Basic Soils CourseCoast Forest Region prepared byPaul Courtin and Mary-Jane Douglas1

This course is as an introduction to forest soils for the Coast Forest Region. It isdesigned to cover those soil characteristics necessary for an adequate descriptionof soils as part of a forest site assessment. Most of the material covered isillustrated with soil pictures that feature soil profiles determined as part of pastsilviculture prescriptions or research projects.

What is Soil?Soil is the unconsolidated naturallyoccurring material at the earth’ssurface. It is important to note thatthe solid components of soil aremineral and/or organic in composition.Some soils are solely made up oforganic matter such as the LFHmaterial over bedrock on some uplandsites or the O horizons of a bogwoodland soil (left). Soil depth,rooting depth and other propertiesare always described from thesurface of the forest floor (= humusform).

SoilComponentsSoils are composed of mineral and/ororganic matter, water and air. Soilporosity is the pore space of a soil. Soilswith good porosity have between 40-60%pore space. Aeration is that % of thepore space not occupied by water. Goodgrowth of forest crops features 50% soilaeration, i.e. an air:water ratio of 50:50.

A horizonsTwo master A horizons are recognized inCanadian Soil Classification. The Ae horizon is alighter coloured (left) leached horizondeveloped through the eluviation of the soil.This is the translocation of Fe, Al and organicmatter usually leaving the Ae lighter in colour.The Ah horizon (below) originates from organicmatter enrichment either by soil organismssuch as in coastal soils or the root growth suchas the Chernozemic Ah associated with interiorgrasslands. Ae and Ah indicate opposite ends ofsoil fertility, the Ah is enriched with organicmatter, the Ae is impoverished by the leachingprocess.

The Ah horizan on the right arose due to earthwormactivity in an alder soil. Alder leaf litter provides foodfor the earthworms and through burroughing andingestion of soil and organic material the surface layerbecomes darkened and enriched.

Cemented layersSoils can be hardened by a number of mechanisms such as the weight of glacialice as in a BC or C horizon yielding what is called hardpan, or by chemical agents inthe soil such as silica, CaCO3, organic matter, sesquioxides, etc. It is important torecognize cemented layers as they often restrict drainage and root growth. Thesoil below has a cemented layer called an Ortstein horizon and would be classifiedas an Ortstein Humo-Ferric Podzol. It is an impediment both to rooting and waterinfiltration

Wow,that’s hard!

Gleyed horizonsPoor drainage and low oxygen levels canresult in reducing conditions and colourchanges in soils. When Fe and Mn arereduced, compounds are formed which givethe grayish and bluish colours of gleyedhorizons. If conditions are part oxidizingand part reducing, perhaps due to afluctuating water table, some of the Fewill be oxidized, and compounds withyellow-brown, brown and red colour will beformed. Quite commonly under fluctuatingmoisture conditions part of the matrix isreduced and part is oxidized, and thecharacteristic colours are mixed; theseintermixed colours are described asmottled. Gleying conditions can bedescribed throughout a soil if the watertable is high or be restricted to only apart of the profile if downward movementof water is impeded. Soils with prominentmottling within 50 cm of the soil surfaceare classified as Gleysols. The soil on theright has an Ah horizon less than 10 cmand therefore is described as an OrthicGleysol.

The bluer soil deeper in this profileindicating longer periods of anaerobicconditions throughout the year. It isfrom the silt loam textured Bg horizon

Parent materials

Soils develop from parent materialsor the weathering of underlyingbedrock. There are many types ofparent materials classified by modeof deposition. The grey basal till onthe left was transported by glaciers.All of the C horizon is consideredunweathered till. The brown soilabove it is the result of theweathering of the till over the past10,000 years since glaciation.

parentmaterial

mode ofdeposition

characteristics

glacial till glaciation unsorted, rounded coarse fragments, can be compactedfluvial,alluvial

glacial meltwater,rivers

sorted, rounded coarse fragments, bedding planes, buried horizons

colluvium gravity unsorted, angled coarse fragmentsglaciallacustrine

water glacial lakebed deposits, uplifted through earth movements,alternating layers of finer, coarser textures

eolian wind sand, silt sized particles, coarse fragment freeglacial-marine

water ocean deposition ranging from coarse to fine materials

organic decomposition accumulation of organic matter either in wetland (e.g. peat bog) orupland (e.g. folisol) environments

A soil withthis colouris brownand has aM ll

Soil colourSoil colour is useful to assess organic matter content anddrainage. Darker coloured soils usually contain higheramounts of organic matter and are more fertile. Converselylighter coloured soils contain less organic matter and areoften less fertile and may have a low base status. Basestatus refers to the amount of cations such as Ca2+, Mg2+

and K2+ occupying exchangeable sites in the soils.

Drainage can be inferred from soil colours. Higher chroma(see below) colours usually indicate free drainage whereaslow chroma (see gleyed soils) indicate poor drainage. Thestandard colour notation for soils is the Munsell system. Abook of colour chips organizes colour by hue, value andchroma (left). Describing soil colour with this systeminsures consistency.

Soil particle size

Soil particles are divided intofines which are less than 2 mmand coarse fragments which aregreater than 2 mm. Fines arecomposed of sand, silt and clay,coarse fragments includegravels, cobbles and boulders.Coarse fragments aredetermined in the field as %volume on a visual basis

Soil texture

Texture refers to the percentages ofsand, silt and clay in the fine fraction.Texture can be approximatelydetermined in the field by handtexturing or exactly by labsedimentation tests. A soil that is 55%sand, 45% silt and 10% clay is a sandyloam ① . A soil that is 15% sand, 65%silt and 20% clay is a silt loam ② .Texture is one of the most importantthings to determine in evaluating siteproductivity and soil management.

Soil Structure

Soil structure is the size, shape andstrength of the naturally occurringsoil aggregates (called peds). Manythings contribute to soil structureincluding texture, organic and mineralcomposition, water content, soilorganisms, biological processes suchas root growth etc. The structure ofthe Ah horizon on the left has beenaffected by earthworm casts,creating what is termed a granularstructure. The peds are rounded andbetween 1 and 10 mm in diameter.This horizon has high pore space andgood aeration.Soils can be structureless with a singlegrained or massive structure. Blockystructures are common in B horizons. Platystrctures can result naturally or can formunder compaction. The horizontalarrangement of platy peds results in reducedpenetration or air, water and roots.