Are all Magnesium fertiliser products created equal?
The short answer – No.
In the New Zealand market, there are several different magnesium fertiliser products available however not all of them behave the same way or do the same job in our soils.
Fertiliser applications – Soil feeders versus Plant feeders
When applying fertiliser, the plan is to provide a level of nutrients needed for plant growth whether it be for pasture or crops. Nutrients that are not immediately uptaken and remain available in the soil can be thought of as Soil Feeders.
The nutrients we apply as fertilisers can either be held strongly by soils commonly referred to as ‘tied up’ such as phosphate or they can ‘leach out’ after application, for example, Urea, these inputs can be considered ‘Plant Feeders’.
A Biological system
The goal under a biological system is to re-mineralise the soil to optimum levels. This method involves building with nutrients that are ‘soil feeding’ creating a system with the very best environment for soil life, pasture growth and stock health. A biological system approach is inherently sustainable: feed the soil – let the soil feed the plant.
Comparatively, if most of the rural sector continues to rely on the use of soluble ‘plant feeding’ products, then soil degradation will acerate and the environment will suffer to the detriment of our agriculture industry.
When do you need magnesium?
There is an important balance between calcium and magnesium when at the correct levels in the topsoil this achieves the optimum soil structure for plant growth.
Ideal magnesium levels are also required for plant photosynthesis and stock health. When using a MAF quick test calcium should be at least 60% and magnesium at 10% base saturation (BS).
While additions of many minerals can be beneficial there is an intricate balance between them - be sure to seek advice as too much of a good thing can cause a negative reaction.
With this in mind, what are the Magnesium options available for your farm?
Soil feeders - Recommended magnesium fertiliser
Magnesium sulphate (Kieserite) 16% mg 20% S
This is a particularly good product when magnesium is required but not calcium, meaning the calcium BS levels are already at or above 60%.
Kieserite also contains essential sulphur that is often under-applied in fertiliser programs.
Dolomite 11% Mg 24% Ca
Dolomite is a calcium/magnesium carbonate product that effectively builds both minerals in the soil making it ideal for many properties. It is advised to regularly soil test and monitor BS levels to make sure optimum levels of calcium and magnesium are achieved.
Plant feeders - Not Recommended
Magnesium Oxide 40% Mg
While high in magnesium it’s not an effective source of magnesium when aiming to correct levels in soil. Most of the magnesium applied is lost which does not build long-term soil levels.
Serpentine (Serpentine super)
A silicate form of magnesium and terribly slow to break down. The magnesium content varies depending on the relevant product which is often combined with superphosphate.
While some data shows increased magnesium uptake by plants post application, this could be due to the ‘fine particle’ component which can be utilised quickly by plants. Again a key consideration is that it does not effectively build magnesium base saturation levels in the soil.
After applying a magnesium fertiliser – Ask the question?
Have your soil Mg levels increased? If not, are you simply in a cycle of ‘plant feeding’ and your soil is missing out on arguably one of the most important minerals for both plant growth and stock health?
The same can be said for many other nutrients recommended to you.