Soils of the Wairau Plain
Soils on the Wairau Plains have developed from a range of parent materials including alluvium and organic deposits on the floodplains, dune sands, beach gravels and estuarine deposits on the coastal margins and older alluvium and loess on river terraces (Laffan and Vincent, 1990).
The soils on the Wairau Plain can be very broadly subdivided on the basis of landform:
- Soils on the unprotected floodplains and river channels
- Soils on the protected and high floodplains
- Soils on the low younger terraces
- Soils on the high older terraces
- Soils on the coastal margins
Descriptions of soils on different land forms
Soils on the unprotected floodplains and river channels
The soils on this land surface lie within the active floodplains of the major rivers (ie; Wairau, Opawa, Omaka, Fairhall and Taylor Rivers). These soils are shallow and stony and comprise raw river gravels and sand or weakly differentiated soils with A/C or AC/C profile forms. Parent material has been deposited in historical times and remains unprotected from frequent flooding.
Soil types:
- Undifferentiated - Shallow (<45 cm) stony soils, well drained developed in stony alluvium
Soils on the protected and high floodplains
The soils on this land surface are developed in alluvium deposited in historical times (< 250 years) but are protected from regular flooding by stopbanks and channel protection works. Soils occur on flat to gently undulating (0-3) slopes. The soils on this land surface are the excessively drained Awatere series which has developed in coarse-grained stony alluvium and has a weakly developed A/C profile form. The Gibson and Gibson (mottle phase) series also have an A/C profile form, but in contrast to the Awatere series are deeper soils developed in finer grained loamy alluvial parent material.
Soil types:
- Awatere soil - Shallow (<45 cm) stony soil, somewhat excessively drained developed in stony alluvium
- Gibson soil - Deep (>75 cm) soil, well drained developed in loamy and sandy alluvium
Soils on the low younger terraces
Four soil series and a phase of a series have been mapped on this land surface. Soils on this land surface include the Rapaura, Wairau, Wairau (mottle phase), Grovetown and Spring Creek soils. Soils have developed on relatively recent alluvium of post-glacial age (300-3500 years). The alluvium is older than that in which the Awatere and Gibson soils have developed and it is located on terrace surfaces about 2-3 m above the floodplain of the Wairau River. Soils occur on flat to gently undulating (0-3) slopes.
The Rapaura soil is a shallow, well-drained soil developed in stony alluvium compared to the Wairau soil which is a deeper soil developed in loamy and sandy alluvium. Both soils have A/BC/C profile forms with better developed A horizons than the Gibson series reflecting their greater age. On the finer-textured alluvial deposits (ie; recent loamy alluvium) on the younger terraces, there are the imperfectly drained Grovetown soil Ah/Bg/Cg and poorly drained Spring Creek soil Ah(g)/Bg/Cg.
Soil types:
- Rapaura soil - Shallow (<45 cm) stony soil, excessively drained, developed from stony alluvium
- Wairau soil - Deep (>75 cm) soil, well drained, developed from loamy and sandy alluvium
- Wairau (mottled phase) soil - Deep (>75 cm) soil, well drained, developed from loamy and sandy alluvium
- Grovetown soil - Deep (>75 cm) soil, imperfectly drained, developed from loamy alluvium
- Spring Creek soil - Deep (>75 cm) soil, poorly drained, developed from loamy alluvium
Soils on the high older terraces
The soils on the high older terraces occur on the higher and older Pleistocene (< 3800 years) terraces west and south of Renwick occurring on undulating (0-3) slopes. Soils on this land surface include the Renwick, Woodbourne, Brancott, Broadbridge, Hawkesbury, Paynter and Burleigh soils and they form a drainage sequence associated with a decrease in particle size of the parent material. Soil types range from the excessively drained Renwick soil developed in stony alluvium to the very poorly drained peat soils of the Burleigh series. All soils show an advanced stage of profile development reflecting the significantly greater age than the soils developed on the younger terraces.
In contrast, the Jordan soil occurs on the gently undulating (0-3) and easy rolling (4-7 ) slopes of the higher terrace remnants. The Jordan soil is a deep, imperfectly drained soil, developed in loess and loess colluvium. The imperfect drainage is a result of a strongly compact fragipan which occurs at around 70 cm in this soil.
Soil types:
- Renwick soils - Shallow (<45 cm) stony soil, excessively drained, developed from stony alluvium
- Woodbourne soil - Deep (>75 cm) soil, well drained, developed in loamy alluvium
- Brancott soil - Deep (>75 cm) soil, moderately well drained, developed in loamy alluvium
- Broadbridge soil - Deep (>75 cm) soil, imperfectly drained, developed in loamy alluvium
- Hawkesbury soil - Moderately deep (50 cm) soil, imperfectly drained, developed in loamy alluvium over stony alluvium
- Paynter soil - Deep (>75 cm) soil, poorly drained, developed in clayey alluvium
- Paynter (peaty phase) - Deep (>75 cm) soil, poorly drained, developed in clayey alluvium
- Burleigh soil - Deep (>75 cm) soil, very poorly drained, peat developed in clayey alluvium
- Jordan soil - Deep (>75 cm) soil, imperfectly drained, developed in loess and loess colluvium
Soils on the coastal margins
These soils occur on sand dunes, beach ridges, alluvium and saline estuarine deposits of Holocene ages (< 2000 years) on the eastern margin of the plain. Soils occur on flat to gently undulating (0-3) and undulating (4-7) slopes. The soils include the Tahunanui, Murrays, Taumutu, Taitapu (equivalent to Spring Creek soil West of SH 1), Waimairi and Motukarara soils. The Tahunanui soil has developed in dune sands, while the Murray soil is found along the margin of the Tahunanui soil where the dune sand has been buried by recent loamy alluvium. East of SH 1 the excessively drained Taumutu soils have developed on gravely beach ridges parallel to the coast. In the swales between the ridges the poorly drained Waimairi peat soils are present. The Motukarara soils are poorly drained soils formed from estuarine sediments adjacent to the Wairau lagoons.
Soil types:
- Tahunanui soil - Deep (>75 cm) soil, somewhat excessively drained, developed in dune sands
- Murrays soil - Deep (>75 cm) soil, moderately drained, developed in dune sands buried by loamy alluvium
- Taumutu soil - Excessively drained, soils developed from gravely beach gravels
- Waimairi soil - Poorly drained peat, overlying beach gravels
- Motukarara soil - Poorly drained in saline loamy alluvium
