An international soil classification system as a simple tool to better manage your soil

Description: An international and common classification of soil is a precious tool for permaculturists that are compelled by a need to understand soil in the context of climate change, greenhouse gases, carbon sequestration, interest in maintaining the planet’s biodiversity and in exploring past cultures.

Permaculturists have raised concerns about how to preserve soil and arable land in a world with a growing population, possible future water crisis, increasing per capita food consumption, and land degradation.

Soil occupies the pedosphere, one of Earth’s spheres that the geosciences use to organize the Earth conceptually. This is the conceptual perspective of pedology and edaphology, the two main branches of soil science. Pedology is the study of soil in its natural setting. Edaphology is the study of soil in relation to soil-dependent uses. Both branches apply a combination of soil physics, soil chemistry, and soil biology.

Soil survey, or soil mapping, is the process of determining the soil types or other properties of the soil cover over a landscape, and mapping them for others to understand and use. It relies heavily on distinguishing the individual influences of the five classic soil forming factors. This effort draws upon geomorphology, physical geography, and analysis of vegetation and land-use patterns. Primary data for the soil survey are acquired by field sampling and supported by remote sensing.

National and international soil survey efforts have given the profession unique insights into landscape scale functions. The landscape functions that permaculturists are called upon to address in the field seem to fall roughly into six areas:

  1. Land-based treatment of wastes
    • Septic system
    • Manure
    • Municipal biosolids
    • Food and fiber processing waste
  2. Identification and protection of environmentally critical areas
    • Sensitive and unstable soils
    • Wetlands
    • Unique soil situations that support valuable habitat, and ecosystem diversity
  3. Management for optimum land productivity
    • Silviculture
    • Agronomy
    • Nutrient management
    • Water management
    • Native vegetation
    • Grazing
  4. Management for optimum water quality
    • Stormwater management
    • Sediment and erosion control
  5. Remediation and restoration of damaged lands
    • Mine reclamation
    • Flood and storm damage
    • Contamination
  6. Sustainability of desired uses
    • Soil conservation.
    • Carbon sequestration

The World Reference Base for Soil Resources (WRB)

An internationally accepted soil taxonomy allows permaculturists to the uniform communication of soil characteristics and functions As of 2006, the World Reference Base for Soil Resources (WRB), via its Land & Water Development division, is the pre-eminent soil classification system. It replaces the previous FAO soil classification.

 The classification is based mainly on soil morphology as an expression pedogenesis. A major difference with USDA soil taxonomy is that soil climate is not part of the system, except insofar as climate influences soil profile characteristics.

The WRB borrows heavily from modern soil classification concepts, including USDA soil taxonomy, the legend for the FAO Soil Map of the World 1988, the Référentiel Pédologique and Russian concepts. The classification is based mainly on soil morphology as an expression of pedogenesis. A major difference with USDA soil taxonomy is that soil climate is not part of the system, except insofar as climate influences soil profile characteristics. As far as possible, diagnostic criteria match those of existing systems, so that correlation with national and previous international systems is as straightforward as possible.

The WRB is meant for correlation of national and local systems. The level of detail corresponds to USDA soil taxonomy subgroups, without the soil climate information. The second edition was not detailed enough for mapping at scales larger than about 1:200 000, and a third edition has been published, improving the system for soil mapping.

To properly classify a soil profile, the detailed procedures explained in the WRB report must be followed.

