Magnesium (Mg) is critical for plant growth. Although it is a common element, poor agricultural practices have led to widespread Mg deficiencies. The total magnesium content in soils may vary from 500-5000 mg/kg, but upwards of 90% of soil Mg may be incorporated into minerals and, thus, unavailable. The availability of Mg depends on environmental conditions like pH, soil organic matter, and parent material. Once released from mineral forms, Mg cations can bind to cation exchange sites on negatively charged clay and organic matter–this improves plant Mg availability by increasing the amount of exchangeable Mg.

In order to be solubilized and taken up by a plant, the positive charge of Mg cations bound to clay particles must be replaced by other cations, such as H+ or K+. Thus, pH can influence Mg availability, and pH <6.0 has significantly increased water-soluble Mg compared to pH >6.5. In soils that are too acidic (pH <5.2), an overabundance of exchangeable Al3+ ions and H+ ions at the rhizosphere can inhibit the ability of the plant to efficiently uptake Mg by interfering with the electrochemical gradient. In reducing conditions in acidic soils, manganese (Mn2+) toxicity may also reduce Mg uptake. Excess K+ and Ca2+ may also restrict Mg uptake in neutral or alkaline soils.

Less than a third of plant Mg is located in the chloroplast. Mg plays a crucial role in protein synthesis and activates enzymes such as carboxylases, phosphatases, and kinases. Mg also activates ATPases, allowing the catabolism of ATP to release energy and power the active transport of other molecules. Optimal Mg nutrition is reported to be 1.5-3.5 g/kg dry weight in vegetative growth. When plants are deficient in Mg, the first symptoms arise in the oldest leaves. Mg is relatively mobile and can be remobilized from old leaves to younger ones. Mg deficiency often presents as interveinal chlorosis, worsening under high light conditions with spreading lesions.

Magnesium deficiencies are most common on acidic, sandy soils, where rainfall can easily leach Mg. Leaching removes high amounts of Mg in these soils, and Al3+ dominates soil cation exchange sites. Deficiencies are also expected in soils with excessive K+ due to competition between the cations. Furthermore, soils may become depleted due to the long-term removal of Mg via grazing without Mg fertilization. Long-term sustainable management requires balancing Mg inputs with all Mg losses, including leaching and crop harvest.

Magnesium deficiency is primarily associated with disrupted photosynthesis, but the deleterious effects can be severe and widespread. When Mg is deficient, chloroplasts do not function properly, and the energy that should be used to capture CO2 instead generates reactive oxygen species (ROS), causing oxidative stress. Mg is associated with more than 300 plant enzymes, and under Mg deficiency, these enzymes are inhibited. As ATPases fail, phloem loading is disrupted, and sugars accumulate in source organs, decreasing starch synthesis. Similarly, protein synthesis is significantly decreased, and amino acids begin to accumulate in the plant. Due to decreased phloem loading and starch and protein synthesis, plant growth can be severely stunted under Mg deficiency. Mg deficiency often results in impaired root growth, which leads to poor water uptake and nutrient uptake. Nitrogen utilization is reported to be significantly affected by Mg deficiency.

Magnesium toxicity in plants is not nearly as common as a deficiency. Toxicity symptoms are described as copper-colored patches along the marginal veins. However, toxicity symptoms are often hardly visible even when cultured at Mg2+ concentrations as high as 60 mM (1.46 g/L). It is suggested that plant tolerance to high Mg concentrations is due to Mg’s sizeable vacuolar storage capacity. When calcium is limited, plants increase the activity of Mg transporter proteins, which can transport Ca2+ as well–this can lead to an excessive amount of Mg accumulating in the plant.