What nutrient is primarily responsible for dark-green coloration in plants?

The nutrient primarily responsible for the dark-green coloration in plants is nitrogen. This can be attributed to nitrogen's crucial role in the formation of chlorophyll, the pigment essential for photosynthesis. Chlorophyll not only gives leaves their green color but also enables plants to capture light energy from the sun to convert into chemical energy. Adequate levels of nitrogen result in lush, green foliage, while a nitrogen deficiency can lead to yellowing leaves, often referred to as chlorosis.

In contrast, while phosphorus, potassium, and zinc are essential nutrients for plant health, they do not directly contribute to the green pigmentation associated with chlorophyll. Phosphorus is vital for energy transfer and root development, potassium regulates water use and overall plant function, and zinc plays a role in enzymatic processes and growth regulation. However, these nutrients do not influence the color of the leaves in the same significant way that nitrogen does.