16.5 Distinguishing Between Ultic Soils
and Related Soil Orders
The Ultic Soils are distinguished by the predominantly high
clay content (except for a relatively minor class of sandy soils
within the order), markedly pale upper subsoils and generally
pale yellowish subsoils, which can also be reddish in colour
in some situations, and evidence of strong leaching and
weathering. The other soil orders with a high clay content are
Granular Soils which, in contrast to most of the Ultic Soils,
are derived mainly from strongly weathered silicic tephra
deposits (especially Hamilton Ash), and Oxidic Soils formed
in dark-coloured mafic-rich volcanic deposits including
basalt. Both Oxidic and Granular Soils have reddish brown or
deep brown colours, in contrast to the dominantly pale, yellowish, upper subsoil colours of Ultic Soils.
16.6 Correlation with Other Classification
Systems
The Ultic Soils generally correlate with the Ultisols of Soil
Taxonomy and the Planosols of the World Reference Base
(Table 16.4). However, Planosols may not correlate well to
most New Zealand Ultic Soils because the New Zealand soils
may not meet strict Planosol requirements for textural change or
pale (gley) colours. In the New Zealand genetic classification,
most Ultic Soils were classified as northern yellow-brown
earths, although some were considered gley soils.
Fig. 16.10 Median, and upper and lower quartiles, of soil pH, organic carbon (%), and P retention (%) for Ultic Soils in the New Zealand Soil
Data Repository
Table 16.3 Example of chemical properties of a Mottled Albic Ultic Soil (Mapua series, SB09292)
Horizon
Depth (cm)
pH (in H 2 O)
Carbon (%)
Nitrogen (%)
CEC
a (cmol
(+) /kg)
Sum bases (cmol
(+) kg)
P retention (%)
Ah
0–14
5.5
3.8
0.25
13.4
7.51
29
Bw(f)
14–20
5.2
1.3
0.08
8.4
2.57
28
Bt(f)1
20–37
5.1
0.8
0.05
9
2.39
28
Bt(f)2
37–65
5
0.6
0.04
15.2
3.24
42
Bt(g)
65–104
5
0.5
0.04
18.1
3.24
48
C(g)1
104–120
5.1
0.4
0.01
14.3
2.37
42
C(g)2
120–180
4.9
0.2
0.01
13.7
2.07
44
a CEC cation exchange capacity
16.5 Distinguishing Between Ultic Soils and Related Soil Orders
261
and Related Soil Orders
The Ultic Soils are distinguished by the predominantly high
clay content (except for a relatively minor class of sandy soils
within the order), markedly pale upper subsoils and generally
pale yellowish subsoils, which can also be reddish in colour
in some situations, and evidence of strong leaching and
weathering. The other soil orders with a high clay content are
Granular Soils which, in contrast to most of the Ultic Soils,
are derived mainly from strongly weathered silicic tephra
deposits (especially Hamilton Ash), and Oxidic Soils formed
in dark-coloured mafic-rich volcanic deposits including
basalt. Both Oxidic and Granular Soils have reddish brown or
deep brown colours, in contrast to the dominantly pale, yellowish, upper subsoil colours of Ultic Soils.
16.6 Correlation with Other Classification
Systems
The Ultic Soils generally correlate with the Ultisols of Soil
Taxonomy and the Planosols of the World Reference Base
(Table 16.4). However, Planosols may not correlate well to
most New Zealand Ultic Soils because the New Zealand soils
may not meet strict Planosol requirements for textural change or
pale (gley) colours. In the New Zealand genetic classification,
most Ultic Soils were classified as northern yellow-brown
earths, although some were considered gley soils.
Fig. 16.10 Median, and upper and lower quartiles, of soil pH, organic carbon (%), and P retention (%) for Ultic Soils in the New Zealand Soil
Data Repository
Table 16.3 Example of chemical properties of a Mottled Albic Ultic Soil (Mapua series, SB09292)
Horizon
Depth (cm)
pH (in H 2 O)
Carbon (%)
Nitrogen (%)
CEC
a (cmol
(+) /kg)
Sum bases (cmol
(+) kg)
P retention (%)
Ah
0–14
5.5
3.8
0.25
13.4
7.51
29
Bw(f)
14–20
5.2
1.3
0.08
8.4
2.57
28
Bt(f)1
20–37
5.1
0.8
0.05
9
2.39
28
Bt(f)2
37–65
5
0.6
0.04
15.2
3.24
42
Bt(g)
65–104
5
0.5
0.04
18.1
3.24
48
C(g)1
104–120
5.1
0.4
0.01
14.3
2.37
42
C(g)2
120–180
4.9
0.2
0.01
13.7
2.07
44
a CEC cation exchange capacity
16.5 Distinguishing Between Ultic Soils and Related Soil Orders
261