horizons may be very dense, forming densipans (Ed horizons). The E or pale B horizons have paler colours than the
lower subsoils, which are enriched with translocated clay,
usually with clay coatings (argillans) and possibly humus
coatings, forming Bt (argillic) horizons that also contain iron
and aluminium ions leached from the upper profile.
The E horizons are usually much less prominent, or
absent, in the Yellow Ultic or Sandy Ultic groups that have
pale B horizons instead. The Bt horizons tend to have pale,
often yellowish, colours. In some soils, rust-coloured mottles
and other redox features such as Fe–Mn concretions point to
restricted drainage and periodic soil saturation, and perched
gleying. However, in the Auckland and Northland regions,
some lower subsoils may be distinctly reddened as a result of
differential weathering of a volcanogenic (Fe-rich) parent
material rather than redox processes, and such
‘red-weathered’ horizons are therefore denoted by the suffix
‘v’, forming Btv horizons. The subsoil dry bulk density is
usually high, forming a root restriction for many plants
(Fig. 16.1). The vertical fissures that define the shapes of
large polyhedral or blocky and prismatic peds in B horizons
are usually easily observed in the profile. The fissures act as
channels for drainage and translocation of clay and/or humus, and provide a habitat for soil fauna as well as playing a
role in mass movement (especially mudsliding—Sect. 16.7).
In the lowermost subsoil, yellow brown colours are
common. With increasing depth, bulk density increases,
fissures are less prominent, and the soil mass is more tightly
packed than above. Thus, porosity diminishes, potential
rooting volume decreases, and permeability becomes slower
with increasing depth.
Ultic Soils are strongly leached with limited natural fertility, low calcium, magnesium, potassium, and sodium
cation contents, and acidic pH values that are <5.5 (some
pHs are sufficiently low for Al-sensitivity or toxicity to
occur). The challenges of the Auckland gardener are shared
by the farmer because the soils become hard, as they dry, in
summer and sodden in winter or spring. It can be a challenge
to establish plants, provide drainage, and maintain soil fertility in the root zone.
16.1.2 Areas of Occurrence
Ultic Soils are most common in the northern North Island
broadly from Hamilton northwards (roughly coincident with
Fig. 16.1 Ultic Soils and landscape. Left: hilly landscape, dominated
by Ultic Soils on stable ridgetops and backslopes, between Warkworth
and Orewa in Northland. Right: strongly weathered Mottled Yellow
Ultic Soil (reproduced with permission of NZ Society of Soil Science)
250
16 Ultic Soils
lower subsoils, which are enriched with translocated clay,
usually with clay coatings (argillans) and possibly humus
coatings, forming Bt (argillic) horizons that also contain iron
and aluminium ions leached from the upper profile.
The E horizons are usually much less prominent, or
absent, in the Yellow Ultic or Sandy Ultic groups that have
pale B horizons instead. The Bt horizons tend to have pale,
often yellowish, colours. In some soils, rust-coloured mottles
and other redox features such as Fe–Mn concretions point to
restricted drainage and periodic soil saturation, and perched
gleying. However, in the Auckland and Northland regions,
some lower subsoils may be distinctly reddened as a result of
differential weathering of a volcanogenic (Fe-rich) parent
material rather than redox processes, and such
‘red-weathered’ horizons are therefore denoted by the suffix
‘v’, forming Btv horizons. The subsoil dry bulk density is
usually high, forming a root restriction for many plants
(Fig. 16.1). The vertical fissures that define the shapes of
large polyhedral or blocky and prismatic peds in B horizons
are usually easily observed in the profile. The fissures act as
channels for drainage and translocation of clay and/or humus, and provide a habitat for soil fauna as well as playing a
role in mass movement (especially mudsliding—Sect. 16.7).
In the lowermost subsoil, yellow brown colours are
common. With increasing depth, bulk density increases,
fissures are less prominent, and the soil mass is more tightly
packed than above. Thus, porosity diminishes, potential
rooting volume decreases, and permeability becomes slower
with increasing depth.
Ultic Soils are strongly leached with limited natural fertility, low calcium, magnesium, potassium, and sodium
cation contents, and acidic pH values that are <5.5 (some
pHs are sufficiently low for Al-sensitivity or toxicity to
occur). The challenges of the Auckland gardener are shared
by the farmer because the soils become hard, as they dry, in
summer and sodden in winter or spring. It can be a challenge
to establish plants, provide drainage, and maintain soil fertility in the root zone.
16.1.2 Areas of Occurrence
Ultic Soils are most common in the northern North Island
broadly from Hamilton northwards (roughly coincident with
Fig. 16.1 Ultic Soils and landscape. Left: hilly landscape, dominated
by Ultic Soils on stable ridgetops and backslopes, between Warkworth
and Orewa in Northland. Right: strongly weathered Mottled Yellow
Ultic Soil (reproduced with permission of NZ Society of Soil Science)
250
16 Ultic Soils