Which of the following is NOT a component of humus?

Humus is a dark, organic material in soil that is formed from the decomposition of plant and animal matter. It plays a crucial role in improving soil structure, fertility, and moisture retention. Components of humus primarily consist of decomposed plant materials and animal materials, which are both rich in nutrients. Nutrient-rich organic matter is also a key aspect of humus, contributing to the overall health of the soil.

Sand and gravel, on the other hand, are inorganic particles and do not form part of humus. They are components of soil texture, affecting drainage and aeration but do not provide the organic quality that characterizes humus. Therefore, identifying sand and gravel as not being part of humus is correct, as they do not contribute to the organic composition essential for enhancing soil fertility and structure.