Which of the following is NOT a soil conditioner?

Soil conditioners are substances added to soil to improve its physical properties, enhance its fertility, and support plant growth. Peat, sawdust, and vermiculite are all commonly recognized as soil conditioners.

Peat improves soil structure, aeration, and moisture retention while also enhancing the organic matter content. Sawdust can improve soil structure and benefit soil moisture retention, although it's important to ensure it's well-aged before mixing it with the soil to prevent nitrogen depletion. Vermiculite is excellent for water retention and aeration, making it beneficial in various soil conditions.

Clay, on the other hand, does not function as a soil conditioner in the same way. Instead, it is itself a soil type characterized by small particle size and compactness. While clay can improve soil structure in some contexts when mixed with sand or organic matter, it generally does not enhance soil drainage or aeration effectively, which are primary goals of soil conditioning. Therefore, identifying clay as not being a soil conditioner aligns with the definition and purpose of these substances in soil management.