根际，也称根际层，是土壤中一个狭窄区域，这一区域主要受到植物根系分泌物以及有关的土壤微生物活动的的直接影响，这一区域之外的土壤通常被称为非根际土壤或散土。一般意义上的根际是指环绕植物根表面直径为40mm的区域，这一区域中生活有许多微生物，他们以植物的根际沉降物质和根部所释放的蛋白质、糖类为食，以细菌为食的原生动物和线虫动物在这一区域的数量也比其它区域多。

根际的物理、化学和生物性质都与周边土壤有所不同，表现最明显的是这一区域的pH值、氧气含量和养分含量等。可以说，植物与土壤的能量和物质交换主要集中于此区域，植物对疾病的对抗也多发生于与土壤接触的区域。

The rhizosphere is the narrow region of soil that is directly influenced by root secretions and associated soil microorganisms. Soil which is not part of the rhizosphere is known as bulk soil. The rhizosphere contains many bacteria that feed on sloughed-off plant cells, termed rhizodeposition, and the proteins and sugars released by roots. Protozoa and nematodes that graze on bacteria are also more abundant in the rhizosphere. Thus, much of the nutrient cycling and disease suppression needed by plants occurs immediately adjacent to roots.