The Second International Symposium on Magnesium was held in São Paulo, Brazil, in November 2014. An overview of the role of magnesium in food production was the theme of the opening session of that meeting. This paper considers agricultural production systems, mainly in terms of sustainability, fertiliser-use efficiency and food security, with emphasis on soils of tropical regions. Magnesium in soils and plants, as well as in relation to other nutrients and to other agricultural production factors, is especially covered. Finally, the role of magnesium is considered in terms of plant and animal health, nutrient management and the need of future research.

In this review, we summarise factors contributing to plant availability of magnesium (Mg) in soils, the role of Mg in plant physiological processes related to yield formation and abiotic stress tolerance, and soil and fertiliser parameters related to Mg leaching in fertilised soils. Mg is a common constituent in many minerals, comprising 2% of Earth’s crust; however, most soil Mg (90–98%) is incorporated in the crystal lattice structure of minerals and thus not directly available for plant uptake. Plants absorb Mg from the soil solution, which is slowly replenished by soil reserves. Duration and intensity of weathering, soil moisture, soil pH, and root–microbial activity in soil are key factors that determine plant-available Mg release from soils. On the other hand, the amount of Mg released from soil minerals is generally small compared with the amounts needed to sustain high crop yield and quality. Thus, in many agro-ecosystems, application of Mg fertilisers is crucial. Magnesium is involved in many physiological and biochemical processes; it is an essential element for plant growth and development and plays a key role in plant defence mechanisms in abiotic stress situations. An early effect of Mg deficiency in plants is the disturbed partitioning of assimilates between roots and shoots because the supply of sink organs with photosynthetic products is impaired, and sugars accumulate in source leaves. Thus, optimal supply of Mg is required to improve crop tolerance to various stresses and to increase yield and quality parameters of harvested products. Unlike other cations, Mg is very mobile in soils because it is less bound to the soil charges. Therefore, Mg losses by leaching might occur in sandy soils with high water conductivity. Leaching of Mg in soils when applied with various water-soluble fertilisers may also vary depending on the fertiliser’s chemical composition, granule size, and effect on soil pH and cation balance, as we discuss in detail.

Magnesium (Mg) plays a vital role in photosynthesis, dry matter production and carbon partitioning in sink organs. Hence, four permanent manurial experiments (20–27 years of duration) under the auspices of All India Coordinated Research Project for Dryland Agriculture (AICRPDA) network centres across diverse agro-ecological regions were carried out to examine the soil exchangeable Mg (ex-Mg), crop uptake and overall Mg balance. Groundnut (peanut), finger millet, rice–lentil sequence and post rainy sorghum were the major crops or cropping systems followed in four permanent manure experiments at Anantapuram, Bengaluru, Varanasi and Solapur, respectively. Nutrient management in all experiments involved control (no addition of nutrients), 100% organic, 100% chemical, and integration of organic and chemical. Except in the finger millet-based system, mean ex-Mg status in the entire profile was higher than the sufficiency level (1.0 cmol( ) kg^–1 as a critical limit). Status of ex-Mg (cmol( ) kg^–1 soil) in soil profiles was in the order: Solapur (3.80) > Varanasi (2.07) > Anantapuram (1.06) > Bengaluru (0.44). A uniform distribution of ex-Mg was observed in plots that received integrated application of organic and chemical fertilisers. In general, improved status of profile ex-Mg (cmol( ) kg^–1) over the control was observed in soils under groundnut (0.19–0.78), finger millet (1.90–3.20), and post rainy sorghum (6.50–7.60, except 4.20 in 100% NPK) cropping. Overall, ex-Mg status and balance of different soil types under diverse crop production systems was influenced by several factors, some of which include soil type with varying mineralogy, particle size distribution, nutrient management strategies and rainfall. Significant positive relationships were observed between ex-Mg status and clay content (R² = 0.94), soil pH (R² = 0.92), cation exchange capacity (R² = 0.98) and mean air temperature (R² = 0.22), whereas a weak relationship was observed with rainfall (R² = 0.01). The study gives an account of Mg balance in major Indian soil types and recommends further attention on Mg nutrition in current intensive agriculture.

Insufficient calcium (Ca) or magnesium (Mg) in the diets of humans and animals has negative effects on health. Knowledge of the concentrations of Ca and Mg in edible crops can help inform the formulation of appropriate diets. There are large differences in shoot concentrations of both Ca ([Ca]_shoot) and Mg ([Mg]_shoot) between angiosperm orders. For example, relative to other angiosperms, commelinid monocot species generally have lower [Ca]_shoot and [Mg]_shoot; species from the Cucurbitales, Malvales and Brassicales generally have higher [Ca]_shoot and [Mg]_shoot; and species from the Oxalidales and Caryophyllales generally have higher [Mg]_shoot but similar [Ca]_shoot, which results in higher [Mg]_shoot/[Ca]_shoot quotients. In this paper the evolution of the combined traits of high [Mg]_shoot and high [Mg]_shoot/[Ca]_shoot quotient in the Caryophyllales was resolved at the family level. All Caryophyllales families had high mean [Mg]_shoot and [Mg]_shoot/[Ca]_shoot quotients, suggesting that both of these traits evolved in an ancient ancestor of all Caryophyllales families.

