Go to NZSoils Home Page

> Topic:- Classifying Soils > Example Soil Groups and Series > Waikato Soils:- Horotiu Series

Waikato Soils:- Horotiu Series


Soil Formation:



Series Definition:

Similar Soils:

Dominant Soil Types:

Chemical and Physical Features:


Distribution of Soil Map Units:

Soils of the Horotiu series have formed from alluvium deposited as low linear ridges  (levees) by the ancient Waikato River system.  Initially, rapid stream currents during  times of flood caused gravels and coarse sand to be deposited along the river banks.  As  stream currents decreased, fine sands, silt and occasionally larger pumice fragments were  deposited over the coarser materials.  This has resulted in the common silty over sandy or  silty over gravelly profile form of these soils and their occurrence on long ridges.  In  some areas the ever-changing stream pattern of the ancient Waikato River system eroded  sections of the ridges so that those parts which remained appear as 'islands' surrounded  by lower ground.

As the ridges occur above the lower, poorly drained areas, and the soil material has rapid  permeability, the soils are well drained.  Under these conditions the volcanic soil  material alters or weathers in a particular way, to form the clay mineral "allophane".   Allophane reacts with phosphate and, consequently, Horotiu soils have very high phosphate  retention values.  This clay mineral also helps to bind soil particles into stable, fine  aggregates giving a friable, low-density, permeable soil material.

The well drained conditions also influence subsoil colour.  Under these conditions, iron  in the soil is oxidised and yellowish brown colours result.  Topsoil colours are dark and,  as in most soils, this is due to the accumulation of decayed organic material.
The Horotiu series is composed of three dominant soil types.  These are the Horotiu silt  loam, Horotiu mottled silt loam and Horotiu shallow silt loam.

Soils of Horotiu series are characterised in the field as follows:

1.They are mineral soils.
2.Subsoils are yellowish brown with no light grey or white colours occurring within  100 cm of the surface.
3.Textures are silt loam or coarser.
4.Soil strength is moderately weak or weaker in situ.
5.The soil is uncemented.
6.The soils are well to somewhat excessively drained with moderate permeabilities.
Horotiu silt loam (Plate 1) and Horotiu mottled silt loam

Both Horotiu silt loam and Horotiu mottled silt loam have a moderately deep rooting depth  and are well drained soils with moderate permeability.  Storage of readily available water  in the root zone is moderately high.  Textures are commonly silt loam on a lower subsoil  of sand or gravelly sand.  Phosphate retention is high.

The soils are extremely versatile, deep and well drained, and suited to a wide range of  uses.  With the addition of fertilizers a wide range of annual and permanent crops are  able to be grown, and the soils are highly suitable for horticulture, cropping, pasture  and forestry.  As these soils dry out in summer, irrigation may be required.  The soils'  excellent physical properties make them of high actual value for food production.

Horotiu mottled silt loam differs from Horotiu silt loam by having rust-coloured mottles  in the subsoil.

A brief description of a typical Horotiu silt loam is:

Topsoil:20 cm of dark brown silt loam; moderately weak soil strength; non-sticky;  moderately developed medium 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 breaking to strongly developed fine  crumb.

Lower subsoil: 20 cm of yellowish brown to brownish yellow sandy loam; very weak soil  strength.
On: Loose yellowish brown sand or gravelly sand.

Horotiu silt loam and Horotiu mottled silt loam occur in close association with each  other.  Horotiu mottled silt loam is common near Bruntwood soils.  Subsoil materials vary  but are silt loam to at least 60 cm over sandy and, less commonly, gravelly material.   There is no obvious pattern to the distribution of the subsoil materials.

Horotiu shallow silt loam

Horotiu shallow silt loam is similar to Horotiu silt loam, but sands or gravel occur  within 60 cm of the surface.  Consequently, the soil has a shallower rooting depth and  stores less water than the Horotiu silt loam.  The Horotiu shallow silt loam commonly  occurs on the edges of terraces of Horotiu silt loam, or as thin narrow ridges.
A significant chemical feature of the Horotiu series is its very high phosphate retention.   This is mainly the result of phosphate reacting with allophane.  With time, this  phosphate is thought to become 'fixed' and unavailable to plants.  Topsoils are moderately  acid, on a near neutral subsoil.  The soils have medium reserves of many nutrients and,  when nutrients are added, they have a medium capacity for retaining and supplying elements  to the plant.

The soils have medium to low soil densities and moderate permeability.  Lower sandy or  gravelly subsoils have rapid permeabilities.  Horotiu silt loams are well drained soils  with a rooting depth of about 80 cm.  Readily available water storage in the root zone is  moderately high.  Topsoil structure is stable and workability is high.  The soil material  is generally non-sticky when wet.

Horotiu shallow silt loam is similar to Horotiu silt loam but is somewhat excessively  drained.  Rooting depth is shallow and readily available water storage in the root zone is  low.  These soils dry out quicker than the Horotiu silt loam but are still of high actual  value for food production.
Land mapped as or dominated by soils of the Horotiu series covers 41 ha or 6.4% of the  total area of Ruakura Agricultural Centre.  The soils commonly occur on sinuous strips 100  to 200 m wide.  Characteristically, they occur on slightly higher land than the  surrounding area, having formed from alluvial silts, sands and gravels deposited as  levees.
Soils of the Bruntwood series are similar to those of the Horotiu series but have a light  grey or white lower subsoil.  Bruntwood soils are moderately well drained and often occur  in slightly lower areas among Horotiu soils, or between Horotiu and Te Kowhai soils.   Where Bruntwood soils are mixed with Horotiu soils, or are so small in area that they  cannot be mapped separately, compound units are used.  Figure 7 shows a diagrammatic  cross-section between Horotiu and Te Kowhai soils.

Soils of the Te Rapa series look similar to those of the Horotiu series but have a humic  or peaty topsoil.  They are chemically quite different and can be distinguished by heating  the yellowish subsoil material (450'C) to bum off the organic material.  Horotiu subsoils  will remain yellowish but the organic stained Te Rapa subsoils will turn grey or white.