6
Granular Soils
Near Pukekohe on Granular Soil
the market gardens feed us all
‘tis not the onions that make one cry
but the expanding city drawing nigh.
Abstract
Granular Soils form on old (c. 45,000 years), strongly
weathered, tephra mainly on rolling hills and terraces in
the northern and western Waikato and South Auckland
areas, and on mafic volcanic rocks in Northland. Granular
Soils develop on the older tephras where young tephra
layers are thin or absent. The soils are dark reddish
brown, moderately acidic, sticky, and clay rich. The clays
dry to form strongly developed aggregates that give the
soils their distinctive granular structure, and their name.
Granular Soils comprise just 1% of New Zealand soils
and are important for vegetable production. ‘Pukekohe
onions’ are a well-known product of these soils along
with potatoes, salad greens, and brassicas. Granular Soils
around Pukekohe provide about one-third of New
Zealand’s fresh vegetable production. Some Granular
Soils have been used continuously for horticulture for
over 70 years. Granular Soils are extraordinarily resilient
to frequent ploughing, although erosion needs to be
managed. They can have reduced workability after heavy
rain and are naturally low in nutrients. The Granular Soils
of Pukekohe and Bombay benefit from a frost-free
climate, allowing vegetable production in late winter.
There is concern that the productive Granular Soils in the
Pukekohe/Bombay area are being lost to urban
development.
6.1 Important Features of Granular Soils
6.1.1 Concept and Key Features of the Soil
Order
Granular Soils are clayey, strongly structured, soils formed
mainly from highly weathered rhyolitic and andesitic
tephras, chiefly the Hamilton Ash beds, which are considerably older than c. 45,000 years. Some Granular Soils are
formed on strongly weathered basaltic to andesitic lava flows
where the Hamilton Ash beds do not occur, either where
tephra was not deposited or has been eroded off. The soils
have high clay contents and when wetted, and rubbed
between the fingers, the soils become sticky and plastic and
may be easily remoulded with little cracking. Granular Soils
are typically strongly structured (as the name implies)
because the iron oxides and aluminium hydroxides are just
sufficient to modify the stickiness and improve the soil
fabric, so that dry or moist materials may be easily parted
into small aggregates.
The subsoils have dark, or bright, reddish brown to brown
colours (Fig. 6.1). At some sites a number of altered tephra
beds of the Hamilton Ash sequence, buried clay-rich soil
horizons (paleosols), are discernible (Sect. 6.2). Clay also
accumulates in the subsoil by moving down through the
profile in suspension (a process known as clay illuviation or
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_6
87
Granular Soils
Near Pukekohe on Granular Soil
the market gardens feed us all
‘tis not the onions that make one cry
but the expanding city drawing nigh.
Abstract
Granular Soils form on old (c. 45,000 years), strongly
weathered, tephra mainly on rolling hills and terraces in
the northern and western Waikato and South Auckland
areas, and on mafic volcanic rocks in Northland. Granular
Soils develop on the older tephras where young tephra
layers are thin or absent. The soils are dark reddish
brown, moderately acidic, sticky, and clay rich. The clays
dry to form strongly developed aggregates that give the
soils their distinctive granular structure, and their name.
Granular Soils comprise just 1% of New Zealand soils
and are important for vegetable production. ‘Pukekohe
onions’ are a well-known product of these soils along
with potatoes, salad greens, and brassicas. Granular Soils
around Pukekohe provide about one-third of New
Zealand’s fresh vegetable production. Some Granular
Soils have been used continuously for horticulture for
over 70 years. Granular Soils are extraordinarily resilient
to frequent ploughing, although erosion needs to be
managed. They can have reduced workability after heavy
rain and are naturally low in nutrients. The Granular Soils
of Pukekohe and Bombay benefit from a frost-free
climate, allowing vegetable production in late winter.
There is concern that the productive Granular Soils in the
Pukekohe/Bombay area are being lost to urban
development.
6.1 Important Features of Granular Soils
6.1.1 Concept and Key Features of the Soil
Order
Granular Soils are clayey, strongly structured, soils formed
mainly from highly weathered rhyolitic and andesitic
tephras, chiefly the Hamilton Ash beds, which are considerably older than c. 45,000 years. Some Granular Soils are
formed on strongly weathered basaltic to andesitic lava flows
where the Hamilton Ash beds do not occur, either where
tephra was not deposited or has been eroded off. The soils
have high clay contents and when wetted, and rubbed
between the fingers, the soils become sticky and plastic and
may be easily remoulded with little cracking. Granular Soils
are typically strongly structured (as the name implies)
because the iron oxides and aluminium hydroxides are just
sufficient to modify the stickiness and improve the soil
fabric, so that dry or moist materials may be easily parted
into small aggregates.
The subsoils have dark, or bright, reddish brown to brown
colours (Fig. 6.1). At some sites a number of altered tephra
beds of the Hamilton Ash sequence, buried clay-rich soil
horizons (paleosols), are discernible (Sect. 6.2). Clay also
accumulates in the subsoil by moving down through the
profile in suspension (a process known as clay illuviation or
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
A. E. Hewitt et al., The Soils of Aotearoa New Zealand, World Soils Book Series,
https://doi.org/10.1007/978-3-030-64763-6_6
87