Which of the following is NOT a factor affecting the rate of photosynthesis?

Soil composition is not directly involved in the process of photosynthesis. Photosynthesis primarily occurs in the leaves of plants, where light, temperature, and water play significant roles.

Light is crucial because it provides the energy needed for photosynthesis. Temperature influences the enzymatic reactions involved, affecting how efficiently plants can carry out this process. Water is essential as well; it is a raw material for photosynthesis and is involved in the photolysis process, where water molecules are split to release oxygen.

Soil composition, while vital for providing nutrients and water absorption, does not directly affect the photosynthetic process itself, making it the correct choice in this context.