After nitrogen, phosphorous and potassium, magnesium is one of the most important plant nutrients. Magnesium is literally of central importance, in that it is essential for the development of leaf green (chlorophyll). Without this, the plant could not carry out photosynthesis.
Magnesium occurs naturally in garden soil. On light, sandy and acidic soils less than on medium-weight and heavy soils. On such light and acidic soils, magnesium is therefore often in short supply. In addition, approx. 4 to 8 grammes of magnesium per square metre of garden soil per year is washed out by precipitation and garden irrigation and the plants extract magnesium from the soil by absorbing nutrients.
When growing and during dry periods, the requirement of the plants for magnesium is at its highest. In addition, potassium is an opponent of magnesium in the soil: excess potassium in the soil and heavy potassium fertilisation can both reduce the presence of magnesium in the garden soil. If a plant lacks magnesium, this is shown by lightening of all leaves or needles: the green leaf veins then stand out from the yellow spaces between the leaves.
A quick remedy is then provided by fertilising the leaves with magnesium sulphate, also known as Epsom salt. This Epsom salt is dissolved in water according to the manufacturer’s specifications and sprayed over the plant suffering from the deficiency, for example a spruce, with a garden syringe. Repeat this procedure at intervals of two to three weeks. In addition, stabilisation of the acidic value (pH value) of the soil to 5.5 through the addition of lime, if necessary, is recommended. Then recovery fertilisation with magnesium can be carried out, most easily by adding Epsom salt (100 to 200 grammes/square metre).
To prevent magnesium deficiency, annual maintenance fertilisation of approx. 50 to 100 grammes of Epsom salt per square metre is recommended – on lighter soils normally add the higher quantity and on heavier soils the lower quantity. If the soil is well adjusted, it is also often sufficient to just supply it with lime, NPK or compound fertiliser containing magnesium.
Tip: Detailed information on the fertiliser requirements of the garden soil for magnesium and other plant nutrients can be obtained from the results of a soil analysis, as provided by gardeners and garden centres.