Soil Drainage Parameters
The dominant soil of a land resource inventory area is mapped according to the following key soil drainage parameters relevant to plant growth: potential rooting depth, depth to a slowly permeable horizon, internal soil drainage, and soil permeability.
Potential rooting depth
Potential rooting depth describes the depth (in metres) to a layer that may impede root extension. Such a layer may be defined by penetration resistance, poor aeration or very low available water capacity. Potential rooting depth is important for plant growth and soil workability. The classes are as defined in Webb and Wilson (1995).
Soil Permeability is described as a class. Profile permeability classes are determined in the soil profile below the A horizon by measuring hydraulic conductivity or by the field measurement methods of Griffiths (1985, 1991). Soil permeability, impacts drainage, water logging, effluent absorption potential, leaching, and water loss. Classes are as defined in Webb and Wilson (1995).
Depth to a Slowly Permeable Horizon
Depth to a slowly permeable horizon (meters). This is defined as an horizon in which the permeability is less than 4 mm/hr. If no slowly permeable horizon is observed, the soil is allocated a null value. Permeability is important for ease of drainage, risk of water logging, effluent absorption potential, leaching, and water loss. These classes are as defined in Webb and Wilson (1995) and the measurement methods in Griffiths (1985).
Soil drainage described as a class. Drainage classes are assessed either using criteria of soil depth and chroma, or from reference to diagnostic horizons. Soil drainage is important for the supply of oxygen to the plant root zone, waterlogging, and water drainage. Drainage classes used here are the same as those used in the NZ Soil Classification (Hewitt 1993), and as defined in Milne et al. (1995).