Allophanic loam soils are formed from silty or sandy glassy volcanic material, have low bulk densities and contain significant amounts of the phosphate-fixing clay mineral, allophane. The excellent physical properties of these soils makes them extremely versatile.
Allophanic loams in the Hamilton Basin are formed either from alluvium derived from a volcanic source, or from airfall volcanic ash. Both of these materials are rhyolitic in composition.
Those from airfall volcanic ash occur mainly to the south of Hamilton City, on low rolling hills. Those from alluvium are common on the plain and occur at Ruakura.
At Ruakura, Allophanic loam soils occur on broad low ridges or levees bordering former braided stream channels of the ancient Waikato River system.
These soils contain significant amounts of allophane in the topsoil and subsoil.
The presence of allophane is associated with a 'greasy' feel (non- or only slightly sticky), high phosphate retention and yellowish brown subsoil colour.
Light grey or white colours due to poor drainage may occur in the lower subsoil. At Ruakura, Allophanic loam soils belonging to the Horotiu series, Bruntwood series and a Bruntwood pale subsoil variant have been recognised.
These are soils that exhibit the properties of gley soils and also have features resulting from podzolisation.
In this process, humus, iron and aluminium are leached by organic acids from the upper part of the soil and, commonly, are deposited as layers in the upper subsoil.
At Ruakura it is thought that Podzol Soils were formerly saturated, allowing the formation of a peaty top, and that this situation was then followed by a change in the local drainage pattern and a lowering of the water table.
Humic material would then have been able to leach through the soil and begin the process of podzolisation.
Consequently, under present day conditions, it is possible to find the unusual combination of well drained soils with peaty tops. Many of these peaty tops contain organic materials resistant to decay and have remained relatively unchanged for at least 40 years.
At Ruakura, Podzol Soils belonging to the Te Rapa series, along with pale subsoil and brown subsoil variants, have been recognised.
The dominant soil types within the series and variants are peaty silt loams and humic silt loams.
Gley soils are formed when the soil is wet at or near the surface for prolonged periods.
Lack of oxygen, resulting from prolonged water-logging, results in conditions that produce pale soil colours.
Rust-coloured mottles may be present along cracks and in root channels down which oxygen is able to move.
Most plant roots are unable to survive in the zone of soil saturation when oxygen is lacking. The roots can, however, penetrate deeper as the water table lowers in summer or if the soil is artificially drained.
Gley soils form in many different materials and may have organic-rich topsoils. They often occur in poorly drained low-lying areas where soil permeability is slow, or at the base of slopes where drainage water accumulates.
At Ruakura, gley soils belonging to the Te Kowhai, Puketaha and Rotokauri series have been recognised.