Ultic Soils
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 or igneous rocks 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 small concentrations of 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.
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 NZ Genetic Soil Classification. Most Ultic Soils are Ultisols in Soil Taxonomy.
Occurrence
Ultic Soils are most common in the northern North Island, and the Wellington, Marlborough and Nelson regions.
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 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. The majority of soils are imperfectly to poorly drained, few are well-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 P 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.
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 | pt. Tangitiki |
| Typic | Omu | ||
| US | Sandy | Albic | Tangitiki |
| Mottled | pt. Tangitiki | ||
| Typic | pt. Red Hill | ||
| UY | Yellow | Magnesic | - |
| Mottled-podzolic | Tennyson | ||
| Mottled | Puhoi | ||
| Podzolic | Opouri | ||
| Buried-granular | Kainui | ||
| Typic | Warkworth |
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 a gley profile form.
US
Other Ultic soils with more than 60% sand in the B horizon.
UY
Other Ultic Soils.
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
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 horizons immediately beneath the topsoil. The surface soil horizons are seasonally wet and the soil is very susceptible to damage from livestock treading.
UEP
Soils that have a reductimorphic horizon below the Eluvial horizons.
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 grey colours in horizons immediately beneath the topsoil. The topsoil is clayey. The wetness is caused by perching on a clayey slowly permeable layer, although a groundwater table may also occur.
UPS
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 relatively lower CEC and available water than is typical of Ultic Soils.
USE
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 E horizons. They are moderately well or imperfectly drained. Few are well drained.
UYG
Soils that have either
- 5% (by volume) or more 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 upper subhorizon of the B, 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 horizons (if present) or upper subhorizon of the B, 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 both
- have silt loam or silty clay texture dominant from the soil surface to a depth of 60 cm, and
- are underlain by a texture contrast to horizons with a clay texture class that have colour values of 4 or less.
Buried-granular Yellow Ultic Soils
UYT
Other soils.
Typic Yellow Ultic Soil