The soil is a rich loam with a subsoil of sand and gravel. Deep ploughing and incorporating the calcareous subsoil into the top soil also helps. To the south the topsoil is brown and sandy with a gravel and clay subsoil. The soil is shallow, with a clay subsoil. The entire subsoil, seabed and waters of the Paracels and Spratlys are claimed by Taiwan. Others have shallow rooting zones and poor drainage due to subsoil cementation. Quartz and iron mineralisation can be detected in outcrops and subsoil. A hard subsoil is impervious to water and prevents proper drainage. The basic construction method begins by digging a trench down to undisturbed mineral subsoil. Figure 3-17 shows a soil profile with prismatic structure in the subsoil. Use of property was divided into topsoil ("tianpi") and subsoil ("tiangu") rights. Aggregation is important in the subsoil, the layer below tillage. The road was constructed by excavating a wide, shallow trench below subsoil level. Also required is sufficient age to exhibit subsoil weathering and development. In some areas, the subsoil is karstic.