Plants dynamically cope with the variability of mineral nutrient distribution in soil by constantly modulating nutrient uptake and shaping root-system architecture. The changes in root morphology in response to major essential elements are largely documented, but little is known about how the root system responds to magnesium (Mg) availability. Thirty-six natural accessions of the model species Arabidopsis thaliana were subjected to an in vitro screen for identifying variation in root system architecture in response to Mg availability. Response of root morphology was observed on 2-dimensional agar plates. Low Mg supply repressed the elongation of the lateral roots more than of the primary root. However, some accessions exhibited higher number and length of lateral roots than the reference Columbia-0. Across all accessions, the root morphological traits did not correlate with tissue Mg concentrations. Interestingly, shoot calcium and root phosphorus concentrations were positively correlated with the number and length of lateral roots, whereas root iron concentration was negatively correlated with the primary root length. The diversity of root phenotypes identified in this report is a useful resource to study the genetic component determining root morphology in response to Mg availability.

Based on established dietary reference intakes (DRIs) (e.g. estimated average requirements, recommended dietary allowances (RDAs), and reference nutrient intakes), magnesium (Mg) deficiency in the range 50–99% of the requirement commonly occurs throughout the world. Yet, Mg is not often considered a major nutrient of concern for health and wellbeing, although deficient intakes and serum concentrations have been associated with numerous pathological conditions including atherosclerosis, diabetes, osteoporosis and some cancers. Probable reasons for this dichotomy are that evidence of Mg deficiency is not consistently found in pathological conditions with which it has been associated, and not all individuals considered Mg-deficient consistently exhibit these pathological conditions. These inconsistencies could be the outcome of chronic inflammatory stress exacerbated or induced by Mg deficiency being alleviated or prevented by other factors that have anti-inflammatory action (e.g. long-chain n-3 fatty acids). Questionable DRIs resulting in the incorrect conclusion that individuals are Mg-deficient when they are not also may be responsible for the inconsistencies. Since 1997, improved balance data have been reported for the determination of DRIs, which suggest that the RDA for a 70-kg healthy adult would be ∼250 mg day^–1. Based on the finding that neutral Mg balance was determined to be 2.36 mg day^–1 kg^–1 bodyweight, the RDA would vary by bodyweight. Even with changed DRIs, a significant number of adults who do not eat recommended amounts of foods of plant origin would not achieve the suggested adequate intake of Mg. Foods of plant origin, including green vegetables, nuts, pulses and whole grains, are good sources of Mg. However, Mg in these foods can be influenced by the availability of Mg to plants from the soil, and plant genotype. Thus, crop breeding and cultural practices, through modifying the amount of Mg in plant-origin foods, can have a significant impact on achieving an adequate dietary intake of Mg for health.

Unlike yield, the plant calcium (Ca) : magnesium (Mg) ratio increases at higher soil Ca : Mg and decreases at lower soil Ca : Mg. Edible plant tissue Ca : Mg at various soil ratios has not been robustly studied. Such studies are appropriate because high Ca : Mg dietary ratios may be associated with increased risk of chronic diseases such as cardiovascular disease and diabetes, and human dietary Ca : Mg ratio is rising as populations integrate more processed foods into traditional diets. This review explores whether increasing the soil Ca : Mg ratio is likely to increase edible plant tissue Ca : Mg ratio, a result that could, if substantial, affect human health.

A literature search gathered published articles reporting Ca and Mg values for plants grown in soils or nutrient solutions with various Ca : Mg ratios. For each study, soil or solution ratio was plotted against plant ratio, and Pearson’s r and 2-tailed P values were calculated. Findings reveal that reporting Ca and Mg content of edible plant tissues is rare in studies assessing the impact of soil Ca : Mg on crop yields, nutrient uptake or crop quality; Ca : Mg of whole plants and most shoots increases as soil Ca : Mg rises; leaf Ca : Mg of some but not all crops increases as soil Ca : Mg rises; Ca : Mg ratios of edible grain, fruit and root tissues are smaller than those of leaves or shoots of the same crop; and Ca : Mg of grain, bean and fruit tissue may not respond to changes in soil Ca : Mg as much as Ca : Mg of plants, shoots and leaves. However, the data are too sparse for conclusions or even speculation. Further measurements of Ca and Mg in edible tissues destined for human consumption are necessary to asses any impact of soil Ca : Mg on the rising dietary Ca : Mg of humans and its health consequences.

