What form of magnesium do plants primarily take up from the soil?

Plants primarily take up magnesium from the soil in the form of the magnesium ion (Mg²⁺). This cation is dissolved in soil water and readily available for plant roots to absorb. Magnesium is an essential macronutrient that plays a crucial role in various physiological processes, including photosynthesis, as it is a central component of chlorophyll. It also contributes to enzyme activation and serves as an important element in plant cell structure.

The other choices represent different nutrients or elements that do not fulfill the role of magnesium in plant uptake. Calcium ions are important for cell wall structure and signaling but do not provide magnesium's specific functions. Copper ions, while also essential, are needed in trace amounts primarily for enzyme function and do not replace magnesium's role. Nitrate ions are a form of nitrogen and are essential for protein synthesis but are not related to magnesium uptake. This reinforces the importance of understanding specific nutrient roles in plant health and development.