Which elements are required in the largest amounts after C, H, and O?

The elements required in the largest amounts after carbon (C), hydrogen (H), and oxygen (O) are nitrogen (N), phosphorus (P), and potassium (K). These three are classified as macronutrients, which are essential for plant growth and development.

Nitrogen is a key component of amino acids, the building blocks of proteins, and is crucial for chlorophyll production, affecting the plant's ability to photosynthesize effectively. Phosphorus is vital for energy transfer through ATP and is essential for nucleic acids, promoting root development and flowering. Potassium plays a critical role in regulating plant metabolism and water use efficiency, aiding in enzyme activation and overall plant robustness.

In contrast, other groups of elements listed do not serve as macronutrients and are required in smaller quantities or are categorized differently. Copper, cobalt, cesium, zinc, chlorine, boron, calcium, magnesium, and manganese are more often classified as micronutrients or have varying functions that do not equate to the significant demand exhibited by nitrogen, phosphorus, and potassium.