The pH of topsoil is, like base saturation, strongly
affected by fertiliser inputs. Subsoil pH values are generally
moderately acid and in many places are extremely acid,
especially in Acid Brown Soils. Where pH values are less
than a threshold of about 4.8, soils are likely to have high
levels of potassium-chloride-extractable labile (mobile) aluminium (KCl–Al). KCl–Al values are usually more than 1.5
cmol
(+) kg
−1 except where clay contents are relatively low.
Brown Soil total P values are frequently very high in
topsoils, again influenced by any fertiliser inputs. In soils
with low fertiliser inputs, Olsen P (a measure of plant -
available P) is likely to be low. Subsoil total P values range
from very low to very high, with only a few sites with low
total P levels indicating high soil weathering. P retention is a
property of the soil mineralogy and less affected by fertility
management. Both topsoil and subsoil P retentions are
generally medium. However, Allophanic Brown Soils have
very high P retentions.
4.4.4 Biological Properties
Brown Soils generally contain active populations of soil
organisms. Except in soils limited by coldness or acidity,
worm casts are common in topsoils, and carbon to nitrogen
ratios are moderate to low. The roots of plants penetrate
deeply. However, the low pH of some subsoils means aluminium becomes soluble and thus there is a risk of aluminium toxicity in many plant species. The rooting depth of
some pasture plants may be limited by aluminium toxicity in
Brown Soils with low subsoil pH.
4.5 Distinguishing Between Brown Soils
and Related Soil Orders
In common with the Raw and Recent Soils, the Brown Soils
are keyed out near the end of the New Zealand Soil Classification key to orders. This means that the soils are relatively
simple and the key for identification filters out soils with
specific notable features, leaving Brown Soils in the remainder. Thus the typifying Brown Soil is distinctive because of the
specific features that it lacks, for example, the absence of gley
profile features, thick allophanic soil material, thick organic
soil material, thick pumice, podzolic features, and so on.
By definition, Brown Soils have a distinct topsoil, and a
weathered B-horizon or cutanic (clay rich) horizon 10 cm
in thickness, specified brown colours, and phosphate retention of 30% or more. They do not have perched-gley features
but may be mottled.
Because they are so extensive, wherever Brown Soils are
recognised they can merge laterally into soils of many other
orders. Brown Soils are more highly leached and weathered
than Semiarid Soils and Pallic Soils, as expressed in lower
natural fertility, lower soil pH, and brownish colours.
Boundaries between Pallic Soils and Brown Soils are
common throughout New Zealand in soils of similar high
water storage capacity. The brunification (thus Brown Soil)
boundary is simply controlled by effective rainfall
with Brown Soils consistently recognised under higher
rainfall.
Brown Soils are ordinarily formed in areas of humid
climate where droughts are rare. It is perhaps surprising then
to find them formed in sites that commonly experience
drought, as on the sandy gravels of the Canterbury Plains, in
association with Pallic Soils. Detailed soil surveys by Ted
Cox and later Trevor Webb have demonstrated the nature of
the Pallic Soil—Brown Soil boundary in the Canterbury
Plains. Areas with varying thicknesses of silty loess overlying sandy gravel demonstrated that Brown Soils were
confined to low water storage capacity sites where loess was
absent or thinner than about 45 cm, and Pallic Soils
(Immature or Argillic soil groups) were confined to high
water storage capacity sites with loess thicker than *45 cm
over gravel. The slower permeability of the loess materials
probably inhibits leaching, and thus supports the development of Pallic, rather than Brown, Soils. Nevertheless, the
mantle of loess does make both soils more versatile than if
formed directly on coarse alluvium.
4.6 Correlation with Other Classification
Systems
The Brown Soil groups mainly correlate with the Inceptisols
of Soil Taxonomy and Cambisols in the World Reference
Base and many were formerly classified as Yellow-brown
earths in New Zealand (Table 4.3).
The World Reference Base correlations are problematic in
that Brown Soils can fit both Cambisols and Umbrisols. The
Umbrisol concept is of soils with weakly developed profiles
with well-structured dark-coloured acidic topsoils. The
emphasis is on the topsoil, and the typifying environment is
cool humid, mountain lands. They are the acidic variant of
Soil Taxonomy’s Mollisols. The Cambisols have moderately
developed profiles and are recognised globally in young
landscapes in both wet humid and dry areas. The cambic
horizon is the equivalent concept to the New
Zealand-defined weathered B-horizon (Bw). The emphasis is
on the subsoil. For New Zealand it is recommended that
4.4 Key Soil Properties
67
affected by fertiliser inputs. Subsoil pH values are generally
moderately acid and in many places are extremely acid,
especially in Acid Brown Soils. Where pH values are less
than a threshold of about 4.8, soils are likely to have high
levels of potassium-chloride-extractable labile (mobile) aluminium (KCl–Al). KCl–Al values are usually more than 1.5
cmol
(+) kg
−1 except where clay contents are relatively low.
