The Basics of Forest Soil

Have you ever given any thought to the relationship between the soil, trees and flora on your property?  If you haven't, this article explains how the various properties of soil influence what grows in your woodlot.

What is the difference between
dirt and soil? 
"Dirt is what you find under your fingernails.  Soil is what you find under your feet."   Source: <www.school.discovery.com>

Soil Basics

Soil productivity is influenced by a number of factors including nutrient levels, moisture, drainage, erosion and compaction. The various trees and plant species found on your property have adapted to a specific range of soil conditions.

What is Soil?

The development of soil from rock is a long-term process. It involves time, physical and chemical weathering and biological activity.

Soils are made of a mixture of natural materials derived from decomposed rocks, organic matter, air and water. The principal components of soil include:

  • minerals from the parent material (stones, gravel, sand, silt and clay);
  • organic material (the remains of plants in various stages of decay);
  • soil organisms (bacteria, fungi, insects, algae);
  • atmosphere (oxygen, carbon dioxide, nitrogen); and
  • water. 

Soil Profile

Soil has four basic layers or horizons. Not all these layers are present in all cases.

LFH layer – often referred to as the duff layer, it consists of three layers of forest litter and organic matter in various stages of decay. Here is a basic description of these layers:

The top layer, known as the litter layer, is an accumulation of parts of decayed leaves, needles, twigs, plants and woody debris.  The structures of these parts are easily recognizable.

Figure 1. The average soil is made up of air, water, mineral particles and organic matter (e.g., organisms, roots and humus). 2

The second layer, known as the fermentation layer, is made up of semi-decomposed litter that may have recognizable material present (e.g., skeletonized midriffs of leaves). 

The third layer, known as the humus layer, is characterized by an accumulation of decomposed organic matter. This material is in such a state of advanced decomposition that its structure is unrecognizable.

“A” horizon – the surface or topsoil (A horizon) is the first layer of mineral soil. It often contains organic material and is rich in nutrients. Water will leach nutrients from this level down to the lower horizons. The depth of the A horizon will depend on the slope of the land and the stage of soil development.

“B” horizon – the subsoil (or B horizon) is where leached minerals, silicates and clay accumulate.  It has a distinct structure, colour and composition. It contains little or no organic matter.  Most of the tree’s root system is found in this horizon. This layer has a more compact structure than the A horizon and in some cases (e.g., heavy clay soils) may restrict water and air movement, affecting tree growth.

“C” horizon – the parent material (or C horizon) contains mostly parent rock material that has been least affected by physical, chemical and biological agents.  It is similar in chemical composition to the original material that formed the A and B horizons.

“R” horizon – underlying consolidated bedrock.

Soil Texture

Soils are made of an infinite number of particles.  Based on their size, soil particles are divided into three categories: sand (2mm-0.05mm), silt (0.05mm-0.02mm) and clay (>0.002mm). 

When we refer to a soil type, we are actually describing the texture of the soil and the relative proportions of sand, silt and clay. The different soil textures are classified by determining the amount by size of each of these primary particles.  For example, a sandy soil contains less than 15 percent silt and clay, while clay soil contains more than 40 percent of fine clay particles.

Figure 2 illustrates the soil texture triangle from the Ontario Institute of Pedology and shows the relationships of the 12 different texture classes.  In the illustration, the percentage composition of sand (horizontal axes) and clay (vertical axes) is shown along the horizontal and vertical axes of the triangle.

There are many different soil texture classes. However, three very broad soil classifications – sand, loam and clay – are all that is necessary to meet the needs of most landowners. Here are a few characteristics of these three soil classes.

Sand soils – This type of soil has easily detectable particles.  It is gritty in texture, and water retention is very poor. Sand soil is more resistant to compaction and has minimal nutrient value.

Loam soils – Loam soils have relatively equal proportions of sand, silt and clay. There is some evidence of graininess. Loam soils have some nutrient value and have fair to good moisture retention.

Clay soils – Clay soils have very fine particles and smooth texture.  Clay contains many nutrients, compacts easily when wet and inhibits the movement of water.

Figure 2. The soil texture triangle showing the different texture classes (for soil material <2mm in diameter).

Here are four field tests that can be done to assist you in determining your soil’s texture.

Feel test – Place a small amount of soil in the palm of your hand. If the sample is wet, dry it by rubbing the soil vigorously in the palm of your hand.  When dry, place some of the soil between your thumb and fingers to measure the percentage of sand. The grittier the texture is, the higher the sand component. 

Moist cast test – Place a sample of the soil in your hand and squeeze tight.  When you open your hand, see whether the soil holds together (forms a cast).  Pass the cast from hand to hand. If the cast holds together, there is a high percentage of clay.

Ribbon test – Roll a handful of moist soil in the palm of your hand into the shape of a cigarette.  Squeeze the soil between your thumb and forefinger to form the thinnest and longest ribbon possible.  Soils with high silt content will form flakes. Soils that form long thin ribbons during this procedure contain higher clay content.

Taste test – Place a small amount of soil in your mouth between your front teeth.  Sand particles will feel gritty against your teeth. Silt particles can be distinguished by fine grittiness; clay has no grittiness.

