17 Semiarid Soils
17.1 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 (i.e., in the absence of irrigation) fail to penetrate deeper than the base of the solum, with consequent accumulation of pedogenic (secondary) carbonate and other soluble salts.
17.2 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 New Zealand Genetic Soil Classification. In Soil Taxonomy, the order corresponds with the Aridisols including Haplargids, Paleargids, Calciargids, and Natrargids, along with Cambids, but also with suborders under a xeric moisture regime (Xerepts, Xerults, Xeralfs, Xerorthents).
17.3 Occurrence
Semiarid Soils occur in the inland basins of Otago and southern Canterbury, where average annual precipitation is less than about 500 mm.
17.4 Accessory Properties of the Order
- Low secondary oxides. Secondary iron and aluminium oxide levels are very low. Phosphate retention levels are also very low (usually less than 15%).
- High base status. Base saturation values in subsoils are high and rise to 100% at the base of the solum.
- Parent materials. The soils occur in materials from non-calcareous quartzo-feldspathic schist and indurated sandstone (greywacke), and fluvial and aeolian derivatives including fan and terrace deposits, and loess, respectively.
- 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.
- 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 machine traffic or stock treading when soils are saturated by irrigation water.
- Drainage. No poorly or very poorly drained soils are included.
- Available water capacity. Available water capacity is limited in most subsoils.
- Low biological activity. In the natural state, biological activity is limited by droughtiness.
- High slaking and dispersion potential. Soil materials are strongly dispersive and will readily slake.
- Erosive. The soils are susceptible to wind and water erosion.
- 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.
- Soluble salts. Soluble salts are present in many soils and land management must consider the risk of salinisation.
- High sodium. Sodium occupies a large proportion of the cation exchange complex, particularly in Semiarid Soils with argillic horizons.
- Climate. Average annual precipitation ranges from about 350 to 500 mm, with zero water surplus and a spring soil moisture deficit.
17.5 Summary of Semiarid Soils Hierarchy
| Code | Group | Subgroup | Example Series |
|---|---|---|---|
| SA | Aged-argillic | Mottled | - |
| Weathered | Clyde | ||
| Alkaline | Drybread | ||
| Thick | Clyde | ||
| Typic | Lowburn | ||
| SZ | Solonetzic | Saline | Chapman |
| Typic | Manorburn | ||
| SJ | Argillic | Mottled | Waenga |
| Saline | Ranfurly | ||
| Alkaline | Blackmans | ||
| Laminar | Manuherikia | ||
| Typic | Ranfurly | ||
| SI | Immature | Mottled | Linnburn |
| Saline | Frazer | ||
| Alkaline | Linnburn | ||
| Typic | - |
17.6 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.
SZ
Other Semiarid Soils containing 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.
SJ
Other Semiarid Soils that have an argillic horizon, or a cutanic horizon that meets the requirements of a slowly permeable layer.
SI
Other Semiarid Soils.
17.7 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 coarse rock fragments are weathered to the extent that clasts 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, resulting in very slow permeability through the clay enriched B horizons.
SZQ
Solonetzic Semiarid Soils that have saline soil material 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 late Pleistocene or early Holocene 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 material 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. They 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 with saline soil material 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