Magnesium (Mg) is an essential nutrient for both plant and animal production. In plants, it plays a central role in photosynthesis as a core component of chlorophyll, and it activates more enzymes than any other nutrient. It is therefore critical for energy transfer, growth and overall plant function.
In livestock systems, magnesium is equally important. A deficiency in grazing animals can lead to hypomagnesaemia (grass tetany), a serious metabolic disorder. This risk is particularly acute in spring, when magnesium uptake from lush grass is low and can be further reduced by high potassium levels in herbage.
Magnesium in soils
Magnesium occurs naturally in soils, largely derived from parent material such as limestone, shale and other Mg-bearing minerals. Total soil magnesium can vary significantly across Ireland, reflecting differences in geology and glacial deposits. However, high total magnesium does not always translate into high plant availability.
Available magnesium is influenced by several factors, including soil type, pH and external inputs. Unlike phosphorus and potassium, magnesium availability is strongly affected by atmospheric and oceanic deposition. Coastal areas, particularly along the western seaboard, often have higher levels due to sea spray carried inland by prevailing winds.
Magnesium is held on soil particles and behaves as a cation (Mg²⁺), similar to potassium and calcium. It is moderately mobile in soil, meaning it can be leached under certain conditions, particularly in light or sandy soils.
The influence of soil type
Soil type plays a key role in magnesium availability. Sandy and free-draining soils are most prone to deficiency due to their lower capacity to retain nutrients. Acid soils and soils with poor structure can also limit magnesium availability, particularly where root growth is restricted.
In contrast, soils derived from limestone or marine materials tend to have higher magnesium reserves. However, availability still depends on soil conditions, and deficiencies can occur even where total magnesium levels are relatively high.
Magnesium availability can also be influenced by interactions with other nutrients. High levels of potassium in soil or fertiliser applications can reduce magnesium uptake by plants, increasing the risk of deficiency.
Availability and crop uptake
Plants take up magnesium from the soil solution, but availability can fluctuate depending on soil conditions. Deficiency is often most noticeable early in the growing season, when root development is limited and soil temperatures are low. In many cases, symptoms are temporary and may disappear as root growth improves.
Magnesium deficiency can affect a range of crops, including cereals and potatoes, and is more likely where soil levels are low or where nutrient imbalances occur. In cereals, responses to magnesium are generally only observed where soil levels are below Index 2.
Magnesium and animal health
Magnesium plays a critical role in livestock nutrition, particularly in grazing systems. Grass tetany is most likely to occur in spring, when herbage magnesium levels are low and the rapid passage of grass through the animal reduces absorption.
The risk is further increased where potassium levels are high, as potassium can interfere with magnesium uptake in the animal. For this reason, magnesium supplementation is often required, particularly for dairy cows during early grazing.
Herbage testing is generally a more reliable indicator of magnesium availability to animals than soil testing.
Loss pathways and soil dynamics
Magnesium is not lost to the atmosphere, but it can be lost through leaching, particularly on light soils with low nutrient-holding capacity. Soil structure also plays an important role, as poor structure can limit root access to magnesium and reduce its uptake.
Because magnesium is held on soil exchange sites, its availability is also influenced by soil pH and the balance of other cations such as potassium and calcium.
Managing magnesium for efficiency
Effective magnesium management begins with soil testing, typically using the Morgan’s Mg index system. The aim is to maintain soils at Index 3, where magnesium supply is adequate for crop and animal needs.
Where soil magnesium levels are low, the most common approach is to apply magnesium limestone, particularly where liming is already required. This provides both pH correction and magnesium supply in a single application. Where soil pH is already optimal, magnesium can be supplied through fertilisers such as kieserite (magnesium sulphate), typically applied every few years.
In cases of acute deficiency, particularly in crops, foliar applications such as Epsom salts can be used to provide a rapid correction. However, these are generally short-term solutions.
In livestock systems, direct supplementation is often necessary. Sources such as calcined magnesite are commonly used to prevent grass tetany, particularly in spring.
A nutrient that requires balance
Magnesium is often overlooked compared to nitrogen, phosphorus and potassium, but it plays a critical role in both crop production and animal health. Its availability depends not only on soil reserves but also on soil type, structure and interactions with other nutrients.
By maintaining appropriate soil levels, managing nutrient balance, and addressing seasonal risks, particularly in spring grazing systems, magnesium can be effectively managed to support productive and resilient farming systems.