Description

How to Describe a Humus Form

Steps in Humus Form Description
[click here for a sample humus form profile description sheet]

  1. Choose the site that you are going to describe
  2. Dig a small soil pit, at least 10 cm into the mineral soil.
  3. Observe the organic horizons closely.
  4. Determine horizon boundaries and designations and record the horizons on your tally sheet. Possible horizons are L, F, H and Ah.
  5. Decide if you are going to split the horizons to provide more detail. For example, you might have a Ln and a Lv horizon. The Ln is new litter and the Lv is decayed litter.
  6. Pay particular attention to determining the type of F horizon as this will be used to classify your humus form. You may designate Fm, Fz or Fa.
  7. Record the horizon depths using the mineral/organic interface as your 0 point. For example, L (7-5 cm), F (5-2 cm), H (2-0 cm), Ah (0-2 cm).
  8. You can then use the methods of humus form assessment to record other properties such as moisture status, color, fabric, consistence, character, and what biota you see in the sample. You can also indicate if you find con-conforming materials such as decaying wood, coarse fragments or charcoal.
  9. You may carry out your description in the field or after recording horizon designations and depths, you can collect an intact sample to take to the lab for further assessment.


video notes

Humus Form Properties

Click below to find out more about each Humus form properties

Horizon Depth

The depth of organic horizons is measured upward from zero depth, with zero being the interface between the organic and mineral horizons, In organic soils the depth is measured downward from the top of the profile, and this is taken as zero depth.

BOUNDARY DISTINCTNESS
Class Description
Abrupt < 5mm
Clear 5 – 10mm
Gradual 11 – 20mm
Diffuse > 20mm
BOUNDARY FORM
Class Description
Smooth Nearly a plane
Wavy Pockets wider than deep
Irregular Pockets deeper than wide
Broken Discontinuous, some parts are unconnected

Moisture Status

Some morphological properties of materials described may be affected by their moisture content.

Class Description
Desiccated Extremely dry condition; organic tissues will crack or snap when broken or crushed
Dry Moisture is not apparent; material will not rub colour out on fingers
Moist Moisture is apparent; colour will rub out on fingers; if material is squeezed in hand, no water will be observed (generally below field capacity)
Wet If material is squeezed or rubbed in hand, water will be observed (generally below field capacity)
Saturated Water is observed without squeezing or rubbing of material

Colour

The colour of materials within the humus form profile may be of less interpretive significance than that for mineral soil horizons. However, the Munsell Color Chart can also be used to assess the colour of organic horizons.

Fabric

The term fabric has been applied to the description of structure, consistence and character of organic materials in the humus profile.

Structure

Classified according to grade, type, kind and size of the macromorphological aggregation of the material within a horizon or layer.

Type Kind
Class Description Class Description
Structureless No observable aggregation Massive A coherent mass showing no
evidence of aggregation
massive
Single Particle An incoherent mass of individual particles (may be of various sizes) with no aggregation massive
Blocklike Materials arranged around a point bounded by flat or rounded surfaces Blocky Faces rectangular and flattened; vertices sharply angular blocky
Granular Spheroidal and characterized by rounded or sub-rounded vertices granular
Platelike Materials arranged horizontally and parallel
to each other, generally bounded by relatively flat horizontal surfaces
Non-Compact Matted Materials arranged along horizontal planes with noevident compaction non-compact_matted
Compact Matted Materials arranged along horizontal planes with evident compaction compacted_matted
Columnlike Materials arranged vertically Erect Materials in vertical position erect
Recumbent Materials in recumbent (reclining) position recumbent
GRADE OF STRUCTURE
Class Description
Weak Disaggregated materials are dominant; <20% distinction of aggregation
Moderate Some disaggregated material is found; 20 – 60% distinction of aggregation
Strong Aggregated materials are dominant; most material conforms to the same arrangement; >60% distinction of aggregation

Consistence

A measure of the strength and nature of forces combining materials together, assessed by deformation or rupture when pressure is applied.

Class Description
Loose There is no consistence of the material
Friable A material that crumbles easily under gentle pressure
Firm A material that can be crushed under moderate pressure; resistance is noticeable
Pliable Material is soft and plastic
Resilient Material that is springy or elastic and assumes original state after forces of deformation have been applied and released
Tenacious Material is cohesive, not easily pulled apart

Character

The distinctive quality, or tactility: how the material feels.

Class Description
Mushy Materials are wet or saturated, soft and spongy
Mucky Materials are usually wet, smooth and sticky; they contain silt and clay sized mineral particles
Greasy Materials are smooth and greasy when moist and easily workable; fine mineral particles are usually absent
Gritty Refers to a rough tactility produced by mineral granules or coarse fragments
Leafy Refers to the tactility of materials produced by deciduous foliage showing a shingle-like layering (banded structure)
Mossy Refers to the tactility produced by bryophytes with more or less preserved vegetative structures
Acerose Refers to the tactility produced by bryophytes with more or less preserved vegetative structures
Felty Refers to the tactility produced by abundant fungal mycelia
Fibrous Refers to tactility produced by and abundance of fibrous plant residues that do not break down upon rubbing (roots)
Ligneous Refers to the tactility produced by coniferous or deciduous wood fibres
Crusty Refers to a hard and brittle tactility of dry or desiccated materials

Roots

For fine and very fine roots, the surface area of the profile viewed is 2.5cm X 2.5cm while for mediuem, coarse and v. coarse roots, a 25cm X 25cm area is viewed. Both living and dead roots are counted, unless it is very clear that a root is dead.

