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Gambica

   

Method F-009A The Determination of Available Potassium in Soils

Equipment Required:

  1. Sherwood Model 410 Flame Photometer
  2. Whatman No.30 filter paper
  3. Potassium Standard ( Sherwood Part 00156621)
  4. Balance weighing to +-0.0005g
  5. Ammonium acetate
  6. Acetic acid.

Method:

  1. Switch on the Model 410 Flame Photometer and allow it to warm up for 30 minutes.
  2. Prepare 0.5M aqueous solution of ammonium acetate/acetic acid by taking 38.55g ammonium acetate and dissolving it in 29mls of glacial acetic acid and diluting to 1 litre with distilled water.
  3. Accurately weigh 10g of soil and transfer to a plastic bottle together with 50ml of acetate/acetic acid solution.
  4. Stopper the bottle and shake using an automatic shaker for 30 minutes.
  5. Remove from the shaker, allow to stand for several minutes. Filter the solution through a Whatman No.30 filter paper.
  6. Prepare standard potassium solutions to cover the range 0-l00 ppm as follows:

    Accurately pipette 10 mls Potassium standard into 100 ml volumetric flask and make to the mark with ammonium acetate /acetic acid solution. This is the 100 ppm standard From this prepare 80,60,40 and 30 ppm standards using the CH3C00NH4/CH3C00H solution as diluent.

  7. Aspirate the 100 ppm standard and set the display to read 100. Without touching the controls aspirate the other standards and confirm that they give a linear response.
  8. Determine the potassium content of the soil extract by spraying the solution without further dilution into the flame photometer and reading the display.

    Calculation:

    1. Concentration potassium (mg/100gms soil)= displayed value( ppm) x vol extractant solution x100 / weight sample x 1000

12 September, 2008