there is also increased pressure on water resources with a
need to balance urban water requirements against irrigation
to support food production.
6.7.3 Soil Water Management
Topsoils are usually free draining with moderate permeability. Subsoils can show some evidence of redox reactions
with veins of low-chroma colours, marking localised
reduced iron common in Granular Soils in the Waikato
region. MnO 2 -rich coatings on ped faces are also common
including in soils in the Pukekohe area. The Perch-gley
group of the Granular Soils can, however, become waterlogged and poorly aerated during wet periods of intense
rainfall. When wet, the soil may be susceptible to livestock
treading and cultivation damage but the risk is usually lower
than in Ultic Soils. There is a risk of soil water deficit in
summer, thus, for reliable crop production, irrigation is
desirable.
6.7.4 Soil Erosion
Soil erosion is not generally an issue on Granular Soils under
pasture because the landforms are commonly gently undulating to rolling rather than hilly or steep. However, when
they are ploughed, given the generally rolling topography,
Granular Soils are prone to surface erosion under heavy
rainfall. Les Basher and Craig Ross have carried out
extensive research into the occurrence, and effects, of soil
erosion in the Pukekohe vegetable growing areas. In 2002
they showed, using
137 Cs as a marker (since the
accumulation of
137
Cs from atmospheric nuclear testing,
starting in the 1950s), that soil had been removed from the
upper slopes of many paddocks and redeposited at the lower
slopes (Fig. 6.8). They found that in the paddocks studied
there was an overall loss of soil of 7–30 t ha
−1
yr
−1 , due
mainly to water erosion of ploughed soil. However, within
individual fields the losses could be up to 92 t ha
−1 yr
−1 from
upland areas with deposition of as much as 100 t ha
−1 yr
−1
near the foot of slopes. The redeposition of the soil reflects
the strong aggregate stability with larger particles redeposited when water flow slowed (Fig. 6.6). Of the soil lost
from the fields only about 1% was estimated to be ‘lost’
from the catchment (to the Manukau Harbour). Much of the
rest was redeposited in areas marginal to the paddocks, or in
roadside ditches. At some sites farmers have moved soil
back from the lower slopes up to the higher areas of the
paddocks.
There was a notable storm in 1999 (considered to be a 2%
annual probability rainfall event) which caused severe runoff. Soil and onions were carried as a slurry and memorably
deposited in the streets of Pukekohe. Although the erosion
from ploughed fields was severe in places, areas under
pasture or cover crops were not badly impacted by the 1999
storm.
Much of the erosion takes the form of surface rill or sheet
erosion. A compacted layer forms at the base of the plough
layer which restricts water infiltration, and the increasing
clay content with depth in the subsoils would exacerbate this
restriction. The soil in the plough layer is often cultivated to
a fine tilth with a high infiltration rate. When a high intensity
rainfall event occurs, the surface soil can become saturated,
and then the water, and soil, flow along the top of the plough
layer (Fig. 6.9). Basher and Ross noted that large areas of
Fig. 6.8 Soil redistribution and
loss from a field in Pukekohe
(adapted from work by Craig
Ross and Les Basher)
6.7 Use and Management of Granular Soils
97
need to balance urban water requirements against irrigation
to support food production.
6.7.3 Soil Water Management
Topsoils are usually free draining with moderate permeability. Subsoils can show some evidence of redox reactions
with veins of low-chroma colours, marking localised
reduced iron common in Granular Soils in the Waikato
region. MnO 2 -rich coatings on ped faces are also common
including in soils in the Pukekohe area. The Perch-gley
group of the Granular Soils can, however, become waterlogged and poorly aerated during wet periods of intense
rainfall. When wet, the soil may be susceptible to livestock
treading and cultivation damage but the risk is usually lower
than in Ultic Soils. There is a risk of soil water deficit in
summer, thus, for reliable crop production, irrigation is
desirable.
6.7.4 Soil Erosion
Soil erosion is not generally an issue on Granular Soils under
pasture because the landforms are commonly gently undulating to rolling rather than hilly or steep. However, when
they are ploughed, given the generally rolling topography,
Granular Soils are prone to surface erosion under heavy
rainfall. Les Basher and Craig Ross have carried out
extensive research into the occurrence, and effects, of soil
erosion in the Pukekohe vegetable growing areas. In 2002
they showed, using
137 Cs as a marker (since the
accumulation of
137
Cs from atmospheric nuclear testing,
starting in the 1950s), that soil had been removed from the
upper slopes of many paddocks and redeposited at the lower
slopes (Fig. 6.8). They found that in the paddocks studied
there was an overall loss of soil of 7–30 t ha
−1
yr
−1 , due
mainly to water erosion of ploughed soil. However, within
individual fields the losses could be up to 92 t ha
−1 yr
−1 from
upland areas with deposition of as much as 100 t ha
−1 yr
−1
near the foot of slopes. The redeposition of the soil reflects
the strong aggregate stability with larger particles redeposited when water flow slowed (Fig. 6.6). Of the soil lost
from the fields only about 1% was estimated to be ‘lost’
from the catchment (to the Manukau Harbour). Much of the
rest was redeposited in areas marginal to the paddocks, or in
roadside ditches. At some sites farmers have moved soil
back from the lower slopes up to the higher areas of the
paddocks.
There was a notable storm in 1999 (considered to be a 2%
annual probability rainfall event) which caused severe runoff. Soil and onions were carried as a slurry and memorably
deposited in the streets of Pukekohe. Although the erosion
from ploughed fields was severe in places, areas under
pasture or cover crops were not badly impacted by the 1999
storm.
Much of the erosion takes the form of surface rill or sheet
erosion. A compacted layer forms at the base of the plough
layer which restricts water infiltration, and the increasing
clay content with depth in the subsoils would exacerbate this
restriction. The soil in the plough layer is often cultivated to
a fine tilth with a high infiltration rate. When a high intensity
rainfall event occurs, the surface soil can become saturated,
and then the water, and soil, flow along the top of the plough
layer (Fig. 6.9). Basher and Ross noted that large areas of
Fig. 6.8 Soil redistribution and
loss from a field in Pukekohe
(adapted from work by Craig
Ross and Les Basher)
6.7 Use and Management of Granular Soils
97