weathering, limited by lack of water and cool average temperatures, clay contents are usually relatively low. However,
the older soils with the well-developed relict argillic horizons can contain nearly 40% clay (Table 15.1). Soil dry bulk
densities are high to very high and total available
water-holding capacities range from low to very high,
depending on the gravel contents of the soils and the amount
of finer interstitial fill.
Soil structure is frequently weakly developed because
there is relatively little organic matter or clay to bind soil
aggregates. The sodic forms of the soils with
sodium-enriched argillic horizons can have well developed
prismatic to columnar structures (Fig. 15.5). Permeability
(saturated hydraulic conductivity) is commonly slow especially in soils with argillic horizons, or horizons with silty or
fine sandy textures. However, permeability may be rapid or
very rapid in gravelly and/or sandy Immature Semiarid
Soils. Infiltration may be reduced by traffic or treading when
soils are saturated by irrigation water. Slow permeability
may induce a perched water table under irrigation management. There are no naturally poorly, or very poorly, drained
Semiarid Soils. Soils with prolonged high water tables, and
grey colours indicative of gleying, are classified as Gley
Soils. However, soils with slow permeability, particularly
towards the toes of fans where water seepage may occur,
may have some mottling in subsoils (e.g. Mottled Argillic
Semiarid Soils).
Root exploration is limited in Semiarid Soils over rock
and also in soils with very low available water-holding
capacity, especially in stony (gravel-rich) soils. The quality
of stony soils depends greatly on the nature of the fine-earth
material that fills the gaps between stones. Stony soils may
have limited fine material between stones, but where the
stone gaps are filled with loamy soil materials the available
water capacity may be a little higher.
15.4.3 Chemical Properties
Topsoil and upper subsoil pH values are generally slightly
acid to near neutral (Fig. 15.8, Table 15.2), although some
Solonetzic groups may have surface salt accumulation and
alkaline topsoils. Deeper subsoils are more likely to be
moderately to strongly alkaline, with a pH as high as 9 in
some soils. Stony (gravelly) Immature Semiarid Soils often
have low clay and organic matter contents and thus low pH
buffering capacity, and hence pH may change markedly in
response to management activities such as fertiliser addition.
Table 15.1 Example of the
physical properties of a Mottled
Argillic Semiarid Soil (Ranfurly
series, SB09891)
Horizon Depth
Sand (%) Silt (%) Clay (%) Dry bulk density (t m
−3 ) RAW
a (%) TAW
a (%)
Ap
0–15
18
53
27
1.40
5
23
BA
15–24
16
53
29
1.68
5
10
Bt1
24–40
15
48
36
1.70
5
7
Bt2
40–52
15
47
37
1.67
5
10
Bw
52–73
19
46
34
1.60
8
13
BCk
73–89
25
47
26
1.57
5
15
Cg
89–107 24
57
17
1.72
5
15
a
RAW = readily available water, TAW = total available water
Fig. 15.8 Median and upper and
lower quartiles of soil pH, organic
carbon (%), and P retention for
Semiarid soils in the New
Zealand Soil Data Repository
15.4 Key Soil Properties
239
the older soils with the well-developed relict argillic horizons can contain nearly 40% clay (Table 15.1). Soil dry bulk
densities are high to very high and total available
water-holding capacities range from low to very high,
depending on the gravel contents of the soils and the amount
of finer interstitial fill.
Soil structure is frequently weakly developed because
there is relatively little organic matter or clay to bind soil
aggregates. The sodic forms of the soils with
sodium-enriched argillic horizons can have well developed
prismatic to columnar structures (Fig. 15.5). Permeability
(saturated hydraulic conductivity) is commonly slow especially in soils with argillic horizons, or horizons with silty or
fine sandy textures. However, permeability may be rapid or
very rapid in gravelly and/or sandy Immature Semiarid
Soils. Infiltration may be reduced by traffic or treading when
soils are saturated by irrigation water. Slow permeability
may induce a perched water table under irrigation management. There are no naturally poorly, or very poorly, drained
Semiarid Soils. Soils with prolonged high water tables, and
grey colours indicative of gleying, are classified as Gley
Soils. However, soils with slow permeability, particularly
towards the toes of fans where water seepage may occur,
may have some mottling in subsoils (e.g. Mottled Argillic
Semiarid Soils).
Root exploration is limited in Semiarid Soils over rock
and also in soils with very low available water-holding
capacity, especially in stony (gravel-rich) soils. The quality
of stony soils depends greatly on the nature of the fine-earth
material that fills the gaps between stones. Stony soils may
have limited fine material between stones, but where the
stone gaps are filled with loamy soil materials the available
water capacity may be a little higher.
15.4.3 Chemical Properties
Topsoil and upper subsoil pH values are generally slightly
acid to near neutral (Fig. 15.8, Table 15.2), although some
Solonetzic groups may have surface salt accumulation and
alkaline topsoils. Deeper subsoils are more likely to be
moderately to strongly alkaline, with a pH as high as 9 in
some soils. Stony (gravelly) Immature Semiarid Soils often
have low clay and organic matter contents and thus low pH
buffering capacity, and hence pH may change markedly in
response to management activities such as fertiliser addition.
Table 15.1 Example of the
physical properties of a Mottled
Argillic Semiarid Soil (Ranfurly
series, SB09891)
Horizon Depth
Sand (%) Silt (%) Clay (%) Dry bulk density (t m
−3 ) RAW
a (%) TAW
a (%)
Ap
0–15
18
53
27
1.40
5
23
BA
15–24
16
53
29
1.68
5
10
Bt1
24–40
15
48
36
1.70
5
7
Bt2
40–52
15
47
37
1.67
5
10
Bw
52–73
19
46
34
1.60
8
13
BCk
73–89
25
47
26
1.57
5
15
Cg
89–107 24
57
17
1.72
5
15
a
RAW = readily available water, TAW = total available water
Fig. 15.8 Median and upper and
lower quartiles of soil pH, organic
carbon (%), and P retention for
Semiarid soils in the New
Zealand Soil Data Repository
15.4 Key Soil Properties
239