Table 1 outlines some of the general properties of the three soil classes.

Table 1:  Soil texture class assessment properties 1

Soil texture class

Visual appearance

Reaction when squeezed in the hand

Ability to form a ribbon

Dry soil

Moist soil

Sand soils

Granular with easily detectable particles

Easily falls apart

Will not form a cast or cast easily

Forms a cast that crumbles

Cannot form a ribbon

Loam soils

Low to moderately granular

Can form clumps/clods when dry

Forms a cast that can be handled relatively easily

Forms a cast that can be easily handled

Pure loam cannot form a ribbon

Loam with more silt and clay will form a fragile ribbon

Clay soils

Fine texture with very few large particles

When dry forms hard clumps

Forms a cast that can be handled freely

Forms a cast that can be worked and is cohesive

Forms a long, flexible ribbon


Soil Structure

Soil structure is how the various soil particles are arranged in a soil (i.e., clumps or aggregates).  The glue or bonding agents that bind the aggregates together is the clay and organic matter in the soil. Aggregates are formed by both physical force (such as drying, freezing, thawing or tillage) and by the binding agents (organic matter). Aggregate composition determines the porosity of the soil.

It is important to note that soil structure can affect plant growth.  A soil's structure will affect the soil's ability to retain moisture, permit air exchange and allow heat transfer. Compacted soil may impede root growth.

Soil pH

A soil's pH is a reference to the level of acidity in the soil. The pH scale goes from 0 to 14, with a soil pH 7.0 considered neutral. A soil with a high pH (alkaline soil) will have a low concentration of free hydrogen ions, while an acid soil (low pH) will have a relatively high concentration of hydrogen ions.

The pH of the soil will influence the solubility of nutrients found in the soil. Most minerals and nutrients are more available in an acid soil versus a neutral or slightly alkaline soil. For example, soil with a pH lower than 5.5 (moderately acidic soil) will have a higher amount of soluble nutrients that can be easily leached from the soil. In soils with high pH (alkaline), nutrients are less soluble and plants cannot easily extract them from the soil. The most fertile soils have a pH ranging from 6.0–7.2 and have the most readily available nutrients for plant growth.

Soil Drainage

Soil texture will influence water movement.  Large-particle soils such as sand have increased pore space, allowing quick water movement.  Fine-textured soils such clay have smaller particle sizes that slow water penetration.

Here are three easy field tests that will assist you in determining soil drainage.

Seasonal observation – Examine your property at various times of the year to identify where water accumulates or drains quickly. 

Existing plant species – Survey the existing plant species for an indication of the amount of soil moisture.  For example, maidenhair fern or red maple trees are generally found in fresh to moist soils. Bracken fern or red pine are normally associated with drier upland sites.

Plant vigour – Compare vegetation from one site to another.  Lush green vegetation on the ground may indicate moister soils versus areas with sparse or no vegetation.

Table 2 provides the description of four basic drainage categories.

Table 2: Soil Drainage Classifications 1

Classification

Description

Well drained

Water drains from the site rapidly

Water seldom pools on the site even during a heavy rain or after snowmelt

Moderately drained

Water may pool, but only for brief periods

Imperfectly drained

Water pools on the site, sometimes for extended periods, especially during the spring or in wet years

Poorly drained

Water drains slowly from the site throughout the year

Soil may appear wet below the surface

Trees have adapted to soil texture; they also have adapted to specific moisture conditions.  Species such as black spruce, willow, tamarack, silver maple, green ash and red maple are better suited to poorly drained areas.  White pine, red pine, red oak, sugar maple, Norway spruce, hickory and black walnut are better suited to well-drained or moderately drained sites.

Soil Erosion

Soil texture also is an important factor that influences soil erosion (along with the slope, ground cover and precipitation). The loss of nutrient-rich topsoil due to erosion will affect growth for years.  Protecting the topsoil and litter layer is important to ensure optimal growth and good regeneration.

Looking for Soil Information?

Soil information for Ontario is available on the Canadian Soil Information's website at <http://sis.agr.gc.ca/cansis/>.   The website offers downloadable soils maps of Ontario’s upper tier municipalities (by county).

Best Management Practices: Soil Management, Ontario Ministry of Agriculture, Food and Rural Affairs.  The guide is available on the Internet at <http://www.omafra.gov.on.ca/english/environment/soil/soilmgmt.htm> or by calling 1-888-466-2372.  Cost $15.00.

Knowing Your Planting Sites is available on the Internet at <http://www.ontariowoodlot.com>.

Acknowledgements

1Choosing the Right Tree: A Landowner’s Guide to Putting Down Roots, Eastern Ontario Model Forest <http://www.eomf.on.ca/media/k2/attachments/Choosing_the_Right_Tree.pdf>.

2 "Introduction to the Lithosphere," University of British Columbia Okanagan, <http://www.physicalgeography.net>

This article was featured in a past edition of the S&W Report, the newsletter of the Ontario Woodlot Association. 

© Ontario Woodlot Association, 2010
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