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.
Zinc deficiency occurs on corn in Ontario.
Visual symptoms on the leaves are the best means of determining deficiency but soil tests are also useful. Response to zinc should not be expected unless deficiency symptoms are quite marked. The higher rate should be sufficient for up to 3 years. A wetting agent should be added. Spray to leaf wetness.