The Calcium Magnesium ratio in Soil

Many people have heard of the Calcium Magnesium ratio, but few people understand what it is all about. The problem is that is has implications for both soil structure and animal health.

Issues also arise in understanding both the importance and use of the Ca:Mg ratio. The desirable levels for this ratio (that are now widely accepted throughout Agriculture in Australia) were one of the first outcomes of Ted Mikhail's research in the 1960s, yet there is still an under appreciation of the significance of this simple measure.

Mostly, when people discuss the calcium magnesium ratio (including many 'experts') they talk about it using plant nutrition terms and so it is often expected that extremes in the ratio will produce either Calcium or Magnesium deficiencies. This is simply incorrect.

However, the problem is that the Calcium Magnesium ratio has implications for both soil structure and animal health. So, strictly speaking, there are really two ratios to talk about.

Also, for soil friability, a proper assessment of the ratio cannot be made on the basis of the Ca:Mg ratio alone. It is important to also realize that the effects of the Ca:Mg ratio occur equally in all soil texture classes (sand and clay alike). To illustrate how things work, here are a few typical scenarios:

1. Soil Friability

For an assessment of how the proportions of Calcium and Magnesium influence soil friability, the Calcium Magnesium ratio should be calculated from the exchangeable cation figures in me/100g. If the Calcium percentage is close to or within its desirable range (60% to 70% of the adjusted CEC) and the Ca:Mg ratio is less than 2:1, then the soil will have poor structure and be classified as non-friable. Under these same conditions, as the ratio increases from < 2:1 to 4:1 the soil will progress through stages of friability from semi-friable to friable and very friable, but above a ratio of 4:1 there will be no further improvement in friability. The table below shows this effect in more detail:

	Ca%	Mg%	Ca:Mg*	Na%	Soil condition
Low/Low	<40%	<12%	>2:1	<5%	Poor structure; semi-friability
Low/High	<40%	>20%	<2:1	<5%	Poor structure; hard setting; non-friable
High/Low	>65%	<12%	>4:1	<5%	Good structure, friable
High/High	>65%	>20%	<4:1	<5%	Good structure, semi-friable (very rare)

* It should be noted that the Ca:Mg ratios given here represent general conditions found in soils with the kinds of Ca% and Mg% shown. The ratio is not and should not be calculated from these percentages.

2. Soil Hardness

As can be seen from the "Low/High" condition in the Table above, if the Calcium percentage is low and the Calcium Magnesium ratio falls below 2:1 then the soil will also become hard-setting when dry and will show poor moisture infiltration and drainage characteristics. However, this effect is not seen if Calcium is above about 55% of the adjusted CEC.

3. Soil Sodicity

The Exchangeable Sodium Percentage (ESP) is simply the proportion of the Adjusted Cation Exchange Capacity that is taken up by Sodium , but it is an important measure for many soils. This is because of its impact on soil condition. When the ESP is in excess of 5% of the adjusted CEC. Under these conditions it has the ability to make soils sloppy and dispersive when wet (resulting in reduced moisture and root penetration). The effect will be especially severe when calcium and magnesium fall within the "Low/High" condition in the table above.

4. Animal health

"Grass Tetany" is a metabolic condition that occurs in lactating cattle and sheep where the Magnesium level in their blood falls below a critical level. Symptoms appear rapidly and animals can die quickly and without warning.

Research by Ted Mikhail showed that if the Calcium Magnesium ratio rises above 5:1, the risk of grass tetany becomes very significant and above a ratio of 6:1 it is extreme. Under these conditions, all other strategies for minimizing this condition in calving or lambing animals are likely to fail and significant numbers can be expected to suffer grass tetany.

However, for animal health purposes, the ratio should be calculated using the Available cation figures rather than the normal exchangeable cation results. Since available cation results are usually given in ppm (or mg/kg) they will need to be converted to me/100g.

An example of how the Calcium Magnesium ratio can differ depending on whether it is calculated from exchangeable or available figures (both as me/100g) is given in the table below.

For the same sample:	Calcium	Magnesium	Ca : Mg
Exchangeable cations	2.95 me/100g	0.72 me/100g	4.1 : 1
Available cations	3.54 me/100g	0.78 me/100g	4.5 : 1

So, in fact, there are really two Calcium magnesium ratios and neither of them are directly associated with plant nutrition. You should also bear in mind that when looking at this ratio, it should never be used alone - the calcium, magnesium and sodium percentages (of the adjusted CEC) should also be taken into account.