Which of the following is a commonly used soil conditioner?

Peat is commonly used as a soil conditioner due to its unique properties that enhance soil structure and fertility. As an organic material, it has the ability to improve soil aeration, water retention, and nutrient availability. Peat helps to balance soil pH and adds organic matter as it decomposes, promoting microbial activity and benefiting plant growth.

In contrast, while lime can adjust soil pH and provide calcium, it is not primarily considered a soil conditioner in the same context as peat. Gravel serves as a drainage material and has limited benefits for conditioning soil, while sand is often utilized to improve drainage and aeration but does not contribute organic matter or significant amendments to soil fertility. Therefore, peat stands out as the most effective choice for enhancing soil quality in various growing conditions.