How to Describe a Humus Form

Steps in Humus Form Description

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

Boundary Form

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

Color

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.

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

Root Size

Root Orientation

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.

Distribution

Size

Abundance

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 – 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

Droppings – Distinction

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