Magnesium is essential for healthy plants and is deemed a secondary macronutrient. It is a constituent of chlorophyll so is required for photosynthesis. It is also a component of many plant enzymes and aids in their function. Magnesium helps plants move phosphorus to where it is needed and to use iron. It is important for the uptake of a variety of nutrients and for nitrogen fixation by bacteria associated with with legumes.
The availability of magnesium in the soil is affected by:
- pH - low soil pH reduces the availability of magnesium, high pH increases it
- Manganese - excess manganese decreases magnesium uptake
- Cation exchange capacity - soil that is high in organic matter and clay will maintain higher levels of magnesium (such soil will absorb magnesium easily and will prevent it from leaching) though if the soil contains little magnesium, it will be harder for plants to take it up
- Other cations - excess levels of other cations, potassium in particular, will prevent the uptake of magnesium
- Soil temperature - low soil temperature reduces magnesium uptake
Ideally, for healthy and productive soil you should aim for a magnesium concentration of at least 1.6 meq/100g (milliequivalents - this is a special term used to describe the amount of some elements in soil).
Magnesium deficiency is most likely to occur in plants grown in acid and/or cold soil. The presence of excess levels of some other nutrients will also increase the chance of magnesium deficiency. In costal sites, areas where the water table has been disturbed and properties where bore water is used, high levels of sodium may also cause symptoms of magnesium deficiency
Symptoms of Magnesium Deficiency
The most obvious symptom of magnesium deficiency in plants is interveinal chlorosis in older leaves. Necrotic spots may also develop on older leaves. In some plants, manganese deficiency may result in leaves that curl over (rather than under).
Treating Magnesium Deficiency
If magnesium deficiencies are detected or suspected in one or more plants, the first step to treating the problem is to check that the pH of the soil is appropriate. Magnesium is in a form most easily uptaken by plants when the soil pH is between 7.5 and 9 but provided the pH is between 6.5 and 9.5 most plants should be able to uptake adequate amounts (as long as the soil contains sufficient magnesium). Plants that require a pH outside this range (such as azaleas and blueberries) may require more magnesium the soil to compensate for reduced availability.
If symptoms of deficiency appear early in the season, cloches can be used to raise the soil temperature. If fertilisers high in other
If these measures do not completely resolve the problem and a soil test confirms that the soil is deficiency in magnesium, a magnesium containing fertiliser can be applied. Most limes contain magnesium and are useful if the soil is too acidic though dolomite contains more magnesium than standard lime. Epsom salts are also a cheap and readily available solution that can be used regardless of soil pH. Rock dust contains a range of nutrients including magnesium.
Magnesium toxicity is rare. Deficiencies in other cations, such as potassium and calcium, are likely to arise before magnesium toxicity itself becomes a problem. If excess magnesium is causing other deficiencies, adding some of the appropriate element will usually be an appropriate shot term fix. For a long term fix, you should investigate the cause of the excess magnesium and aim to fix the problem.