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Application Note

Potassium Extraction from Soil

Based on Method 52 in MAFF (1986).
The Analysis of Agricultural Materials
(3rd ed) HMSO.


Potassium was extracted from a soil sample with acidified ammonium lactate to determine plant available potassium ionic concentration present in the sample.


Bottle - 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.

5 (+ Number of soil samples) 100ml polyethylene volumetric flasks

100mg/l K stock solution


Acidified Ammonium lactate extractant 2M, 215.2g ammonium lactate was dissolved into 500ml of DI water, 50ml 1M acetic acid solution was then added and then made up to the mark in a 1L volumetric flask with DI water.


The soil was air dried to remove moisture content and caution was given to drying in dusty areas which could contaminate the sample. Soil was ground to a fine powder which was then passed through a 2mm mesh sieve. 20g of dried soil was transferred into a clean dry 125ml graduated bottle and 50ml of acidified ammonium lactate solution was added. This solution was then agitated on the shaker machine for 30 minutes. The solution was then filtered through 125mm Whatman No. 2 filter paper and the filtrate retained in a polyethylene 100ml volumetric flask.


The concentration of potassium in the extract was determined by flame photometry; significant interference by other elements does not occur, however the addition of ammonium lactate was accounted for in the development of a series of potassium standards.

A series of potassium standards were formed from 100mg/l stock solution. To a series of polyethylene volumetric flasks 0, 10, 20, 30 and 40ml of 100mg/l K stock solution was added. To each of these solutions 21grams of ammonium lactate were added and then 5ml of 1M acetic acid solution following the addition. These bottles were then labelled with 0-40mg/l K standard solutions with 2M acidified ammonium lactate.

Preparation of Standard Graph

The flame photometer was set to Multipoint/Single Ion calibration setting found on page 24 of the BWB-Tech installation and Operations manual. The potassium standards were then nebulised starting with the lowest concentration solution to form a single ion calibration graph.

Examination of Extract

Adjust the flame photometer was set to READ mode and the sample nebulised until a stable reading was acquired. It was recommended that multiple samples are analysed from different parts of the soil collection site for an overall representative sample of soil area.

Calculation of Results

A mean result was calculated from the different soil samples and the range of results recovered (The lowest result was subtracted from highest result then halved, this number was then used as a plus or minus from the mean average to give indication of deviation from the mean average) to improve data analysis of the sample site.

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