Analytical Methods for Available Potassium, Magnesium, and Calcium

In a routine analysis for advisory purposes, attention is paid mainly to available –P,-K-Mg, pH, and lime requirements.

Potassium and Magnesium

Exchangeable Potassium K+ is measured with:

1M NH4 NO3 extraction method. This method is popularly used in England and Wales.

Extraction with 0.5M CH3 COOH (Acetic acid) is recommended for acid soils as for Scotland soils which are more acid than soils of England and Wales.


Interest is not on available Ca but on the indirect availability of Ca through pH. pH is measured:

Read Also: Forms and Functions of Calcium in Plants

In water (pH H2O)

In 0.01M CaC12 or KCL (pH CaC12) whereby pH CaC12 < pH H2O by about 0.5 – 1.0 unit. When the pH measurement is low, liming desirable to a final pH of 6.2 – 6.5.

In conclusion, the sufficiency or deficiency of calcium in soils is associated with soil acidity. Calcium is usually low in acid soils, soils low in organic matter, and in sandy soils where leaching of calcium minerals is usually pronounced.

The use of Ca CO3 to correct soil acid levels simultaneously adds calcium (Ca++) elements to the soil. Calcium (Ca++) and magnesium (Mg++) dominate the exchange complex of most salt-affected soils. Minerals such as serpentine rocks and dolomite are sources of both magnesium and calcium.

The behavior of sulphur in plants and soil is very much related to that of nitrogen in that they both form essential parts of proteins and other organic compounds in soil organic matter. The release of sulphur in the inorganic forms is mainly through the mineralization of soil organic matter mediated by soil microorganisms.

Sulphur also causes acid rain or acid deposition as an environmental problem in soils. The deficiency of sulphur easily shows in legumes such as groundnuts, soya beans, and cowpea, especially in savanna zones with coarse-textured soils subjected to repeat annual burning.

Read Also: Forms and Functions of Sulphur in Plants

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