Brown Soil total P values are frequently very high in
topsoils, again influenced by any fertiliser inputs. In soils
with low fertiliser inputs, Olsen P (a measure of plant -
available P) is likely to be low. Subsoil total P values range
from very low to very high, with only a few sites with low
total P levels indicating high soil weathering. P retention is a
property of the soil mineralogy and less affected by fertility
management. Both topsoil and subsoil P retentions are
generally medium. However, Allophanic Brown Soils have
very high P retentions.
4.4.4 Biological Properties
Brown Soils generally contain active populations of soil
organisms. Except in soils limited by coldness or acidity,
worm casts are common in topsoils, and carbon to nitrogen
ratios are moderate to low. The roots of plants penetrate
deeply. However, the low pH of some subsoils means aluminium becomes soluble and thus there is a risk of aluminium toxicity in many plant species. The rooting depth of
some pasture plants may be limited by aluminium toxicity in
Brown Soils with low subsoil pH.
4.5 Distinguishing Between Brown Soils
and Related Soil Orders
In common with the Raw and Recent Soils, the Brown Soils
are keyed out near the end of the New Zealand Soil Classification key to orders. This means that the soils are relatively
simple and the key for identification filters out soils with
specific notable features, leaving Brown Soils in the remainder. Thus the typifying Brown Soil is distinctive because of the
specific features that it lacks, for example, the absence of gley
profile features, thick allophanic soil material, thick organic
soil material, thick pumice, podzolic features, and so on.
By definition, Brown Soils have a distinct topsoil, and a
weathered B-horizon or cutanic (clay rich) horizon 10 cm
in thickness, specified brown colours, and phosphate retention of 30% or more. They do not have perched-gley features
but may be mottled.
Because they are so extensive, wherever Brown Soils are
recognised they can merge laterally into soils of many other
orders. Brown Soils are more highly leached and weathered
than Semiarid Soils and Pallic Soils, as expressed in lower
natural fertility, lower soil pH, and brownish colours.
Boundaries between Pallic Soils and Brown Soils are
common throughout New Zealand in soils of similar high
water storage capacity. The brunification (thus Brown Soil)
boundary is simply controlled by effective rainfall
with Brown Soils consistently recognised under higher
rainfall.
Brown Soils are ordinarily formed in areas of humid
climate where droughts are rare. It is perhaps surprising then
to find them formed in sites that commonly experience
drought, as on the sandy gravels of the Canterbury Plains, in
association with Pallic Soils. Detailed soil surveys by Ted
Cox and later Trevor Webb have demonstrated the nature of
the Pallic Soil—Brown Soil boundary in the Canterbury
Plains. Areas with varying thicknesses of silty loess overlying sandy gravel demonstrated that Brown Soils were
confined to low water storage capacity sites where loess was
absent or thinner than about 45 cm, and Pallic Soils
(Immature or Argillic soil groups) were confined to high
water storage capacity sites with loess thicker than *45 cm
over gravel. The slower permeability of the loess materials
probably inhibits leaching, and thus supports the development of Pallic, rather than Brown, Soils. Nevertheless, the
mantle of loess does make both soils more versatile than if
formed directly on coarse alluvium.
4.6 Correlation with Other Classification
Systems
The Brown Soil groups mainly correlate with the Inceptisols
of Soil Taxonomy and Cambisols in the World Reference
Base and many were formerly classified as Yellow-brown
earths in New Zealand (Table 4.3).
The World Reference Base correlations are problematic in
that Brown Soils can fit both Cambisols and Umbrisols. The
Umbrisol concept is of soils with weakly developed profiles
with well-structured dark-coloured acidic topsoils. The
emphasis is on the topsoil, and the typifying environment is
cool humid, mountain lands. They are the acidic variant of
Soil Taxonomy’s Mollisols. The Cambisols have moderately
developed profiles and are recognised globally in young
landscapes in both wet humid and dry areas. The cambic
horizon is the equivalent concept to the New
Zealand-defined weathered B-horizon (Bw). The emphasis is
on the subsoil. For New Zealand it is recommended that
4.4 Key Soil Properties
67