Plant and Soil

, Volume 341, Issue 1–2, pp 461–472 | Cite as

An improved procedure for routine determination of reserve-K in pastoral soils

  • Peter L. Carey
  • Denis Curtin
  • Catherine L. Scott
Regular Article


Non-exchangeable K (reserve K) can be an important source of K for plant uptake in micaceous soils but testing for reserve K is laborious and expensive and involves use of potentially hazardous chemicals. Our objectives were to (1) identify a cheaper and more reliable reserve K test that is amenable for routine use by commercial laboratories, and (2) quantify the response of grass-clover pasture to applied K in soils differing in reserve K. We measured the release of reserve K using different combinations of the K-precipitant, sodium tetraphenyl boron (0.033 to 0.2 M), and extraction time (1 to 16 h). Copper chloride was used to decompose the K-tetraphenyl boron precipitate rather than the more toxic mercuric chloride or acetone. Based on data obtained for soils with a wide range of reserve K values, a test involving extraction of soil (1-g) in 0.1 M Na-tetraphenyl boron for 4 h was selected as the best option for routine use because it generally gave similar values to the current test (thus the same diagnostic criteria may be applicable) and it provided a better fit with operational requirements of commercial laboratories. Pasture K responses, in the presence and absence of applied N, were measured over a single growing season (pasture harvested at intervals of 1–2 months, depending on growth rate) at seven sites in Canterbury, New Zealand with reserve K (measured using the current test) ranging from 0.5 to 2.3 cmolc/kg (all sites were low in exchangeable K, <0.33 cmolc/kg). The size of the K response depended on the clover content of the pasture and only one site with a high clover content responded consistently to K throughout the season. Clover content increased at all sites following K addition, confirming that a major benefit of pasture K fertilization is in improving clover production. Our results suggest that there may not be a unique critical threshold for reserve K that can be applied to all pastures; rather, the value below which a production response to K is obtained may depend on the amount of clover in the pasture.


Potassium Non-exchangeable Micaceous Tetraphenylboron 



We thank New Zealand Fertiliser Manufacturers’ Research Association (FMRA), Hill-Laboratories, NZ Labs, ARL, Summit-Quinphos, and Agrow-Canpotex for financial and in-kind support for this project. We are grateful to David Marsh, Murray Corlette, Marvin Pangborn, Derek Tyson, Mike Dewhirst, John Kirk and Hayden Watson for hosting the trials on their properties. Thanks also to Ruth Butler and Esther Meenken (Plant & Food Research), who provided statistical advice and analysis, and to Robert Boyd, Don Patten, Peg Gosden, Lesley Corbett and Simon Kelly who provided technical assistance in the field and laboratory. Thanks to Peter Lorentz of Analytical Research Laboratories (ARL) Ltd who undertook the nitric acid extractable and total-K analyses for the soil mineralogy and finally, but not least, Dr Jock Churchman of Adelaide University who kindly provided interpretive assistance for the soil mineralogy.


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Copyright information

© Springer Science+Business Media B.V. 2010

Authors and Affiliations

  • Peter L. Carey
    • 1
  • Denis Curtin
    • 2
  • Catherine L. Scott
    • 2
  1. 1.LRS (Land Research Services) Ltd, Arts WorkshopLincoln UniversityCanterburyNew Zealand
  2. 2.New Zealand Institute for Plant & Food Research LimitedChristchurchNew Zealand

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