What term is used to describe elements required by plants in small amounts such as manganese, boron, copper, and zinc?

The term that describes elements required by plants in small amounts, such as manganese, boron, copper, and zinc, is micronutrients. Micronutrients are essential for plant growth and development, even though they are needed in much smaller quantities compared to macronutrients like nitrogen, phosphorus, and potassium. These micronutrients play critical roles in various physiological functions, including enzyme activity, photosynthesis, and overall plant health.

While the other terms may seem related, they reflect different categories of nutrients. Primary nutrients refer to the major nutrients that plants require in larger quantities. Macronutrients encompass the primary nutrients as well as secondary nutrients that are still needed in larger amounts than micronutrients. Trace elements, while a synonym for micronutrients, are typically used in a different context and do not have as widely accepted a definition as the term micronutrients does in relation to plant nutrition. Thus, micronutrients is the most fitting term for the elements listed in the question.