Where soil levels of magnesium are marginal, apply only enough potassium to meet current crop requirements as indicated by a soil test. Few soils that do not need lime will require magnesium. Magnesium application is recommended only if the magnesium soil test is less than 20. On these soils magnesium can be supplied either by magnesium sulphate or, if potassium is also required, by sulphate of potash magnesia. It usually appears as broad, light-colored strips on either side of the midrib of the younger leaves.
Zinc deficiency is common in spots where erosion or deep plowing has brought subsoil to the surface.
Symptoms are most pronounced under cool conditions because release of zinc from the soil is slowed. If the deficiency is slight, corn often recovers without adverse effects once the soil warms up. In more severe eases maturity is delayed and yields may be reduced. Zinc deficiency Zinc deficiency can also be induced by over-application of phosphorus. This effect is especially noticeable with phosphorus applied in a band at planting.