18 Ultic Soils
18.1 Concept of the Order
Ultic soils are acidic soils with clayey and/or organic illuvial features in subsoil horizons. They are developed in clayey weathering products of felsic sediments, igneous rocks or strongly weathered tephra deposits, and usually contain mixtures of clay minerals including kaolinite, halloysite, aluminium-interlayered vermiculite and smectite. A few are developed in the weathering products of limestones and greensands. They have low potassium, magnesium and phosphorus reserves and contain mostly highly weathered minerals. Eluvial horizons or other features such as skeletans in the upper parts of the Bt horizon are indicative of clay destruction/removal processes. Argillic horizons are usually present.
18.2 Correlation
The order comprises most northern yellow-brown earths, podzolised northern yellow-brown earths, many central yellow-brown earths (particularly those described as derived from pre-weathered parent materials) and some northern podzols and yellow-brown sands of the New Zealand Genetic Soil Classification. Most Ultic Soils are Ultisols in Soil Taxonomy.
18.3 Occurrence
Ultic Soils are most common in the northern North Island, and the Wellington, Marlborough and Nelson regions.
18.4 Accessory Properties of the Order
- Acidity. KCl-extractable aluminium levels of more than 1 cmolc/kg are usual in B horizons, and indicate toxic aluminium which may inhibit root function and may contribute to shallow rooting habits in aluminium-sensitive plants.
- Clayey subsoils. CEC values are medium to high. Ultic Soils cover a wide range of mineralogy classes, Kandic/Halloysitic and Smectitic being the most common.
- Low magnesium and potassium. Low concentrations of reserve magnesium and potassium resulting from strong weathering.
- Strongly weathered with low nutrient reserves. Low levels of 0.5M H2SO4- extractable phosphorus (usually less than 3 mg/100 g) and total phosphorus (usually less than 20 mg/100 g) are characteristic. The former reflects low solubility and/or a low level of inorganic phosphorus reserves. Phosphorus supply from inorganic sources under zero phosphorus input grassland farming is negligible after about 2 years. Strong weathering is indicated by low ratio of 0.5M H2SO4--extractable phosphorus to inorganic phosphorus, usually 0.25 or less.
- Slow permeability. A slowly permeable layer occurs in clayey profiles, in some cases marking a buried paleosol beneath a lithological discontinuity. The majority of soils are imperfectly to poorly drained. Soil water movement is mainly along planar voids.
- Susceptibility to livestock treading damage. Clayey or low strength, silty surface horizons are susceptible to treading damage or compaction during wet periods.
- Dispersible surface horizons. Surface horizons, especially silty ones, are dispersible according to the test of McQueen (1981), and are prone to erosion where the surface cover has been removed. Although phosphate retention is usually moderate or high in B horizons, it may be low in A and E horizons consistent with low contents of secondary iron oxides.
18.5 Summary of Ultic Soils Hierarchy
| Code | Group | Subgroup | Example Series |
|---|---|---|---|
| UD | Densipan | Perch-gleyed | Wharekohe |
| Mottled | Hukerenui | ||
| UE | Albic | Perch-gleyed | Waikare, Okaka |
| Mottled | Rangiora | ||
| Yellow | Riponui | ||
| UP | Perch-gley | Sandy | Tangitiki |
| Typic | Omu | ||
| US | Sandy | Albic | Tangitiki |
| Mottled | Tangitiki | ||
| Typic | Red Hill | ||
| UY | Yellow | Magnesic | - |
| Mottled-podzolic | Tennyson | ||
| Mottled | Puhoi | ||
| Podzolic | Opouri | ||
| Buried-granular | Kainui | ||
| Typic | Warkworth |
18.6 Key to Groups of Ultic Soils
UD
Ultic Soils that have a densipan (Ed horizon).
UE
Other Ultic Soils that have an Eg or Er horizon overlying a firm, clayey B horizon that has prismatic peds with humus and/or clay coatings in some part.
