Which one of the following is not an essential micronutrient for plants?

Micronutrients are essential elements that are used by plants in small quantities. For most micronutrients, crop uptake is less than one pound per acre. In spite of this low requirement, critical plant functions are limited if micronutrients are unavailable, resulting in plant abnormalities, reduced growth and lower yield. In such cases, expensive, high requirement crop inputs such as nitrogen and water may be wasted. Because of higher yields, higher commodity prices and higher costs of crop inputs, growers are reviewing all potential barriers to top grain production, including micronutrient deficiencies. Boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn) are the essential micronutrients.