Potassium extractants

Potassium occurs in soil clay minerals, feldspars and micas. The unweathered illite region of the clay mineral contains non-exchangeable K+, the weathered vermiculite region has exchangeable K+, while the intermediate region has slowly exchangeable K+. There is also available potassium in the soil solution. The extractant will leach the free potassium ions and displace the exchangeable and some slowly exchangeable K+ by replacing the K+ with Na+, H+ or NH4+, depending on the extractant. Various extractants are listed in Table 4.4.

Table 4.4. Some extractants for potassium in soils used by various regional laboratories.

Regional methodology Extractant

Comments

North Central USA

North Eastern USA

MAFF/ADAS UK

Mehlich 3

1 M ammonium acetate

Morgan, Modified Morgan, Mehlich 1 and Mehlich 3

1 M ammonium nitrate

Non-calcareous soils Calcareous soils if Ca and Mg also to be extracted All are acidic and could extract some non-exchangeable K+. Ammonium based reagents extract more K+ than H+ or Na+ based ones Also used for Mg and Na

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