these soils have effectively lost the full depth of their topsoil.
The extent of this loss is visually obvious quite commonly
because the cultivated fields can look brown or reddish
brown because upper subsoils (Bt horizons) rather than
darker A horizons (topsoils) are exposed (Fig. 6.1). However, in effect a new topsoil is established, albeit with a
reduced soil carbon content and without the darkish colours.
The use of fertiliser, and the largely favourable friability and
porosity of the upper subsoils, appears to mask any discernible impact on productivity.
Compacted wheel tracks, especially when they run up and
down the slope, rather than along contours, are also sources
of runoff and erosion during intense rainstorms. Ripping
with a tined implement (or cultivation) can break up the
compacted soil and increase infiltration and has been shown
to reduce erosion by 95% at some sites. Cover crops, on
wheel tracks, and during periods of fallow, can help reduce
erosion, both through direct protection of the soil from
erosion and by helping build soil organic matter and
aggregate stability.
Grass filter strips on lower edges of paddocks can help
trap eroded soil, and thus prevent it from being lost to the
wider environment. Sediment retention ponds are widely
used to prevent sediment from escaping from earthwork sites
and there could be potential to apply them also to cropping
sites. One study by the Franklin Sustainability Project estimated that when an appropriately sized sediment retention
pond was used, the annual soil loss to downstream environments following high intensity rainfall events was
reduced by two thirds.
6.7.5 Sustainable Soil and Environmental
Management
Maintaining soil structure, preventing compaction and erosion, and maintaining soil fertility are all important aspects
of sustainable management of the Granular Soils under
intensive horticulture. Although Granular Soils are exceptionally resilient as already noted, frequent ploughing leads
to reduced soil organic matter (Fig. 6.10) and degraded soil
structure. Incorporating compost, green manure crops, and
other organic materials, or including periods of pasture
within the crop rotation, could help ameliorate the loss of
soil carbon.
When wet, the dominant sticky clays can also reduce
workability. Under intensive market gardening there has been
a tendency for high fertiliser use leading to some soils having
excessively high Olsen P values. Thus soil tests should be
undertaken before fertiliser addition. Cadmium accumulation
is also high at many sites as a result of past high phosphorus
fertiliser use. Cadmium occurs naturally in some of the rock
deposits from which phosphate fertilisers are made. Cadmium is strongly adsorbed in the soil, hence amounts can
increase gradually as more phosphate fertiliser is applied. At
sites with high cadmium there is now a requirement to use
only fertilisers that are low in cadmium to prevent further
accumulation in the soil. Most recently, accumulations of
fluorine, derived from long-term applications of phosphate
fertilisers to farmlands (including on Granular Soils) in the
Waikato region have essentially doubled the average total F
concentrations in surface soils (from *220 to 440 mg/kg).
Fig. 6.9 Granular Soils under horticulture near Pukekohe. Left: rill erosion following a rainstorm. Right: soil profile showing compaction at base
of plough layer. Water saturates the soil above the compacted layer and then it moves downslope. Photos: Craig Ross
98
6 Granular Soils
The extent of this loss is visually obvious quite commonly
because the cultivated fields can look brown or reddish
brown because upper subsoils (Bt horizons) rather than
darker A horizons (topsoils) are exposed (Fig. 6.1). However, in effect a new topsoil is established, albeit with a
reduced soil carbon content and without the darkish colours.
The use of fertiliser, and the largely favourable friability and
porosity of the upper subsoils, appears to mask any discernible impact on productivity.
Compacted wheel tracks, especially when they run up and
down the slope, rather than along contours, are also sources
of runoff and erosion during intense rainstorms. Ripping
with a tined implement (or cultivation) can break up the
compacted soil and increase infiltration and has been shown
to reduce erosion by 95% at some sites. Cover crops, on
wheel tracks, and during periods of fallow, can help reduce
erosion, both through direct protection of the soil from
erosion and by helping build soil organic matter and
aggregate stability.
Grass filter strips on lower edges of paddocks can help
trap eroded soil, and thus prevent it from being lost to the
wider environment. Sediment retention ponds are widely
used to prevent sediment from escaping from earthwork sites
and there could be potential to apply them also to cropping
sites. One study by the Franklin Sustainability Project estimated that when an appropriately sized sediment retention
pond was used, the annual soil loss to downstream environments following high intensity rainfall events was
reduced by two thirds.
6.7.5 Sustainable Soil and Environmental
Management
Maintaining soil structure, preventing compaction and erosion, and maintaining soil fertility are all important aspects
of sustainable management of the Granular Soils under
intensive horticulture. Although Granular Soils are exceptionally resilient as already noted, frequent ploughing leads
to reduced soil organic matter (Fig. 6.10) and degraded soil
structure. Incorporating compost, green manure crops, and
other organic materials, or including periods of pasture
within the crop rotation, could help ameliorate the loss of
soil carbon.
When wet, the dominant sticky clays can also reduce
workability. Under intensive market gardening there has been
a tendency for high fertiliser use leading to some soils having
excessively high Olsen P values. Thus soil tests should be
undertaken before fertiliser addition. Cadmium accumulation
is also high at many sites as a result of past high phosphorus
fertiliser use. Cadmium occurs naturally in some of the rock
deposits from which phosphate fertilisers are made. Cadmium is strongly adsorbed in the soil, hence amounts can
increase gradually as more phosphate fertiliser is applied. At
sites with high cadmium there is now a requirement to use
only fertilisers that are low in cadmium to prevent further
accumulation in the soil. Most recently, accumulations of
fluorine, derived from long-term applications of phosphate
fertilisers to farmlands (including on Granular Soils) in the
Waikato region have essentially doubled the average total F
concentrations in surface soils (from *220 to 440 mg/kg).
Fig. 6.9 Granular Soils under horticulture near Pukekohe. Left: rill erosion following a rainstorm. Right: soil profile showing compaction at base
of plough layer. Water saturates the soil above the compacted layer and then it moves downslope. Photos: Craig Ross
98
6 Granular Soils