壤界 (rǎng jiè) literally combines 壤 (rǎng), meaning 'soil' or 'land', and 界 (jiè), meaning 'boundary' or 'limit'. Together, it refers to a territorial boundary—especially one defined by natural land features such as rivers, mountains, or soil types—rather than purely political or administrative lines. It emphasizes the physical, geographical basis of a border, often used in contexts involving ecology, geography, or historical land demarcation.

This term carries a formal, somewhat literary or technical tone and appears frequently in academic writing, environmental reports, or discussions about regional ecosystems and habitat ranges. While less common in everyday conversation than 边界 (biānjiè) or 国界 (guójiè), 壤界 is precise when highlighting how terrain itself shapes jurisdiction or ecological zones—such as where soil composition changes across a region, marking a transition between distinct biomes or animal habitats.