All plants require magnesium to capture the sun's energy for growth and production through photosynthesis.

Magnesium oxide

MgO is a high-analysis product containing 40% Mg which dissolves to release plant-available Mg over a period of several months. The material itself is highly alkaline, but at normal rates of application it is unlikely to have much effect on soil pH. The main factor influencing the rate at which MgO dissolves in the soil is particle size, i.e. finer material will release Mg more quickly.

MgO can be mixed with most fertilisers but being alkaline, should not be mixed with ammonium sulphate which is acidic (they react, leading to ammonia loss, which can create a health hazard for workers in confined spaces).

Mixing MgO with superphosphate is ideal for capital dressings where the purpose is to boost soil Mg levels, enhancing the supply of pasture Mg to cows and reducing the incidence of hypomagnesaemia during early lactation. The best time for such dressings is in autumn to allow sufficient time for the MgO to dissolve and be taken up by the pasture. Magnesium Super is available as a standard blend containing MgO.

Serpentine Super

Because of its 5.5% Mg content and the fact that not all the magnesium is readily available, Serpentine Super is suitable for use as a maintenance fertiliser rather than for supplying capital inputs of Mg. When added to Super prior to granulation, finely ground magnesium silicate rock reacts to the acid in the Super, converting about 70% of the Mg to a plant-available form. The other 30% or so has rather low plant availability in the short term, but at least some of it will become available over the medium to long term at a rate dependent on the fineness of grinding.

Dolomite

Finely-ground Dolomite is ideal in situations where maintenance inputs of magnesium are required in addition to a liming effect. Dolomite (11% Mg) is a naturally occurring material containing both magnesium and calcium carbonates (lime contains only calcium carbonate). As with lime, particle size has a big influence on the dissolution rate and therefore on how quickly the Mg becomes available as well as how quickly it increases soil pH.

Like lime, Dolomite is a slower-release material, typically delivering a benefit over a three to 24-month period with the finer particles dissolving within a few months.

Lloyd Glenny is Product Manager, Fertiliser at Ravensdown.​