A calcium/magnesium ratio is provided by most commercial soil testing laboratories, but there is debate about what it really indicates. There is evidence that the ratio has little impact on plant growth (mimimum quantities of each element seems to be more important), but does seem to correlate to soil structure.
Soil colloids (as well as sites on soil humus/carbon) are negatively charged, with calcium and magnesium making up the bulk of the cations attracted to the colloid surface. Calcium ions (Ca2+) flocculate (open) soil and magnesium ions (Mg2+) coagulate (close) soil. A balance between the two is needed to maintain ideal soil structure. Note: the following explanations are mineral oriented and ignore the possible links to biology.
The ionic charge and ionic radius of different cations has been used to explain how calcium and magnesium may affect soil structure.
Calcium has a 2+ charge and a large ionic radius - therefore 'holds' onto soil colloids securely, but at a distance - hence flocculation.
Magnesium also has a 2+ charge but a small ionic radius - therefore 'holds' onto soil colloids securely, but closely - hence coagulation.
Interestingly, this hypothesis also provides an explanation for the collapsible/non-structural nature of high sodium or potassium soils. They both have a 1+ charge, so 'hold' onto soil colloids less securely.
How are Calcium/Magnesium Ratios Measured?
The ratio is based on the exchangable/saturation (meq/100gm) level of calcium and magnesium as measured in regular soil tests*. Ca meq/100gm is divided by Mg meq/100gm. If cations have been measured in ppm, they can be readily converted to meq/100gm.
As a rough rule of thumb:
Ca/Mg Ratio <2 5-7 >10
Soil Structure sticky in wet, hard in dry ideal often dispersible
Low Ca/Mg Ratio soils, commonly referred to as high magnesium soils, are known to build up 'platform shoes' in the wet.
While these ratios underpin soil structure, not necessarily its nutrient value, very low Ca/Mg Ratio soils often do have poor calcium and/or magnesium plant uptake. Perhaps biology is the missing link where tight closed (low Ca/Mg Ratio) soils with inherently poor aeration simply cannot support enough beneficial/aerobic soil microbes - the key to nutrient availability and uptake... this could also explain why some ideal ratio soils can have poor nutrient status - minerally it may be sound, but biologically deficient.
How Do You Interpret Calcium/Magnesium Ratios?
The ratio provides a quick reference guide to current soil structure constraints. While calcium and magnesium levels can be compared to ideal levels for deficiencies, the ratio helps point to the cation needing the most attention - proportionately, particularly when there are cost-constraints or where excessive inputs are of concern. For example, both cations may be low along with a low ratio indicating calcium needs proportionately more attention, or vice versa, if both cations are low but the ratio high, then magnesium would need proportionately more attention. This is particuarly relevant to Albrecht advocates who design fertiliser blends that work towards improving soil structure (along with soil fertility) by using mixtures of bulk calcium and magnesium materials.
Beware though: using Ca/Mg Ratios in isolation can lead to erroneous interpretations:
1) calcium and magnesium levels can both be low, yet have an ideal ratio; or
1) both can be high, yet have an idea ratio.
Calcium Magnesium Ca/Mg Ratio
Ideal meq/100gm 70 10 1 √
Actual meq/100gm 23.3 (low) 3.33 (low) 1 √
Actual meq/100gm 84 (high) 12 (high) 1 √
* In the past this ratio was often based on total calcium and magnesium levels in a soil - as well as proving to be a cumbersome and time-consuming extraction method, it did not relate to the 'functional' fraction of calcium and magnesium in the soil, ie. that proportion actively being exchanged between soil colloids, soil solution, plant roots, microbes etc.
For More Information
Search Google for 'Soil Calcium Magnesium Ratio' or 'Soil Structure Calcium Magnesium Ratio'
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