Which of the following is considered a miscellaneous organic soil additive?

Peat moss is considered a miscellaneous organic soil additive due to its ability to improve soil structure, moisture retention, and nutrient availability in planting mediums. As an organic material, peat moss contributes essential organic matter to the soil, helping to create a more favorable environment for plant roots and beneficial microorganisms. Its lightweight and fibrous nature helps aerate the soil while also retaining moisture, making it an ideal addition for potting mixes and garden beds.

On the other hand, sand, concrete, and gravel are inorganic additives that do not provide the same benefits associated with organic matter. Sand can improve drainage but does not offer nutrients; concrete is typically not used in soil mixes; and gravel is primarily used for drainage purposes. Therefore, among the given options, peat moss stands out as a valuable organic additive.