Magnesium, with the symbol Mg and atomic number 12, is the fifth chemical element in our biochemical sequence of essential plant nutrients. As the positively charged  ion (Mg⁺⁺) at the centre of chlorophyll, no plant could survive without magnesium. Photosynthesis is not possible without it.  Due to its presence in chlorophyll, which converts sunlight into the plant’s energy, magnesium can be likened to an engine room. Magnesium is a highly mobile element within the plant and the soil, and is therefore easy to leach and lose.  It works closely in conjunction with nitrogen, calcium and phosphorus, and being in balance with these other elements is vital to healthy plant growth.


  • synthesizes with amino acids, vitamins, oils, fats, sugars, pigments and carotene
  • helps to regulate the plant’s uptake of calcium, nitrogen and phosphorus
  • acts as a carrier for phosphorus in the plant
  • is vital for seed germination

In the plant we are looking for a constant supply throughout the growing season, specifically in both a slow release (stable form) and a more plant available form.

Unfortunately a lot of the Western Cape’s soils are seriously deficient in magnesium, which is easily leached away.  Ideally, we want luxury levels of magnesium in the soil all year round.

Here are a couple of pointers on magnesium management, and things to be aware of:

  • Excessive potassium in the soil can restrict the magnesium uptake. Bear in mind, that this situation is very common in chemical fertilization regimes which focus on soluble potassium sources. Mg : K ratio should be approximately 1:1 in ppm
  • Even in high magnesium soils it is possible to experience deficiencies, as excessive magnesium blocks itself.  A way to bypass this, is to consider using folia applications
  • Nitrogen stimulates magnesium uptake
  • Excessive calcium in soil displaces magnesium and visa versa, hence the focus on balance in all soils. The ideal Ca : Mg ratio should be 7:1 in heavy, tight soils and 4:1 in sandy, loose soils.

On Avondale, when we first started to balance and restore our soils, we mostly made use of slow release forms of magnesium such as dolomitic lime.  You can also use magnesium oxide if your certification body allows it.  We rely on the micro-life in our soils to digest this slow release form and make it available to our vines.  There is a quick release form of magnesium, magnesium sulphate, which is very soluble and can leach away easily, but it may be appropriate to use at times.  Today, the magnesium levels in our soils are stable and the high levels of organic matter in the soils help to prevent leaching.

The biochemical sequence is an explanation of the cycles, and the roles that biochemicals play in the development of plants.  You can read more about the fascinating interplay of boron, silicon, calcium, sulphur and nitrogen.