Magnesium – the big loser

Better soils
with Brett Petersen
Kiwi Fertiliser & Golden Bay Dolomite

Lack of magnesium in NZ soils has caused losses of millions of dollars every year. Little research is being done to solve the problem. Under the heading of ‘Magnesium deficiency’, one authorative magazine listed the common options for supplementing magnesium are…It went on to list many options; each option temporary, time consuming and repetitive. The options were in fact, treating or trying to prevent symptoms of grass tetany, (hypomagnesaemia) particularly prior to calving.

There was not a single reference to the soil. The soil is where the cause is seated, so that is where the remedy must be applied. The pasture and animals are a reflection of what is in, or not in, the soil. Kiwi Fertiliser’s approach is based on cause centred science; the usual approach is based on symptom oriented science. This is the science that transfers profit from farmers to corporations.

Soil samples

Kiwi Fertiliser sends its soil samples to PAL laboratories in Missouri. The test results of new clients show more than 90 per cent of samples are deficient in magnesium. That test often shows values of six per cent to eight per cent of base saturation when those levels need to be between 10.1 per cent and 12 per cent for most soils. That deficiency may translate to 150-200kg/ha of magnesium.

Some NZ publications cite 25kg/ha of magnesium as being an adequate annual application. NZ research is often faulty as it looks at products, while not supplying what the soil needs. Some trials use only mag oxide then wrongly assume that accurately represents magnesium behaviour and results.


There is only one source of Dolomite and that is in Golden Bay, so it has to be shipped north. This adds cost, but as a calcium/magnesium carbonate, it works exceptionally well. It is a semi-soluble calcium-magnesium source. Dolomite can also be direct-fed to cows as a temporary but highly effective measure to counter hypomagnesemia.

Mag carbonate, mag oxide and serpentine – a hard silica rock – are also semi-soluble. Many thousands of tonnes of mag oxide and serpentine have been applied to NZ soils over the last 60 years, but magnesium deficiencies are still rife. Soluble sources are Mag sulphate and Kieserite. Kieserite releases Mg quickly and is useful where Mg is required for short term crops. Dolomite and Keiserite build soil Mg levels, whereas other materials may not. Your choice of weapon is critical and should not be based on the cheapest per unit or nearest source. It must be based on what works and what doesn’t. Dolomite releases Mg during a period of up to two years.


Do not consider magnesium and calcium on its own. Rather, they must be calculated in tandem. If there is ample Ca in the soil, Dolomite may not be an option, but Kieserite will be. If calcium is not adequate – the optimum being 60 per cent to 68 per cent on a PAL soil test – then Dolomite is a preferred option. However, Mg and Ca have a one-to-one relationship when the soil supply of each is high, so for each 1 per cent of Ca added, 1 per cent of Mg can be displaced – and vice versa – and needs to be added back into the calculations. This relationship tapers off when there are deficiencies of either.

A relationship overlooked

If calcium plus magnesium add up to less than 80 per cent of base saturation on the PAL soil test, then Mg is most likely deficient, regardless of whether it shows a high reading – for example, 18 per cent. In this situation, the plants find it difficult to take up Mg. It can only be considered to be excess when those numbers exceed 80 per cent. This relationship is often overlooked and probably why NZ farms suffer so much hypomagnesemia. High Mg may not be high at all. Once the calcium is added, the magnesium deficiency is exposed. The message is clear. Do not just apply calcium (lime) without considering magnesium.

Other nutrients also have a direct relationship with magnesium, especially potassium, nitrogen and phosphorus. The closer the figures above are to correct, the less negative impact each nutrient will have on each other, and less of each will be required to achieve superior results.


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