A modified sodium tetraphenylboron method for the routine determination of reserve-potassium status of soil

Abstract

A method was developed and tested which is suitable for the routine determination of the reserve-potassium (reserve-K) level of soil — i.e., K which is plant-available but not exchangeable to 1M ammonium acetate. The method is simple, relatively quick, and uses 2 g soil and 20 ml 0.03M sodium tetraphenylboron (NaTPB) in 1M sodium chloride with a 16 h extraction time. The KTPB precipitated is dissolved in an acetone medium and analysed directly by atomic absorption spectrophotometry. The NaTPB method was compared with 4 other measurements of soil K status: New Zealand Ministry of Agriculture and Fisheries Quick Test for K (STK), K exchangeable to ammonium acetate by leaching (K_ex), boiling nitric acid extractable K (K_c), and K extracted by 0.05M barium acetate at pH 5.0. These methods were tested by correlation of the extraction results with dry matter yield, total K uptake, and % K in ryegrass (Lolium perenne L.) grown to exhaustion on 13 soils of varying K status. The new method was then validated against the same ryegrass parameters for a further 25 soils exhausted of K in a separate glasshouse trial. K_TPB, the quantity of K extracted by the NaTPB method, was highly correlated with K uptake by ryegrass (n = 13, r ² = 0.90***; n = 25, r ² = 0.94^***), with yield (n = 13, r ² = 0.74^***; n = 25, r ² = 0.56***), and with % K in herbage (n = 13, r ² = 0.77^***; n = 25, r ² = 0.85^***). It was generally superior to the other K measurements in all correlations examined. The method is sensitive to changes in K status caused by K uptake by plants and can identify K uptake from soil reserves after minimum levels of K_ex (or STK) are reached. K_TPB levels are less likely to be affected by seasonal variation than are STK levels. The NaTPB method is not suited to very large batch sizes and would be best used as a supplement to STK level measurements to establish the reserve-K content of soil. The method determines short-to mediumrterm availability of K, rather than the long-term availability indicated by the K_e level. It would be most useful for soils which have significant reserve-K and in the low-medium STK range (<8).

Keywords:

• potassium
• soils
• analytical methods
• soil K
• reserve-K
• sodium tetraphenylboron
• plant-available K
• ryegrass
• exchangeable-K