Semiarid Soils

Concept of the Order

Semiarid Soils are high base status soils in which a soil water deficit prevails over most of the growing season. Wetting fronts under natural climate conditions fail to penetrate deeper than the base of the solum, with consequent accumulation of pedogenic carbonate and other soluble salts.

Correlation

The order comprises brown-grey earths, associated steepland soils and intergrades between brown-grey earths and yellow-grey earths and recent soils of the NZ Genetic Soil Classification. In Soil Taxonomy, the order corresponds with the Aridisols but also with suborders under a xeric moisture regime (Xerepts, Xerults, Xeralfs, Xerorthents).

Occurrence

Semiarid Soils occur in the inland basins of Otago and southern Canterbury, where precipitation is less than about 500 mm per year.

Accessory Properties of the Order

  1. Low secondary oxides. Secondary iron and aluminium oxide levels are very low. P retention levels are also very low (usually less than 15%).
  2. High base status. Base saturation values in subsoils are high and rise to 100% at the base of the solum.
  3. Parent materials. The soils occur in materials from non-calcareous quartzo-feldspathic schist and indurated sandstone (greywacke).
  4. Illitic clay mineralogy. Mica/illite clay minerals predominate with trace primary chlorite and kaolinite. Semiarid Soils almost always have an Illitic mineralogy class unless they have a clay content of less than 10% and fall into the Mixed class.
  5. Permeability. Saturated hydraulic conductivity is slow in soils with argillic or cutanic horizons, and in some other silty or fine sandy soils. Infiltration may be reduced by pugging.
  6. Drainage. No poorly or very poorly drained soils are included.
  7. Available water capacity. Available water capacity is limited in most subsoils.
  8. Low biological activity. In the natural state, biological activity is limited by droughtiness.
  9. High slaking and dispersion potential. Soil materials are strongly dispersive and will readily slake.
  10. Erosive. The soils are susceptible to wind and water erosion.
  11. Weakly buffered. The soils are weakly buffered because of low CEC and anion adsorption capacity. They are consequently very sensitive to management, showing rapid changes in response to fertilisation, irrigation and cultivation.
  12. Soluble salts. Soluble salts are present in many soils and land management must consider the risk of salinisation.
  13. High sodium. Particularly in soils with argillic horizons, sodium occupies a large proportion of the cation exchange complex.
  14. Climate. Precipitation ranges from about 350 to 500 mm per year, with zero water surplus and a spring soil moisture deficit.

Summary of Semiarid Soils Hierarchy

Table 19.1: Semiarid Soils
Code Group Subgroup Example Series
SA Aged-argillic Mottled -
Weathered Clyde
Alkaline pt. Drybread
Thick Clyde
Typic Lowburn
SZ Solonetzic Saline pt. Chapman
Typic Manorburn
SJ Argillic Mottled pt. Waenga
Saline pt. Ranfurly
Alkaline Blackmans
Laminar Manuherikia
Typic pt. Ranfurly
SI Immature Mottled pt. Linnburn
Saline pt. Frazer
Alkaline pt. Linnburn
Typic -

Key to Groups of Semiarid Soils

SA

Semiarid Soils that have an argillic horizon in which the matrix has, in part, 7.5YR or redder hues or 10YR hue with chroma 6 or more.

AGED-ARGILLIC SEMIARID SOILS

SZ

Other Semiarid Soils that have an argillic horizon that has prismatic or blocky peds with more than 10% coatings of colour value 4 or less, and pH of 8.6 or more in some part.

SOLONETZIC SEMIARID SOILS

SJ

Other Semiarid Soils that have an argillic horizon, or a cutanic horizon that meets the requirements of a slowly permeable layer.

ARGILLIC SEMIARID SOILS

SI

Other Semiarid Soils.

IMMATURE SEMIARID SOILS

Key to Subgroups of Semiarid Soils

SA - AGED-ARGILLIC SEMIARID SOILS

Aged-Argillic Semiarid Soils occur mainly on fans, terraces or hill slopes that have been relatively stable since mid to early Pleistocene. The argillic horizons are redder in colour than those in younger soils, and are usually slowly permeable.

SAM

Aged-Argillic Semiarid Soils that have a mottled soil profile form.

Mottled Aged-argillic Semiarid Soils

SAW

Other soils in which the majority of rock fragments are weathered to the extent that they may easily be broken by hammer or spade.

Weathered Aged-argillic Semiarid Soils

SAK

Other soils that have pH of 8.6 or more in some part within 60 cm of the mineral soil surface.

Alkaline Aged-argillic Semiarid Soils

SAH

Other soils with an argillic horizon 30 cm or more thick.

Thick Aged-argillic Semiarid Soils

SAT

Other soils.

Typic Aged-argillic Semiarid Soils

SZ - SOLONETZIC SEMIARID SOILS

Solonetzic Semiarid Soils have a higher proportion of exchangeable sodium than other Semiarid Soils. This tends to promote the dispersion of clay and organic matter and establish very slow permeability through the clay enriched B horizons.

SZQ

Solonetzic Semiarid Soils that have saline soil materials within 60 cm of the mineral soil surface at some time of the year.

Saline Solonetzic Semiarid Soils

SZT

Other soils.

Typic Solonetzic Semiarid Soils

SJ - ARGILLIC SEMIARID SOILS

Argillic Semiarid Soils occur on land surfaces of early Holocene or late Pleistocene age. The argillic horizon is brown, dark brown or olive brown in colour and usually slowly permeable.

SJM

Argillic Semiarid Soils that have a mottled profile form.

Mottled Argillic Semiarid Soils

SJQ

Other soils that have saline soil materials within 60 cm of the mineral soil surface at some time of the year.

Saline Argillic Semiarid Soils

SJK

Other soils that have pH of 8.6 or more in some part within 60 cm of the mineral soil surface.

Alkaline Argillic Semiarid Soils

SJL

Other soils that have an argillic horizon that is predominantly in the form of lamellae.

Laminar Argillic Semiarid Soils

SJT

Other soils.

Typic Argillic Semiarid Soils

SI - IMMATURE SEMIARID SOILS

Immature Semiarid Soils occur on Holocene land surfaces and do not have argillic horizons but usually have accumulations of calcium carbonate.

SIM

Immature Semiarid Soils that have a mottled profile form.

Mottled Immature Semiarid Soils

SIQ

Soils that have a horizon withsaline soil materials within 60 cm of the mineral soil surface at some time of the year.

Saline Immature Semiarid Soils

SIK

Other soils that have pH of 8.6 or more in some part within 60 cm of the mineral soil surface.

Alkaline Immature Semiarid Soils

SIT

Other soils.

Typic Immature Semiarid Soils