Which organic material is commonly added to soil to improve structure and fertility? Compost. Compost is decomposed organic matter made from materials such as leaves, grass clippings, vegetable scraps, and small branches. Microorganisms break these materials down into a dark, crumbly substance that gardeners mix into soil or spread as mulch.
Compost improves soil structure by helping sandy soils hold water and allowing heavy clay soils to drain and accept air more easily. It also adds organic matter and releases nutrients gradually, supporting earthworms and beneficial soil organisms. Its exact nutrient content varies according to what went into the pile.
Compost is often confused with fertilizer, but they are not identical. Fertilizer is formulated mainly to supply specific nutrients, while compost primarily improves the soil environment and provides modest, slow-release nutrition. Finished compost should smell earthy and contain no recognizable food scraps; fresh or unfinished compost can temporarily compete with plants for nitrogen.