Potassium is extracted from soil with ammonium nitrate.
Bottles - 125 ml, wide mouth, screw cap.
Scales - accurate to 1 DP.
Shaking machine with platform - approx. 275 strokes per min, length of travel 25 mm.
Ammonium nitrate, M - Dissolve 400 g of ammonium nitrate in water and dilute to 5 l. Standardization is unnecessary.
Grind soil and pass through a 2 mm mesh sieve. Transfer 20 g of air dried soil into a bottle and top up to 50 ml of M ammonium nitrate. Cap the bottle and shake on the shaking machine for 30 mins. Filter through a 125 mm Whatman No. 2 filter paper and retain the filtered extract (this sample may also be used for the determination of magnesium; and sodium if carried out immediately or the bottle is stored in a polythene bottle). Carry out a blank determination with ammonium nitrate solution.
The concentration of potassium in the extract is determined by flame photometry; significant interference by other elements does not occur.
Potassium working standard solutions, 0-50µg/ml of potassium - Prepare solutions containing 0, 10, 20, 30, 40 and 50 µg/ml of potassium in 1M ammonium nitrate.
Preparation of Standard Graph
Set the flame photometer in accordance to MultiPoint/Single Ion Calibration found on page 24 of the BWB Technologies Installation and Operation Manual, to measure potassium emission. Nebulize the potassium working standard solutions containing 0 and 50 µg/ml of potassium and adjust the controls until steady zero and maximum readings are obtained. Nebulize the intermediate working standard solutions and construct a graph relating meter readings to µg/ml of potassium in all the standard solutions.
Examination of Extract
Adjust the flame photometer until the steady zero and maximum readings are obtained with potassium working standard solutions containing 0 and 50 µg/ml of potassium. Nebulize the extract and note the meter reading.
Calculation of Results
Read from the standard graph the number of µg of potassium equivalent to the meter readings of the sample and the blank determinations. Multiply the difference by 5. The result gives the mg/l of extractable potassium in the soil.
Potassium stock standard solution, 1 mg/ml of potassium - Dry potassium nitrate at 102°C for 1 hr and cool in a desiccator. Dissolve 1.293 g of the dried salt in water and add 1 ml of hydrochloric acid, approx. 36% m/m HCl to reduce the pH and aid dissolution of the sample. Dilute to 500 ml and add 1 drop of toluene.