soil, mineral material at the Earth’s surface formed by weathering of underlying bedrock (see geology), or transported sediments, which form the parent material of a soil. The main distinction between soil and parent material is a soil’s enrichment with plant and animal remains that undergo decomposition to form soil organic matter. Because soil formation is an ongoing process, determined by climate, vegetation, topography, and time, the boundary between a soil and its parent material is usually indistinct. Over very long periods, episodes of erosion and deposition followed by soil formation have led to layered soils, common in old weathered landscapes such as in Australia and southern Africa. However, where soils have formed on one parent material in situ, such as in north-west Europe and North America during the 11,000 years since the last Ice Age, distinctive zones called horizons can form in a vertical array, which is called a soil profile. These horizons can be differentiated by their organic matter, colour, thickness, texture, structure, and stoniness. In addition to organic matter, soils are distinguished from rock materials by their structure (see soil structure), which influences the rate of water infiltration (from rainfall or irrigation), their resistance to soil erosion, and ease of root penetration. The combined effect of soil texture and structure determines a soil’s water-holding capacity, aeration, and drainage, and hence the soil’s suitability for healthy root growth and vine performance.
