Soil Formation:
Topics:
Series Definition:
Similar Soils:
Dominant Soil Types:
Chemical and Physical Features:
Variant:
Distribution of Soil Map Units:
Soils of the Bruntwood series and its variant have formed from mainly sandy and silty alluvium on the sides of the low ridges upon which Horotiu soils are found. They occur slightly above the lower, poorly drained areas. In many cases the lower subsoil is at the same level as the poorly drained areas or has a slowly permeable layer which restricts drainage of water through the soil. In either case, a poorly drained subsoil
results. The upper subsoil is well drained and in it allophane has formed; this gives the upper subsoil very high phosphate retention, stable fine aggregates, a friable nonsticky feel and low soil density. Under such conditions iron is in an oxidised form which gives the upper subsoil a yellowish brown colour.
Under conditions of poor drainage in the lower subsoil, sands and silts have altered to form the clay mineral "halloysite". This clay is sticky and has low phosphate retention. The soil material is firmer and the aggregates are coarser when halloysite is present than when allophane is dominant. The aggregates are also less stable. Under such conditions oxygen is lacking and iron is converted (or reduced) to a form which can be leached from the soil; as a result, poorly drained subsoils are white. Yellowish mottles may occur, around old root channels or pores where oxygen is present. Topsoils are dark owing to accumulations of decaying organic material.
The Bruntwood series and a Bruntwood pale subsoil variant have been recognised.
The Bruntwood series is composed of one dominant soil type, the Bruntwood silt loam. Soils of the Bruntwood series are characterised in the field as follows:
1. They are mineral soils.
2. The upper subsoil is yellowish brown.
3. Lower subsoils have light grey colours beginning between 60 and 100 cm from the surface.
4. The soil is uncemented.
5. The soils are moderately well drained with an upper subsoil of moderate permeability on a poorly drained lower subsoil of moderately slow permeability or one of rapid permeability but with a high water table.
Bruntwood silt loam (Plate 2)
Bruntwood silt loam has a moderately deep rooting depth, is moderately well drained and has moderate permeability. It has a well drained, yellowish brown upper subsoil on a light grey, poorly drained, permeable or slowly permeable lower subsoil. Readily available water storage in the root zone is high. Textures are silt loam in the topsoil and upper subsoil, and sand or silty clay loam in the lower subsoil. Phosphate retention in the topsoil and upper subsoil is high, owing to the significant content of allophane.
The soil's favourable physical properties make it wen suited to a wide range of uses, and renders it of high actual value for food production.
A brief description of a typical Bruntwood silt loam is:
Topsoil:20 cm of very dark greyish brown silt loam; moderately weak soil strength; non-sticky; moderately developed fine nut structure breaking to strongly developed fine crumb.
Upper subsoil: 40 cm of yellowish brown silt loam, moderately weak soil strength, moderately developed medium nut structure.
Lower subsoil: 20 cm of light yellowish brown silt loam with yellowish brown mottles.
On: Light grey loose sand or sticky moderately firm silty clay loam.
Bruntwood silt loam is the predominant soil type within the series; silty clay loams and humic silt loams may occur but are rare. Subsoil materials vary but are silty over sandy material or, less commonly, silty material, or the soil has clayey material in some part of the profile. Clayey or silty material in the lower subsoil may be firm with slow permeability, in which case downward water movement is restricted and poor drainage results. In other cases, particularly where there is a sandy subsoil, subsoil permeability is rapid and poor drainage results from high regional water-table levels in winter.
The soil is moderately well drained with white or light grey colours due to poor drainage below 60 cm depth. These pale colours may be in the form of mottles or, more commonly, they may be the main soil colour.
Like those of the Horotiu series, soils of the Bruntwood series have very high phosphate retention. The lower subsoil may, however, have low phosphate retention as the abundance of allophane decreases. Topsoils are moderately acid and subsoils slightly acid. The soils have low reserves of many nutrient elements but, if nutrients are added, the upper half of the soil has a high capacity for retaining and supplying the elements for plant use.
The topsoil and upper subsoil of Bruntwood soils have low soil densities and rapid permeabilities. Lower subsoils may have medium soil densities and slow permeabilities. The soils have poorly drained lower subsoils which can restrict rooting depth but which may be a source of extra water for plant growth in summer. Readily available water storage in the root zone is high. Soil structure is stable and workability high.
Bruntwood silt loam, pale subsoil variant
Bruntwood silt loam, pale subsoil variant differs from Bruntwood silt loam in having white or light grey colours due to poor drainage less than 60 cm from the surface. Land mapped as dominated by this soil occupies 13 ha. This variant often occurs in areas which are only slightly higher than the lowest parts of the land-surface, or where firm, slowly permeable subsoil material is near the surface. The Bruntwood silt loam, pale subsoil variant is an imperfectly drained soil with shallow rooting depth.
A brief description of a typical Bruntwood silt loam, pale subsoil variant is:
Topsoil:20 cm of dark brown silt loam; moderately weak, non-sticky; moderately developed fine nut structure breaking to strongly developed fine crumb.
Upper subsoil: 30 cm of brownish yellow silt loam with yellowish brown mottles; moderately weak in situ; moderately developed medium nut structure breaking to fine nut.
Lower subsoil: Light grey silt loam with yellowish brown mottles; slightly sticky, moderately weak.
On: Moderately weak sand or moderately firm silty clay loam.
Imperfect drainage and shallow rooting depth makes the soil well suited to intensive grassland and mixed cropping but only marginally suited to deeper rooting crops. It has high potential value for food production.
Land mapped as soils of Bruntwood series and variant covers 41 ha or 6.4% of the total area. The soils commonly occur over a narrow band, 100 to 150 m wide. Characteristically, Bruntwood soils occur on gently sloping edges of land that is slightly higher than the surrounding area, having formed from alluvial silts, sands and clays.
Soils of the Bruntwood series have properties intermediate between those of the Horotiu and Te Kowhai series. Horotiu series is similar to the Bruntwood series but lacks the light grey or white colours in the subsoil. Horotiu series is well drained, and soils of this series often occur in slightly higher areas adjacent to Bruntwood soils. In lower areas of the landscape poorly drained Te Kowhai soils occur, in which light grey subsoils begin immediately below the topsoil. Figure 7 shows the position of Bruntwood soils in the landscape and their relationship to Horotiu and Te Kowhai soils.
Soils of the Te Rapa series look similar to those of the Bruntwood series but they have a humic or peaty topsoil. Chemically, they are quite different and the soils can be distinguished from each other by heating the yellowish subsoil material to bum off any organic material. Bruntwood subsoils will remain yellowish but the organic-stained Te Rapa subsoils will turn grey or white.