6.9 Exchangeable Cations

Exchangeable cations 

Exchangeable cations refer to the positively charged ions which are loosely attached to the edge of clay particles or organic matter in the soil. The cations include Calcium, Magnesium, Potassium, Sodium, Hydrogen and Aluminium.

The total number of these positively charged ions is known as the Cation Exchange Capacity (CEC)

The cation exchange capacity gives an indication of the number of sites where +ve ions can attach and hence the higher the figure the greater the potential fertility of a soil.

Where the cation exchange capacity of the soil is <5 the soil is considered inherently infertile. Sandy soils in SA often have values less than 5

Plant and Livestock Requirements

Most of the exchangeable cations are required by both plants and animals. The level of cation present and the balance between the cations can lead to imbalances, deficiency or toxicities. In most cases dryland crops and pastures are particularly tolerant of low levels of cations and cation imbalances.

Of particular importance when assessing soils for cations is to recognise the large differences in levels which may occur down root zones. This is very important as soils with underlying clays have much higher levels of cations than surface soils which are tested.

Observations from pasture, grazing and horticulture

Calcium
Deficiency in pastures and crops has not been recorded in the Mt Lofty Ranges. High Ca:Mg ratio can make plant Mg levels lower however pasture plant growth has not been affected to date. Calcium deficiency is more common in horticulture where some plants have high requirements, eg apples, brassicas

Magnesium 
Deficiency in pastures on acid soils is rare. It has not been observed in situations where soils do not have an acidity problem.

Low magnesium in livestock causes grass tetany (hypmagnesaemia) and can be induced by high potassium relative to calcium plus magnesium and/or acid soils. Often liming can only partially reduce the potential for grass tetany when acidity is an issue. Other techniques such as improved clover content, careful grazing regime and magnesium supplements to livestock are also recommended to counteract grass tetany.

High Magnesium Percentage (>20%) is often associated with hard setting clays.

Particularly high levels of magnesium can induce potassium deficiency in more intensive horticultural crops

Sodium
High sodium (>6%) can lead to soil dispersion and/or sodicity. Gypsum will reduce the effect of sodicity by replacing sodium with calcium on the cation exchange

Potassium
Potassium deficiency/low potassium levels - common in some soils, particularly sandy soils and where considerable hay cutting has occurred. 

High potassium induces low Mg problems in horticulture (and potential grass tetany in livestock).

Aluminium
High/toxic levels occur on most soils with an acidity problem. If Aluminium % is greater than 5% it will affect most plants

Desirable Cation Ratio

There are several proposed ratios for optimum soil production. For dryland crops and pastures there is little evidence to support a particular ratio. However in table 8 are "ball park" figures for pH and cation levels and ratios which can be used to give an indication of issues.

Figure 1:
Desirable soil pH and cations in clay loam - sandy loam soils

• Low pH values must be treated with liming products ASAP.

• Suspected exchangeable cation imbalances or low figures need to be confirmed by other means - eg soil physical problems, livestock disorders, plant tests. In many cases particularly with dryland crops and pasture they do not effect the growth of the plant.

Effect of Liming Product on Exchangeable Cations

from limited data

If 2.5 tonnes/ha of a high quality product is applied:

• PURE LIME (35-40% Calcium, no Mg) increase Exchangeable Calcium by 500-600 mg/kg , no effect on Mg

• LOW MAG (29% Calcium, 4% Mg) increase Exchangeable Calcium by 400-500mg/kg and Exchangeable Magnesium by 60 mg/kg 

• HIGH MAG (DOLOMITE) (24% Ca, 9% Mg) increases Exchangeable Calcium by 260 mg/kg and Exchangeable Magnesium by 120 mg/kg

Gypsum will increase exchangeable Calcium at the expense of sodium, however gypsum will not change the pH.

Other sources of magnesium are Magnesium oxide and magnesium sulphate.

6.9 Exchangeable Cations

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