant uptake
of K below the soil sampling zone (0-75 mm). Prediction of a dry matter
K response when exchangeable-K levels are near or below critical levels i.e.,
at Quicktest K (QTK) values < 6-7, is governed by the buffering capacity
of soil K reserve. This was linked in this study to measures of both readily
available (short incubation; 1-20 h) and long-term available (168-h) K. Soils
with a large long-term K reserve can still exhibit a K response under high
K demand, but this occurred in only one trial when QTK values were 3 or less,
and when short-term available K was also low. The biphasic nature of K release
underlies the complications of predicting K response in many sedimentary
soils.The main means of classifying and predicting K response in New Zealand
has historically been from Metson’s soil class Kc values. This
study showed that K reserve status for a significant number of New Zealand
soils is not adequately represented by these values, and that better prediction
of K response could be achieved using the new TBK-incubation method after
optimisation against field trial data. Implications for using this method
to advance routine soil testing of K are discussed.
Keywords TBK; QTK; long-term; pasture; field trials;
Kc; soil testing; potassium
A02051; Received 12 August 2002; accepted 22 April 2003; online publication
date 9 September 2003
New Zealand Journal of Agricultural Research, 2003, Vol. 46: 199-213
0028-8233/03/4603-0199 $7.00/0 © The Royal Society of New Zealand
2003
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