Magnesium dynamics in the soil
Besides the Mg2+ ions occurring in the soil solution, magnesium is either adsorbed to cation exchangers such as organic matter or clay particles in the exchangeable fraction or it is bound inside the crystals of soil silicates. Only the first two fractions are available to plants.
The strength with which the Mg ions are bound to the exchange surfaces is relatively low because of the large hydrate sphere of the magnesium ion. This results in an increased risk of leaching, especially on soils with a low Cation Exchange Capacity (CEC) coupled with a low pH.
- Plant-available Magnesium derived from the weathering of silicates is made available only very slowly over geological timescales
- Magnesium is present in some soils as magnesites and dolomites. At pH values >6, this magnesium is largely insoluble and therefore unavailable
- Many soil types are inherently low in Magnesium. Light textured and acidic soils are often Mg depleted and the supply is often insufficient for many agricultural and horticultural crops.
- The uptake of Mg by the plant is negatively affected by large K:Mg and large Ca:Mg ratios as well as a low pH. This means that even at high magnesium concentrations in the soil a latent or even severe deficiency of magnesium in plants is possible.
Supply in the soil
The fraction of magnesium easily available from the soil solution is important for the nutrition of plants. Soil analysis can reveal the current supply in the soil and allow the calculation of required fertilisation. Most countries have a scale of Mg index with a recommended value for the different types of crop depending on their Mg requirement. These indices are also greatly affected by the soil.
Magnesium in the plant
Plants take up magnesium from the soil solution as Mg2+ ions. Mg is highly mobile in the plant and is important for the correct functioning of many important metabolic pathways.
Functions of magnesium in the plant
- central atom of the chlorophyll molecule and therefore essential for the light dependant reaction of photosynthesis.
- essential for synthesis, transport and storage of important plant substances such as carbohydrates, proteins and fats.
- magnesium activates more enzymes than any other plant nutrient
- regulates the energy balance of plants, as it is important for facilitating reactions between enzymes and ATP, the energy currency in plants.
- effects RNA synthesis and therefore the translation of genetic information into proteins.
- component of pectin, important for stability of cells and phytin, an energy rich phosphate store hugely important for seed germination
- integrated part of ribosomes and the cell matrix as well as aiding stabilisation of cell membranes.
- is required for cell wall synthesis.
- has hydrating characteristics, and therefore affects water balance and effectiveness of enzymes.
- Magnesium and manganese increase the concentration of valuable components such as citric acid and vitamin C. They increase the nutitional quality of frozen vegetables and the resistance of potatoes against discoloration during processing to mash and potato powder.
Magnesium deficiency symptoms
- Deficiency symptoms first occur on older leaves as chlorotic spots between leaf veins.
- necrosis and red discoloration of stems occur during prolonged periods of deficiency
- the entire plant looks wilted and limp during intense sun radiation, similar to wilting seen as a result of K deficiency. This relates to an imbalance of water in the plant. Single leaves look stiff and brittle.
- Chlorophyll content and the number of chloroplasts in the plant are decreased.