N Soil profile Type of soils
1. Soils with thick organic layers: Histosols (HS)
2. Soils with strong human influence
Soils with long and intensive agricultural use: Anthrosols (AT)
Soils containing many artefacts: Technosols (TC)
3. Soils with limited rooting due to shallow permafrost or stoniness
Ice-affected soils: Cryosols (CR)
Shallow or extremely gravelly soils: Leptosols (LP)
4. Soils influenced by water
Alternating wet-dry conditions, rich in swelling clays: Vertisols (VR)
Floodplains, tidal marshes: Fluvisols (FL)
Alkaline soils: Solonetz (SN)
Salt enrichment upon evaporation: Solonchaks (SC)
Groundwater affected soils: Gleysols (GL)
5. Soils set by Fe/Al chemistry
Allophanes or Al-humus complexes: Andosols (AN)
Cheluviation and chilluviation: Podzols (PZ)
Accumulation of Fe under hydromorphic conditions: Plinthosols (PT)
Low-activity clay, P fixation, strongly structured: Nitisols (NT)
Dominance of kaolinite and sesquioxides: Ferralsols (FR)
6. Soils with stagnating water
Abrupt textural discontinuity: Planosols (PL)
Structural or moderate textural discontinuity: Stagnosols (ST)
7. Accumulation of organic matter, high base status
Typically mollic: Chernozems (CH)
Transition to drier climate: Kastanozems (KS)
Transition to more humid climate: Phaeozems (PH)
8. Accumulation of less soluble salts or non-saline substances
Gypsum: Gypsisols (GY)
Silica: Durisols (DU)
Calcium carbonate: Calcisols (CL)
9. Soils with a clay-enriched subsoil
Albeluvic tonguing: Albeluvisols (AB) replaced by Retisols (RT)
Low base status, high-activity clay: Alisols (AL)
Low base status, low-activity clay: Acrisols (AC)
High base status, high-activity clay: Luvisols (LV)
High base status, low-activity clay: Lixisols (LX)
10. Relatively young soils or soils with little or no profile development
With an acidic dark topsoil: Umbrisols (UM)
Sandy soils: Arenosols (AR)
Moderately developed soils: Cambisols (CM)
Soils with no significant profile development: Regosols (RG)

Following is a highly simplified description of each reference soil group.

Code Soil type Brief description
AC Acrisols Red, brown or yellow coloured soil, develops in areas of intense weathering, has a clay rich B horizon
AB Albeluvisols Obsolete, replaced in 3rd edition by Retisols (c.f.)
AL Alisols
AN Andosols Soil developed from volcanic material, are young immature soils, characteristics depend on type of volcanic material
AT Anthrosols Soils that have been modified profoundly through human activities, such as addition of organic or mineral material, charcoal or household wastes, or irrigation and cultivation.
AR Arenosols Sandy soil with no more profile development than an A horizon
CL Calcisols Soil with a substantial secondary accumulation of lime
CM Cambisols Transformation of soil matter (Fe particularly) in situ without moving in profile. Mostly brownish color.
CH Chernozem Fertile black-coloured soil containing a high percentage of humus, phosphoric acids, phosphorus and ammonia
CR Cryosols Soil in permafrost areas, exhibits cryoturbation and is usually rich in organic matter
DU Durisol Soil of some arid and semi-arid environments, contains cemented secondary silica
FR Ferralsols Red to yellow soil rich in iron and aluminium, common in temperate to tropical humid areas
FL Fluvisol Soil developed above flood plain sediments, A horizon is commonly directly above C horizon
GL Gleysols Saturated with groundwater for long period of time
GY Gypsisols Soils with substantial secondary accumulation of gypsum (CaSO4.2H2O)
HS Histosols Soil consisting primarily of organic materials, common in wetlands
KS Kastanozem
LP Leptosols Shallow soil over bedrock, calcareous material or a deeper soil that is gravelly or stony, common in mountains
LX Lixisols
LV Luvisols
NT Nitisols
PH Phaeozem Sod organic-accumulative
PL Planosols
PT Plinthosols
PZ Podzols Soil that presents significant podzolization, common in coniferous forests
RG Regosols
RT Retisols Soils having a clay illuviation horizon (similar to Luvisols), but with an interfingering of bleached coarser- textured soil material into the illuviation horizon forming a net-like pattern.
SC Solonchaks
SN Solonetz
TC Technosols Soils whose properties and pedogenesis are dominated by their technical origin.
UM Umbrisols Soil with a dark topsoil and in which organic matter has accumulated significantly within the mineral surface soil
VR Vertisols Shows significant and recurrent swelling with water, high content of expansive clay

The hope for regenerative agricultural development really rests on the integration of all experiences rather than reliance on one tradition at the expense of the other. With a common soil classification knowledge, permaculturists can have a support to strongly act now to stop the threatening of soil degradation.

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