UP
Other Ultic Soils that have BOTH
- A gley profile form, AND
- Perch-gley features.
US
Other Ultic soils with more than 60% sand in the B horizon.
UY
Other Ultic Soils.
18.7 Key to Subgroups of Ultic Soils
UD - DENSIPAN ULTIC SOILS
Densipan Ultic Soils have a high density but uncemented pan at shallow depth which severely limits root penetration and water movement. Surface soil horizons are seasonally wet and the soil is very susceptible to livestock treading damage.
UDP
Densipan Ultic Soils that have a reductimorphic horizon below the densipan.
Perch-gleyed Densipan Ultic Soils
UDM
Other soils that have a redox-mottled horizon immediately underlying the densipan.
Mottled Densipan Ultic Soils
UE - ALBIC ULTIC SOILS
Albic Ultic Soils have an eluvial horizon immediately beneath the topsoil. The surface soil horizons are seasonally wet and the soil is very susceptible to damage from livestock treading.
UEP
Albic Ultic Soils that have a reductimorphic horizon below the eluvial horizon.
Perch-gleyed Albic Ultic Soils
UEM
Other soils that have a redox-mottled horizon below the eluvial horizons.
Mottled Albic Ultic Soils
UEY
Other soils.
Yellow Albic Ultic Soils
UP - PERCH-GLEY ULTIC SOILS
Perch-gley Ultic Soils have seasonal wetness close to the soil surface, indicated by low-chroma colours in horizons immediately beneath the topsoil. The wetness is caused by perching on a clayey, slowly permeable layer, although a groundwater table may also occur. The topsoil is clayey.
UPS
Perch-gley Ultic Soils that have more than 60% sand in the B horizon.
Sandy Perch-gley Ultic Soils
UPT
Other soils that have a reductimorphic horizon with an upper boundary within either 15 cm of the base of the A horizon, or 30 cm of the mineral soil surface.
Typic Perch-gley Ultic Soils
US - SANDY ULTIC SOILS
Sandy Ultic Soils occur in weathered aeolian sands. They have lower CEC and available water than is typical of Ultic Soils.
USE
Sandy Ultic Soils that have an Ea or Er horizon.
Albic Sandy Ultic Soils
USM
Other soils that have a mottled profile form.
Mottled Sandy Ultic Soils
UST
Other soils.
Typic Sandy Ultic Soils
UY - YELLOW ULTIC SOILS
Yellow Ultic Soils are clayey and lack densipans or thick eluvial horizons. They are moderately well or imperfectly drained. Few are well drained.
UYG
Yellow Ultic Soils that have EITHER
- 5% (by volume) or more coarse rock fragments that consist mainly of ultramafic rocks, OR
- Have an exchangeable calcium/magnesium molar ratio of 0.2 or less and exchangeable magnesium of 1.5 cmolc/kg or more in some part of the B horizon to 60 cm from the mineral soil surface.
Magnesic Yellow Ultic Soils
UYMZ
Other soils that have BOTH
- pH of 4.8 or less in the eluvial horizons (if present) or uppermost B subhorizon, and 10% or more humus coatings or coatings of colour value 4 or less on peds in some part of the B horizon, AND
- A mottled profile form.
Mottled-podzolic Yellow Ultic Soils
UYM
Other soils that have a mottled profile form.
Mottled Yellow Ultic Soils
UYZ
Other soils that have pH of 4.8 or less in the eluvial horizon (if present) or uppermost B subhorizon, and 10% or more humus coatings or coatings of colour value 4 or less on peds in some part of the B horizon.
Podzolic Yellow Ultic Soils
UYBG
Other soils that have BOTH
- A silt loam or silty clay texture dominant from the soil surface to a depth of 60 cm, AND
- A texture contrast to clay-textured horizons that have colour values of 4 or less.
Buried-granular Yellow Ultic Soils
UYT
Other soils.
Typic Yellow Ultic Soil