Which of the following is NOT a soil component?

The correct choice identifies metals as not being a soil component. Soil is primarily composed of four essential components: mineral material, organic matter, water, and air. The mineral material forms the basic structure of the soil and provides nutrients necessary for plant growth. Organic matter, which includes decomposed plant and animal residues, enriches the soil and supports a healthy ecosystem of microorganisms.

Air is crucial for the respiration of plant roots and soil organisms, while water is vital for the transport of nutrients and maintaining plant health. Although some metals may be present in trace amounts within the soil due to the mineral content, they are not classified as a fundamental component of soil. Instead, soil components refer more directly to the foundational elements that make up the soil and support its function in agriculture and ecology. Hence, metals do not fit into the basic categories that define what constitutes the soil.