ROOT ABUNDANCE
Class Number of roots/unit surface area
Very few < 3
Few 3 – 10
Common 11 – 20
Plentiful 21 – 30
Abundant >30
ROOT SIZE
Class Size (diameter in mm)
Very fine < 1
Fine 3 – 10
Medium 11 – 20
Coarse 21 – 30
Very Coarse >30
ROOT ORIENTATION
Class Description
Random Roots are oriented in all directions
Oblique Roots are oriented along oblique planes
Horizontal Roots are oriented along horizontal planes
Vertical Roots are oriented along vertical planes

Non-Conforming Materials

Usually charcoal, decaying wood and mineral coarse fragments. The kind, distribution, size and abundance are described. They may have: Regular Distribution throughout, or Irregular Distribution only in certain parts of the horizon. Abundance refers to the volume by percentage occupied by non-conforming materials for each size class.

NON-CONFORMING MATERIALS – DISTRIBUTION
Class Description
Random Materials are distributed randomly; there is no recognizable, specific pattern of distribution
Clustered Materials are distributed in clusters or groups
Banded Materials are distributed in bands, sheets or layers
NON-CONFORMING MATERIALS – SIZE
Class Diameter(mm) Length(mm)
Very fine < 5
Fine 5 – 20
Medium 21 – 100
Coarse 101 – 500
Very Coarse > 500
NON-CONFORMING MATERIALS – ABUNDANCE
Class Description
Few Occasional and scatted pieces; volume < 5%
Common Frequent occurrence; volume 5 – 20%
Many Numerous, coarse pieces; volume > 20%

Biota

The activity of soil flora and fauna paly such an important role in humus formation that their description cannot be overlooked.

Soil Fauna

Soil fauna are those that pass one or more active stages wholly or largely in the soil or surface litter, excluding those species which occur there only in passive stages such as eggs, cysts or pupae.

Direct observation of fauna or indirect observation through the presence of casts can be used to determine faunal presence – castes provide a good indication of their activity

Size Distribution, shape, colour and level of humification indicate the type of soil fauna that produced casts.

  • Mite-type: Small (<0.1mm diameter) spherical/oval, humified, rust to dark brown, lacking mineral grains
  • Enchytraeid-type: Small (0.05-0.2mm diameter) sub-spherical, rugose, generally well-humified, brown with varying amounts of mineral grains and clay
  • Arthropod-type (and small surface-feeding earthworms): Visible without hand lens (1-3mm long) well-humified, dark brown, containing mineral grains but low in clay
  • Worm casts: Well-humified, generally brown or greyish brown, containing mineral grains and clay, spongy or coarse rugose granular structure (5-10mm diameter)

Soil Fauna Figures

A. Mites (Acarina)
B. Springtails (Collembola)
C. Spiders (Areneida)
D. Fly larvae (Diptera)
E. Beetles and Larvae (Coleoptera)
F. Ants (Hymenoptera)
G. Woodlice (Isopoda)
H. Centipedes and millipedes (Myriapoda)
I. Termites (Isoptera)
J. Earthworms (Lumbricida)
K. Potworms (Enchytraeida)
L. Nematodes (Nematoda)

SOIL FAUNA – SIZES
Class Description
Macrofauna Animals with a body size greater than 1cm (in longest dimension); earthworms, vertebrates, molluscs and large arthropods
Mesofauna Animals ranging in body size from 1cm to 0.2mm including some mites, springtails, potworms and most of the larger nematodes. The lower limit is about the limit of viewing with a 10x hand lens.
Microfauna Organisms less than 0.2mm in size; includes the protozoa as well as many of the smaller mites and nematodes
DROPPINGS – ABUNDANCE
Class Description
None No visible droppings
Few Droppings occasional observed but scattered
Common Droppings commonly observed
Abundant Droppings frequently observed in relatively large numbers throughout the horizon
DROPPINGS – DISTINCTION
Class Description
Random Droppings are distributed randomly, there is no recognizable pattern of distribution
Clustered Droppings are distributed in clusters or groups
Banded Droppings are distributed in bands, sheets or layers

Soil Flora

Bacteria, actinomycetes, algea and fungi. Description is usually narrowed to fungi in the field. Fungi include: Rusts, moulds, yeasts and mushrooms. Presence is detected by observing a mass of hyphae (thread-like filaments) called mycelium. Colours include brown, black, grey, white, red, yellow and blends of these or transparent. Description includes abundance, colour and distribution.

FUNGAL MYCELIA – ABUNDANCE
Class Description
None Mycelia not visible
Few Mycelia occasionally present but scattered and not easily observed
Common Mycelia commonly observed
Abundant Mycelia observed continuously throughout the horizon, often “matting” materials together and creating a “felty” tactility
FUNGAL MYCELIA – DISTRIBUTION
Class Description
Random Mycelia are distributed randomly, there is no recognizable pattern of distribution
Clustered Mycelia are distributed in clusters or groups
Banded Mycelia are distributed in bands, sheets or layers