Monday, 01 August 2016 07:54

Connecting calcium and magnesium

Written by  Ross Wright, Golden Bay Dolomite
New Zealand's's only dolomite quarry – Golden Bay Dolomite. New Zealand's's only dolomite quarry – Golden Bay Dolomite.

Magnesium is an extremely important but almost forgotten mineral in New Zealand soils.

It is inextricably linked to Calcium, a mineral that is also neglected, or at least used for the wrong reasons. They both should form the basis of fertility programmes. Other minerals can be more effective when Calcium and Magnesium are at optimum levels and are vital for soils, plants and animals. Deficiencies of either or both can result in serious economic consequences.

The percentages of Ca and Mg on a soils base saturation are critical to the success of any crop, including grapes vines. Don't let anyone tell you it is ratio, it is absolutely not.

An ideal soil contains 45% minerals, 5% Humus and 50% pore space occupied equally by air and water; it is Calcium and Magnesium that are responsible for this configuration and in turn will determine the physical water holding and drainage characteristics of your soil. When the optimum pore space has been achieved then an ideal environment is created for soil biology to thrive. It is imperative a soil has good biology levels because we rely heavily on these little critters to process and supply nutrient to the vines root system.

Vine quality and yield are not only determined by soil nutrients but also by soil microbes and root depth. Without the correct soil structure vine roots cannot access nutrients and the biology struggles to survive. Without correct soil structure top yield and vine health cannot be achieved. It is therefore mandatory that Calcium and Magnesium be correct.

In most New Zealand soil the target base saturation is 68% Calcium and 12% Magnesium (PAL test).

Calcium loosens soil by flocculating particles and increasing pore space while Magnesium tightens soil by separating particles and decreasing pore space. Therefore if your soil is a lighter sandy type you may well need slightly higher Magnesium to improve water holding capacity (65%Ca – 15% Mg), while a heavier clay type would require a little more Calcium (69%Ca – 11%Mg). The total Ca + Mg should equal 80% BS.

If Magnesium, a determinant of Nitrogen and Phosphorus efficiency, is outside the 10 – 15% 'sweet spot' vine performance is hindered and deficiencies may start to show. Magnesium is found at the centre of the chlorophyll and so is essential for vines to carry out photosynthesis. It gives the healthy deep green colour to the leaves.

We do have areas of high Magnesium soils in New Zealand but most are deficient. A common mistake is to apply Calcium (Ag Lime) alone to lift pH, where in fact all four cations - Calcium, Magnesium, Potassium and Sodium affect pH. The latter three to a greater degree than Calcium, with Magnesium lifting 1.66 times more. Calcium inputs on there own will displace Magnesium on the soil colloid. With every 1% Calcium added 1% Magnesium can be lost!

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