Soil cations - Soil Essentials: Managing Your Farm's Primary Asset - page 50

How to apply soil cations
Calcium . Calcium is usually applied in the form of lime when used to raise soil pH,
or as gypsum if used to help counteract sodicity. One tonne of lime per hectare
will usually raise soil pH about half of one unit and help improve soil structure.
Finely ground lime is best.
Gypsum may be required in rates between 5 and 20 tonnes per hectare to
counteract sodicity and to make clay more friable. Gypsum can also be used to
increase calcium in nonsodic soils.
Both additives may be top dressed on, but lime is best incorporated into the
soil for faster results. Gypsum is most effectively applied dissolved in irrigation
water but has to be mechanically mixed due to its low solubility.
Magnesium . If magnesium is required, it is usually applied as a top dressing of
dolomite or magnesium oxide. This material will also raise soil pH, and its
neutralising value is about 1.7 times that of lime. If pastures are deficient in
magnesium, stock health may be adversely affected causing hypomagnesemia. Top
dressing with dolomite or magnesium oxide will help solve this problem.
Potassium . Potassium is required by plants for cell growth, and it is involved in
regulating plant water uptake and use, thus helping them cope with dry
conditions. Potassium is usually applied in blended fertilisers, usually in
conjunction with nitrogen and phosphorus, but can be added as a nitrate, chloride
or sulphate. Potassium availability is dependent on soil conditions, and in excess it
can be rapidly leached out of the root zone if it is not used. If high rates are
required (for example by fruit trees) it is probably best applied in split applications
to avoid leaching losses.
Sodium . This nutrient is not usually applied; rather, too much is the usual
problem. Excess sodium may be leached out in time by fresh water if there is
somewhere for it to leach to. Applications of gypsum are often used to help remove
exchangeable sodium from the soil in conjunction with leaching. One situation
where sodium may need to be applied is when growing plants in the beet family
(for example, sugar beet and mangolds).
Aluminium . Again, the toxin aluminium is usually a problem of excess rather
than deficiency. It is a problem in acidic soils, where it becomes soluble and
available to plants. Addition of lime to reduce soil acidity above pH(w) 5.5 will
solve toxic aluminium problems.
Manganese . The same applies for manganese as for aluminium; reducing soil
acidity by adding lime will reduce its solubility, and toxic effects.
Hydrogen . Excessive hydrogen ions in the soil are an indication of the level of
soil acidity. (pH is the number of hydrogen ions in solution.) Hydrogen is not
added to or removed from soil, but is the result of chemical changes that happen in
the soil from adding carbonates, oxides or hydroxides of calcium and/or
magnesium, or from the results of microbial activity and root exudates.