Comparing Soils

© 2007 Donald G. McGahan (aka soilman) All Rights Reserved

Soil Taxonomy

Soil Taxonomy
a systematic hierarchy of soil classification in which the selection of properties is based on concepts of soil genesis and soil management.
  • based primarily on soil properties
  • has a bias for agricultural soils and management practices

Soil Taxonomy Categories

  • Order (12)
  • Suborder (47)
  • Great Groups (231)
  • Subgroups (1250)
  • Families
  • Soil Series (18,000)
major influence or properties influencing soil-forming processes (e.g., most important property).
additional properties resulting from the major control of the current soil-forming processes (e.g., moisture regime, drainage)
Great group
properties resulting from additional, or subordinate controls of the current soil-forming processes (e.g., hardpans, presence of sodium, calcium carbonate).
properties which represent two different kinds of genesis (thick vs. thin A horizons)
additional properties which reflect the potential for change of the soil materials (e.g., broad textural classes, mineralogy, temperature).
groupings of soils which have experienced a similar genesis and which will behave in similar ways to use and management.

Soil Taxonomy: Orders

>30% clay in all horizons and cracks to 50 cm
no significant soil development - young soils
minimal soil development - young soils but more highly developed than Entisols
occur in arid regions (desert soils)
translocation of organic matter, Fe, & Al - forest soils in cold climates
clay-rich subsurface horizon; Base Saturation < 35%
clay-rich subsurface horizon; Base Saturation > 35%
dark, organic-rich A horizon; Base Saturation > 50%
highly weathered soils; only Fe/Al oxides/hydroxide minerals remain - tropical soils
organic soil materials to a depth of 40 cm
formed in volcanic materials, low bulk density and an abundance of noncrystalline minerals
soils with permafrost

Soil Taxonomy Construction of Names

Soil Taxonomic names are constructed of formative units. This is accomplished by starting the most inclusive –highest category– Order and adding succesively more exclusive –lower categories– to the left.

The following is an example of a soil with a Soil Taxonomy classification:

very-fine, montmorillonitic, thermic Vertic Haplaquoll

Category Categorical Name Formative Unit
Order Mollisol oll
Suborder Aquic aqu
Great group no special feature hapl
Subgroup Vertic vertic
Family temperature class thermic (59–72° F)
mineralogy montmorillonitic montmorillonitic
texture very-fine very-fine (>60% clay)

With each of the Categorical Name are definitions and limitations that exclude other soils.

Storie Index

Storie Index
A land rating system for evaluating the suitability of a particular soil for agricultural production. Developed for use in California agricultural soils.
  • based on soil properties
  • developed for irrigated agriculture
  • calculates a numerical value (0–100); ratings of 60–100 indicate good agricultural soils

Storie Index: Important Soil Factors

Storie Index is a parametric indices.

Factor A × Factor B × Factor C × Factor X

Factor A Depth of rooting zone
Factor B Surface texture
Factor C Slope
Factor X drainage × salt affected × nutrient level × erosion × microrelief

USDA Land Capability Classification (LCC)

USDA Land Capability Classification
Uses soil and climate data to group soils with similar management options or problems. Thirteen criteria are considered: effective soil depth, surface texture, permeability, drainage class, available water-holding capacity, slope, erosion, flooding hazard, salinity, sodium affected, toxic substance, frost-free days, and climate indices.

Seven Classes: I, II, III, IV, V, VI, VII

I - IV = suitable for cultivation (Class I & II generally considered Prime Farmland)

V - VII = not suited for cultivation

Four Subclasses

  • c - climate
  • s - soil condition (e.g., salts, low H2O holding capacity, etc.)
  • e - erosion
  • w - wetness