Typically, Atomic Absorption (AA) or Inductivity Coupled Plasma-Optical Emission Spectrometry (ICP-OES) is used to measure potassium ion, by first extracting the potassium ion from sample soils by 1 mol/L ammonium acetate (CH3COONH4). These are the methods performed in laboratories.
A simpler method for a rapid measurement of potassium ion in soil uses the LAQUAtwin potassium ion meter B-731. The extraction method is the same as the lab method. The following procedure explains how you can measure K+ with good correlation to analytical lab tests.
Put 1g each of air-dried soils (four samples) in 100mL glass beakers, two beakers per soil sample.
Prepare two kinds of extraction per soil sample, one by adding 20 ml of 1mol/L CH3COONH4 to one beaker, and 20ml of 0.01mol/L CH3COONH4 to another beaker.
Shake the beakers around 1 hour to extract K+ from the soil using a bench top shaker.
Calibrate LAQUAtwin B-731 with 150mg/L and 2000mg/L K+ standard solutions included in the product.
Measure potassium ion concentration of the filtrated solution with calibrated B-731 and with ICP-OES (e.g. HORIBA Jobin Yvon. Model ULTIMA2).
Perform this measurement with 4 different samples.
Results and Benefits
The Laqua Twin B-731 allows for a simple on site determination of potassium which provides accuracy close to laboratory techniques.
Based on table 1, higher value against ICP-OES is detected by LAQUAtwin K+ with 1 mol/L CH3COONH4 extraction, due to strong interference by NH+ of CH3COONH4. However, with 0.01 mol/L CH3COONH4 extraction, although the extraction efficiency is reduced by approximately 80%* (Figure 1), very good correlation (R=0.981, R2=0.962) is obtained between ICP-OES and LAQUAtwin (Figure 2).