A soil sample should be taken from the same area and at the same time as a plant sample. For more information see Plant Analysis section in Soils . The symptoms appear first as yellow striping of the lower leaves. As the deficiency worsens, the upper leaves may become striped while the lower leaves turn reddish-purple. This test is a reliable guide for determining magnesium requirements.
Dolomitic lime is an excellent source of magnesium.
Where limestone is required to correct soil acidity, dolomitic lime should be used whenever the magnesium test is less than 100. Few soils that do not need lime will require magnesium. Magnesium application is recommended only if the magnesium test is under 20. On these soils magnesium can be supplied either by magnesium sulphate or, if potassium is also required, by sulfate of potash magnesia. Over-application of potassium can induce magnesium deficiency.