Soil Formation:
Topics:
Series Definition:
Similar Soils:
Dominant Soil Types:
Chemical and Physical Features:
Variant:
Distribution of Soil Map Units:
The Te Rapa series and its variants have formed from mainly silty and sandy alluvium on low ridges (levees) or sides of ridges, and on broad flat to gently undulating land. Such material often borders former stream channels or the edges of former lakes now filled with peat. Gravelly and sandy materials in the lower subsoils were alluvially deposited by streams flowing through the area. As stream currents decreased, silty, fine sandy and occasionally lighter pumice materials (which could contain large pieces of pumice) were deposited over the sands and gravels. This resulted in the common silty over sandy profile form. The material deposited around former lake edges is often very pumiceous and may have stone sized pumice (about 10 cm diam.) in the silty matrix of the upper profile.
These alluvial deposits changed the drainage pattern, creating wet conditions suitable for peat formation. This possibly resulted from alluvium blocking former channels and gullies. There is evidence of beach ridges at Ruakura, showing the existence of a lake at a level that was several metres above present day Te Rapa and Motumaoho soils. During this time a thin layer of volcanic ash fell over the area and is preserved in the peat to the present day. Dating of the peat above and below the ash layer, together with information on the ashes sand mineralogy, indicates that the ash is the c. 7000 year old Mamakau Ash.
A further change resulted in a lowering of water levels and the exposure of soils with peaty tops. This may have been caused by a general decline in rainfall which began at or before c. 2000 years ago, as reported by Green and Lowe (1985), or by an alteration of the drainage system.
With improved drainage conditions, rainwater, and possibly seasonal fluctuations in water-table levels, leached humic material from the decaying topsoil peat into the subsoil. Much of this material was-deposited at the contact between peat and minerals material to form a thin, dark brown to reddish brown layer rich in humus-aluminium complexes. Such complexes react with phosphate and these soils have very high phosphate retention characteristics. Some humus complexes also leached into the subsoil, staining it yellowish brown. This leaching was less where the water table or a slowly permeable silt or clayey layer was encountered; here light grey colours characteristic of poorly drained conditions remained.
In Te Rapa pale subsoil variant, poorly drained conditions occur near the surface and only a thin humus-stained yellowish brown upper subsoil exists over a light grey lower subsoil. In Te Rapa brown subsoil variant there is no restriction to downward water movement through the soil, and the ridges on which this variant occurs are well drained. Consequently, humic material leaches well down into the soil and yellowish colours continue into the lower subsoil.
Under some conditions an iron pan can begin to form in the lower subsoil. This usually occurs where there is a sharp boundary between sand and gravel. Puketaha soils have a well developed iron pan and are common on the edges of peats where there is a fluctuating iron-rich water table (see p. 49).
The peaty and humic topsoils of Te Rapa soils are unusual in that they are relatively resistant to decay. This is thought to reflect the accumulation of stable humus-Al compounds during the soil-forming process. It is notable that the peaty tops of Te Kowhai soils have decayed and disappeared after 40 years while those of Te Rapa soils remain. This may be the result of the lower humus-Al contents and the higher natural fertility of Te Kowhai soils which favours microbial activity and therefore rapid decomposition of organic material.
Vertical root channels extending well into the subsoil are a common feature in Te Rapa soils. They are about 2 mm in diameter and are coated with reddish brown material thought to be decayed root material. These channels provide a means of access by grass roots into the subsoil and result from a former vegetation such as Raupo.
The Te Rapa series is composed of the Te Rapa peaty silt loam and the Te Rapa humic silt loam.
Soils of the Te Rapa series are characterised in the field as follows:
1.They are mineral soils with a completely decomposed peaty or humic top less than 40 cm thick.
2.In most cases a dark brown or reddish brown layer occurs immediately below the topsoil.
3.The upper subsoil is yellowish brown, but the colour changes to white or grey when ignited at 4500C, indicating that the colour is mainly organic staining.
4.Subsoils have light grey colours beginning 60 to 100 cm below the surface.
5.The soil is uncemented.
6.The soils are moderately well drained and have moderate upper subsoil permeabilities and moderate to slow lower subsoil permeabilities.
Te Rapa peaty and humic silt loams
Te Rapa peaty silt loam (Plate 3) and Te Rapa humic silt loam have a moderately deep rooting depth and are moderately well drained soils with moderate permeability. They
have a well drained yellowish brown upper subsoil on a poorly drained light grey lower subsoil. The poorly drained lower subsoil is commonly the result of a high regional water table, but it can also be caused by a subsoil layer of slow permeability which restricts downward water movement. Readily available water storage in the root zone is medium. Topsoils are commonly peaty silt loam, over a silt loam upper subsoil and sandy lower subsoil. Phosphate retention in the topsoil and upper subsoil is very high.
The soils have the limitations of acid conditions but have favourable physical properties for plant growth. Te Rapa soils are well suited for a wide range of uses and are of high actual value for food production.
A brief description of a typical Te Rapa peaty silt loam is:
Topsoil:20 cm of black to very dark greyish brown peaty silt loam; moderately weak soil strength in situ; non-sticky; moderately developed medium nut or granular structure breaking to fine nut and crumb.
A 15 cm dark brown to dark reddish brown layer occurs immediately below the topsoil; this is silt loam in texture and has moderately weak soil strength.
Upper subsoil: 35 cm of yellowish brown silt loam; moderately weak soil
strength; moderately developed medium nut structure.
Lower subsoil: 30 cm of light grey sand to sandy loam.
