In plants, deficiency of Zn are usually associated with concentrations of less than . . . . . . . .

In plants, deficiency of Zn is usually associated with concentrations of less than 20 ppm. Zinc is a micronutrient element which means it is required in the least proportions by plants for their proper functioning.
Deficiencies of Zn are observed with concentrations of less than 20 ppm. Its normal range is 25 - 150 ppm in plants. However, toxicities will occur when the Zn leaf concentration exceeds 400 ppm.