HomePhotometry & ReflectometryAnalytical Method: Potassium in soils

Analytical Method: Potassium in soils

Instruments and Reagents

Product No. 173016 Spectroquant® VIS Spectrophotometer Prove 100 or
Product No. 173017 Spectroquant® UV/VIS Spectrophotometer Prove 300 or
Product No. 173018 Spectroquant® UV/VIS Spectrophotometer Prove 600 or
Product No. 109748 Spectroquant® Photometer NOVA 30 or
Product No. 109751 Spectroquant® Photometer NOVA 60 or
Product No. 173632 Spectroquant® Colorimeter Move 100 Product No. 114946 Rectangular cells 10 mm and/or
Product No. 114947 Rectangular cells 20 mm and/or
Product No. 114944 Rectangular cells 50 mm
Product No. 114562 Spectroquant® Potassium Cell Test
Product No. 100615 Spectroquant® Potassium Cell Test
Product No. 105590 Sodium hydroxide 32% for analysis
Product No. 102103 Calcium lactate for soil analysis
Product No. 109325 Calcium acetate for soil analysis
Product No. 116754 Water for analysis
Product No. 100063 Acetic acid 100% for analysis

CAL-Extraction Solution

Dissolve 15.4 g calcium lactate for soil analysis and 7.9 g calcium acetate for soil analysis separately in 300 mL of water for analysis whilst warming. Combine the two solutions in a 1-l volumetric flask and add 17.9 mL acetic acid 100 %. Allow to cool, make up to the mark with water for analysis and mix well.

Sample Preparation

Air-dry the soil sample, subsequent to removing course particles such as stones and plant material. Sieve the sample through 2-mm mesh. Place 5 g of sample in a separating flask of approx. 300 ml volume and add 100 ml of extraction solution. Shake the closed flask for 90 minutes at 175 shakes per minute. Filter the extract through a folded filter and dispose of the first 10 ml filtrate. Adjust the clear filtrate to pH 5 - 7 with sodium hydroxide solution 32%. Samples containing more than 35 mg/l K2O should be diluted with extract solution.


Determine with the above mentioned test kits.
Measurement should be made against the pure extraction solution. If the sample extract has a yellow color, it should be measured, without the addition of reagent, as a blank and the result subtracted from the sample value.

Potassium content in mg/kg K2O = analysis value in mg/l K x 1.2 x 100
weight of sample in g
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