Soils of the Te Rapa series have predominantly peaty silt loam topsoils; humic silt loams are less common. For both soil types, at a depth of 15 to 20 cm, coarse pumice sand can occur as a thin layer or be dispersed through the topsoil. Where the topsoil is thin, ploughing may have destroyed the underlying dark brown to dark reddish brown layer. Subsoil materials vary but are commonly silty over sandy material; less commonly there may be silty or clayey material in some part of the subsoil. The clayey material is often firm with slow permeability. Vertical root channels stained a reddish brown colour are a common feature in many Te Rapa soils. These extend deep into the subsoil. Yellowish brown mottles may also occur in the subsoil.
Soils of the Te Rapa series commonly have a moderately acid topsoil on a strongly acid subsoil. Phosphate retention is very high in the topsoil and upper subsoil, reducing to low with depth. The soils have low reserves of many nutrient elements but, when nutrients are added, the topsoil and upper subsoil have a very high capacity for retaining and supplying the elements for plant use.
The soils have low soil density in the topsoil and upper subsoil, becoming medium with depth. Permeability is moderate and the soils are moderately well drained. Rooting depth is to about 70 cm and there are medium amounts of readily available water stored in the root zone. Topsoil structure is stable and workability is high.
Te Rapa peaty or humic silt loam, pale subsoil variant
The Te Rapa, pale subsoil variant is distinguished from the Te Rapa series by having white or light grey colours due to poor drainage, less than 60 cm from the surface.
Land mapped as dominated by these soils occupies 43 ha or 6.6% of the total area. The Te Rapa, pale subsoil variant often occurs on the lower sides of low ridges and on low, broad, flat to gently undulating areas where the water table is near the surface (Figure 8). The water table may be either the regional water table or a "perched' water table resulting from water being trapped above a slowly permeable subsoil layer. Two dominant soil types have been recognised: the Te Rapa peaty silt loam, pale subsoil variant and the Te Rapa humic silt loam, pale subsoil variant.
Soils of the Te Rapa, pale subsoil variant have a shallow rooting depth and are imperfectly drained. They have a slowly permeable subsoil near the surface or have moderate permeabilities and a high regional water table.
A brief description of a typical Te Rapa pale subsoil variant is:
Topsoil:20 cm of black peaty silt loam or humic silt loam; moderately weak soil strength in situ; non-sticky, moderately developed medium nut or granular structure breaking to fine crumb and nut.
A 15 cm brown to dark yellowish brown layer occurs immediately below the topsoil; this is silt loam in texture and has moderately weak soil strength.
Upper subsoil: 20 cm of light yellowish brown silt loam with moderately weak
soil strength, or light grey silty clay loam with brownish yellow mottles and moderately firm soil strength.
Lower subsoil: Light grey sandy loam with brownish yellow mottles.
The imperfect drainage, shallow rooting depth and acid conditions of these soils are limitations for some uses. However, they have high potential value for food production.
Te Rapa peaty or humic silt loam, brown subsoil variant
Soils of the Te Rapa, brown subsoil variant are distinguished from those of the Te Rapa series by having yellowish brown subsoil colours to one metre depth; also, white or light grey subsoil colours are absent.
Land mapped as being dominated by these soils occupies 51 ha or 7.8% of the total area. This variant often occurs on the tops of low ridges where the lower subsoil material is commonly permeable sand or gravel (Figure 8). Two dominant soil types have been recognised, the Te Rapa peaty silt loam, brown subsoil variant (Plate 4) and the Te Rapa humic silt loam, brown subsoil variant. A Te Rapa shallow brown subsoil variant has also been recognised, where gravels and sand are less than 60 cm from the surface.
Soils of Te Rapa, brown subsoil variant have a moderately deep rooting depth and are well drained with moderate subsoil permeabilities. Readily available water storage in the root zone is medium. Textures are commonly peaty silt loam in the topsoil, with silt loam on sand or gravely sand subsoils. The Te Rapa, shallow brown variant is somewhat excessively drained, has a shallow rooting depth and low readily available water storage in the root zone.
A brief description of a typical Te Rapa, brown subsoil variant is:
Topsoil:20 cm of black peaty silt loam or humic silt loam; moderately weak soil strength in situ; non-sticky; moderately developed medium granular or nut structure.
A 20 cm brown to dark brown layer occurs immediately below the topsoil; this is silt loam in texture and has moderately weak soil strength.
Upper subsoil: 40 cm of dark yellowish brown silt loam, moderately weak soil strength.
Lower subsoil: Yellowish brown sand or gravelly sand.
The favourable physical properties of these soils makes them of high actual value for food production, and they are highly suitable for horticulture, cropping pasture and forestry. However, they have the disadvantages of high phosphate retention and acid conditions, and may require irrigation for some crops.
Land mapped as soils of the Te Rapa series covers 40 ha or 6.1% of the total area. The soils commonly occur on broad flat to gently undulating areas, or as narrow bands, both slightly higher than the lowest surrounding areas. Characteristically they occur adjacent to or in the proximity of peat soils and have formed from alluvial sands, silts and less commonly clays with a thin covering of completely decomposed peat.
Bruntwood soils look similar to Te Rapa soils but lack peaty or humic tops. They can be further distinguished from each other by heating the yellowish subsoil material (450'C) to bum off any organic material. Te Rapa subsoils will turn grey or white but Bruntwood subsoils remain yellowish. Some Motumaoho soils look similar to Te Rapa soils but they have peaty tops more than 40 cm thick.