Magnesium (Mg) is an essential mineral micronutrient in humans. Risks of dietary Mg deficiency are affected by the quantity of Mg ingested and its bioavailability, which is influenced by the consumption of other nutrients and ‘anti-nutrients’. Here, we assess global dietary Mg supplies and risks of dietary deficiency, including the influence of other nutrients. Food supply and food composition data were used to derive the amount of Mg available per capita at national levels. Supplies of Mg were compared with estimated national per capita average requirement ‘cut points’. In 2011, global weighted mean Mg supply was 613 ± 69 mg person^–1 day^–1 compared with a weighted estimated average requirement for Mg of 173 mg person^–1 day^–1. This indicates a low risk of dietary Mg deficiency of 0.26% based on supply. This contrasts with published data from national individual-level dietary surveys, which indicate greater Mg deficiency risks. However, individuals in high-income countries are likely to under-report food consumption, which could lead to overestimation of deficiency risks. Furthermore, estimates of deficiency risk based on supply do not account for potential inhibitors of Mg absorption, including calcium, phytic acid and oxalate, and do not consider household food wastage.

Potassium (K), calcium (Ca) and magnesium (Mg) are three macro-elements essential for plants and animals. The ratios K : Mg or K :  (Ca   Mg) are viewed as indices of physiological status in livestock animals. In plants, Ca, Mg and K concentrations can vary with climate in terrestrial ecosystems. Here, with a widespread tree species (Chinese cork oak, Quercus variabilis Blume) and an acorn predator (the weevil Curculio davidi Fairmaire), we investigate how K, Ca and Mg vary in soils, plant tissues (leaves and acorns) and a consumer (herbivore insects) with climatic variables induced by latitude (LAT) across the temperate–subtropical areas of eastern China. Concentrations of K, Ca and Mg in soils, leaves, acorns and weevil larvae showed different degrees of variation across the study area, but only Mg concentration increased significantly with rising LAT across all four trophic levels, albeit with varying slopes. With rising mean annual temperature (MAT) and precipitation (MAP), soil Ca significantly decreased, as did leaf and acorn K concentrations, whereas all four tropic levels showed significant decreases in Mg content with both MAT and MAP (P < 0.05–0.001). Leaf and acorn Ca : Mg showed significant relationships with LAT and MAT (P < 0.05–0.01). The K : (Mg   Ca) ratio in soils and weevil larvae increased linearly with MAP (P < 0.05), and acorn K : (Mg   Ca) ratio varied in a concave manner (P < 0.001). Our results suggest that variations of Ca, Mg and K in plant tissues and weevil larva across a study area of 20° LAT range were largely driven by climatic factors, and that Mg concentration changes in all four trophic levels with climate (and LAT) largely drive changes in soil, plant and consumer ratios between Mg, Ca and/or K. These results provide information on possible effects of climate change on nutrient dynamics in terrestrial ecosystems.

Magnesium (Mg) is an essential nutrient that can alleviate soilborne toxicity of many ions. This review paper critically assesses the literature on interactions and mechanisms influencing Mg alleviation of aluminium (Al) and heavy metal toxicity. Hydrated radii of Mg² and Al³ are similar; therefore, these two ions compete for binding to ion transporters and other important biological molecules. In monocotyledonous species such as rice and wheat, millimolar concentrations of Mg alleviate Al toxicity, mainly by decreasing Al saturation and activity at cell wall and plasma membrane binding sites. In dicotyledonous legume species such as soybean (Glycine max), rice bean (Vigna umbellata) and broad bean (Vicia faba), micromolar concentrations of Mg may enhance biosynthesis of organic ligands and thus underpin alleviation of Al toxicity. Resistance to Al may be enhanced by increased expression of the genes coding for Mg transporters, as well as by upregulation of activity of Mg-transport proteins; intracellular Mg² activity may thus be increased under Al stress, which may increase the activity of H -ATPases. In Vicia faba, Mg-related enhancement in the activity of plasma membrane H -ATPase under Al stress was found to be due to post-translational modification (increased phosphorylation of the penultimate threonine as well as association with regulatory 14-3-3 proteins), resulting in increased resistance to Al stress. Magnesium can alleviate heavy metal stress by decreasing negative electrical potential and thus metal ion activities at the plasma membrane surface (physico-chemical competition), by enhancing activities of enzymes involved in biosynthesis of organic ligands, and by increasing vacuolar sequestration of heavy metals via increasing H -pumping activity at the tonoplast. Future work should concentrate on characterising the role of intracellular Mg² homeostasis and Mg transporters in alleviating metal stress as well as in transcriptional, translational and post-translational regulation of H -pumps and enzymes involved in biosynthesis and exudation of organic ligands.