plants. Organic carbon is generally medium in topsoils and
very low in subsoils (Fig. 16.10, Table 16.3).
Ultic Soils are strongly weathered and leached with low
nutrient reserves. Phosphorus reserves are low with total
phosphorus characteristically less than 20 mg/100 g. However, total P values may be very high in topsoils as a result of
fertiliser inputs. Subsoil P values are usually less influenced
by fertilisation and are frequently low indicating the marked
weathering of the soil parent material. Low levels of
0.5M H 2 SO 4 -extractable phosphorus (usually less than
3 mg/l00g) are a characteristic feature. Strong weathering is
indicated by low ratios of 0.5M H 2 SO 4 -extractable phosphorus to inorganic phosphorus, usually 0.25 or less.
Potassium, magnesium, and molybdenum reserves are also
low, again reflecting the strong weathering and leaching.
Although P retention is usually medium in B horizons, it
may be low in A and E horizons consistent with generally
low contents of secondary iron oxides. Thus P added as
fertiliser remains available for plant uptake. Subsoil base
saturations are low or very low and indicate that a high
proportion of the cation exchange is occupied by acid
cations (Al
3+ and H
+
).
16.4.4 Biological Properties
Ultic Soils in their natural state have low pH, relatively low
nutrient availability, potential for aluminium toxicity, and
periods of drought in summer and saturation in winter or
early spring, thus they are a relatively difficult environment
for plant growth. The natural vegetation comprises
slow-growing native forest with plants such as manuka and
kanuka, which are tolerant of the poor conditions, dominant
in early stages of succession.
Fig. 16.9 Median and upper and lower quartiles of clay content, soil dry bulk density (t m
−3
), and total available water-holding capacity for Ultic
Soils in the New Zealand Soil Data Repository
Table 16.2 Example of the
sand, silt and clay content of a
Mottled Albic Ultic Soil (Mapua
series, SB09292)
Horizon
Depth
Sand (%)
Silt (%)
Clay (%)
Ah
0–14
19
56
24
Bw
14–20
15
50
33
Bt(f)1
20–37
14
42
43
Bt(f)2
37–65
6
27
66
Bt(g)
65–104
9
23
66
C(g)
104–120
7
40
52
260
16 Ultic Soils
very low in subsoils (Fig. 16.10, Table 16.3).
Ultic Soils are strongly weathered and leached with low
nutrient reserves. Phosphorus reserves are low with total
phosphorus characteristically less than 20 mg/100 g. However, total P values may be very high in topsoils as a result of
fertiliser inputs. Subsoil P values are usually less influenced
by fertilisation and are frequently low indicating the marked
weathering of the soil parent material. Low levels of
0.5M H 2 SO 4 -extractable phosphorus (usually less than
3 mg/l00g) are a characteristic feature. Strong weathering is
indicated by low ratios of 0.5M H 2 SO 4 -extractable phosphorus to inorganic phosphorus, usually 0.25 or less.
Potassium, magnesium, and molybdenum reserves are also
low, again reflecting the strong weathering and leaching.
Although P retention is usually medium in B horizons, it
may be low in A and E horizons consistent with generally
low contents of secondary iron oxides. Thus P added as
fertiliser remains available for plant uptake. Subsoil base
saturations are low or very low and indicate that a high
proportion of the cation exchange is occupied by acid
cations (Al
3+ and H
+
).
16.4.4 Biological Properties
Ultic Soils in their natural state have low pH, relatively low
nutrient availability, potential for aluminium toxicity, and
periods of drought in summer and saturation in winter or
early spring, thus they are a relatively difficult environment
for plant growth. The natural vegetation comprises
slow-growing native forest with plants such as manuka and
kanuka, which are tolerant of the poor conditions, dominant
in early stages of succession.
Fig. 16.9 Median and upper and lower quartiles of clay content, soil dry bulk density (t m
−3
), and total available water-holding capacity for Ultic
Soils in the New Zealand Soil Data Repository
Table 16.2 Example of the
sand, silt and clay content of a
Mottled Albic Ultic Soil (Mapua
series, SB09292)
Horizon
Depth
Sand (%)
Silt (%)
Clay (%)
Ah
0–14
19
56
24
Bw
14–20
15
50
33
Bt(f)1
20–37
14
42
43
Bt(f)2
37–65
6
27
66
Bt(g)
65–104
9
23
66
C(g)
104–120
7
40
52
260
16 Ultic Soils