What are organic materials placed on top of the soil to reduce weed growth and conserve moisture called?

The correct answer is mulch. Mulch consists of organic materials, such as straw, wood chips, leaves, or grass clippings, that are spread over the surface of the soil. The primary functions of mulch are to suppress weed growth by blocking sunlight, which prevents weed seeds from germinating, and to retain soil moisture by reducing evaporation. This creates a more stable environment for plant roots, promoting healthy growth.

Fertilizer is a substance added to soil or plants to provide essential nutrients but does not have the functions of moisture conservation or weed suppression. Compost is a form of organic matter that improves soil structure and fertility when mixed into the soil, rather than being applied on top for coverage. Soil refers to the upper layer of earth in which plants grow, which does not specifically address the question of surface treatment for moisture and weed management.