What factor primarily determines soil permeability?

The primary determinant of soil permeability is the soil's grain size distribution. This characteristic describes the range and proportion of different grain sizes present in the soil, which significantly influences how easily water can flow through it.

Larger particles, such as sand, create larger void spaces and allow water to move through more freely, resulting in higher permeability. Conversely, smaller particles, such as those found in clay, create smaller voids and offer more resistance to water movement, resulting in lower permeability. The arrangement of these particles, including how well they are sorted by size, also plays a crucial role in the ease of water movement.

While other factors like soil moisture content, compaction level, and temperature influence overall soil behavior, grain size distribution is the fundamental property that dictates how effectively water can percolate through the soil structure.