Analytical Method: Potassium in soils

Instruments and Reagents:

Cat.No. 173016  Spectroquant® VIS Spectrophotometer Prove 100 or

Cat.No. 173017  Spectroquant® UV/VIS Spectrophotometer Prove 300 or

Cat.No. 173018  Spectroquant® UV/VIS Spectrophotometer Prove 600 or

Cat.No. 109748  Spectroquant® Photometer NOVA 30 or

Cat.No. 109751  Spectroquant® Photometer NOVA 60 or

Cat.No. 173632  Spectroquant® Colorimeter Move 100

Cat.No. 114946  Rectangular cells 10 mm and/or

Cat.No. 114947  Rectangular cells 20 mm and/or

Cat.No. 114944  Rectangular cells 50 mm
Cat.No. 114562  Spectroquant® Potassium Cell Test
Cat.No. 100615  Spectroquant® Potassium Cell Test
Cat.No. 105590  Sodium hydroxide 32 % for analysis
Cat.No. 102103  Calcium lactate for soil analysis
Cat.No. 109325  Calcium acetate for soil analysis
Cat.No. 116754  Water for analysis
Cat.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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