Soil

What is soil?

Below: a clump of soil- packed with living organisms.

For a civil engineer, soil can be a substrate on which to build roads; for a botanist, it can be a substrate to grow plants. But in general terms, soil is considered to be an unconsolidated material source of nutrients. It is a living body: from 10⁸ to 10¹⁰ bacteria can be found per gram of dry soil! Soil is also part of the hydrologic cycle since it mediates the flow of water to streams, lakes and ground water and is central to the nutrient cycle (C, N, P, S, K, micronutrients).

Soil formation

Soil formation is a long and complex process. The oldest soils in Québec are approximately 8 000 years old, which is relatively young when compared to those of other parts of the planet. Soil formation is driven mainly by factors such as climate, topography, living organisms and parent materials. Parent materials are the materials from which soil is formed. They come from the weathering of underlying rock or from alluvial deposits (carried by streams and rivers), marine sediments (carried by seas and gulfs), glacial till (moraine), colluvial debris (material which fell from hills), eolian deposits (deposited by wind) or organic plant residues.

Over time, with the action of physical weathering (freezing, thawing, wetting, drying, heating, cooling, erosion, plants, animals) and chemical weathering (chemical reactions with water, oxygen, organic and inorganic acids, and organic matter with low molecular weight), the parent material gets to form layers that are called "horizons". The top horizon is the one containing most of the organic matter and biological activity and on which plants can grow. The underlying horizons are mainly the parent material, but slightly altered. The younger the soil is, the less developed are these underlying horizons and the thinner the top horizon is.

Soil composition

Soil is composed of mineral matter (about 45 %), water and air (about 50%), dead organic matter (about 1,6%) and living organisms (less than 1%). The solubilization of mineral matter and organic matter provide most of the nutrients required by plants. Elements required by plants in large amounts are C, H, P, K, N, S, Mo, Ca. Micronutrients such as Fe,Mn,Cu, Zn,Cl and Co are only needed in small amounts and, if too abundant in the soil, they can become toxic. However, as micronutrients are removed by crops, so micronutrient deficiency is more likely in soils that have been cultivated for a long time.

Organic matter is made up of decomposing plant and animal residues. Although cultivated soils commonly contain between 1 and 5 % organic matter, this small amount has a tremendous influence on soil's physical properties (aeration, infiltration of water, colour of soil), chemical properties (very reactive material, promotes chemical reactions) and biological properties (organic matter is food for soil microorganisms). Organic matter must be constantly replenished by plant residues, animal manure, logging and wood manufacturing refuse, industrial organic residues and food processing residues.

Some air in the soil is essential since the roots, microbes, fauna, etc., need to breath. The air found in the soil is called 'soil atmosphere'. Soil atmosphere is heterogeneous and depends on the amount and quality of soil pores, soil water and biological activity.

Water affects soil temperature, aeration, plasticity, swelling-shrinking, consistency, ease of compaction. It lubricates soil for root penetration, allows movement of soil micro-flora and nutrients, and is involved in many chemical reactions. Water is also mainly responsible for erosion and run-off (refer to later sections for more details.)

Soil's physical properties

• soil texture (sand, silt, clay)
• soil structure
  • structural form
  • structural stability and strength
  • porosity
  • bulk density
• soil colour
• soil temperature

Soil Texture

Refers to the size of inorganic soil particles and to the relative proportion of the class sizes in the soil. The three main class sizes are clay (< 0,002 mm), silt (0,002 to 0,05 mm), and sand (0,05 to 2mm). Gravel (> 2mm) is not considered a class of soil.

There are two ways to determine soil texture: you can either use your hands, knowing that sand feels coarse and abrasive when rubbed between two fingers, silt feels smooth like powder and when wet feels like soap, and clay is sticky and plastic when moist and forms very hard clods when dry. You can also use laboratory methods involving sieving and sedimentation analysis techniques.

Soil texture cannot be altered by management practices: sand remains sand. It is an inherent soil property.

Soil Structure

Structural form

The structural form of soil refers to how the particles of sand, silt, and clay are bound together into stable structural units known as peds or aggregates by organic matter and clay.

Structure is very important, as it affects the influence of texture with reference to water and air movement and ease of root penetration. Clay and silt are like cement and inappropriate for life unless their structure supports it, despite the fact that they are nutrient-rich.

Soil conservation (the backbone of sustainable agriculture) is achieved by using management practices minimizing erosion while maintaining fertility: the success of such practices is usually judged by the ability to maintain and create stable soil aggregates.

Structure influences:
• Water infiltration and drainage
• Water retention
• Aeration
• Soil erodability
• Seed germination
• Root growth
• Soil porosity

Soil stability and strength

Stability refers to the ability of the soil to maintain its structural form when subjected to stresses such as tillage, traffic and rainfall. Factors that affect soil stability are wetting, drying, freezing, thawing, plant root growth, tillage, earthworms and other soil life.

Strength measures the soil capacity to withstand stress without giving way to those stresses by collapsing or becoming deformed. Factors affecting soil strength are the soil water content, the texture and the structure.

Soil porosity

Refers to the amount of pore space within the soil.

Soil pores are the open spaces between and within aggregates, and are filled with air and water.

Pore space is about 30% to 60% in most soils. Coarse soils are less porous but have more large pores: they drain well, are well aerated but drought prone. Fine-textured soils are more porous, but contain mostly micropores that do not drain well. Hence, the soil is not well aerated. Pore size is more important than total number of pores (%) and a mixture of large and small pores is necessary for optimal plant growth.

Bulk density

Soil bulk density is useful to estimate the porosity of a soil. Bulk density is expressed as grams of soil per cubic centimeter.

Here is an example: a plow layer that has a bulk density of 1,33 g/cm³ has a total porosity of 50 %. This means that half the soil volume is available for occupation by water and air. As soil become more compacted (see section on soil compaction), the bulk density will increase, meaning the porosity will decrease.

Soil colour

Soil colour affects the temperature of soils exposed to the sun; darker soils absorb more of the sun's energy. When the soil is dark, it indicates the presence of organic matter. When it is whiter, it indicates salt or carbonate deposits. Mottles or spots are signs of an inadequate aeration (red or yellow indicate well-drained soil, bluish-greyish indicates inadequate aeration).

Soil temperature

Soil temperature varies from permafrost at a shallow depth to tropical soil surface temperature of 40°C (day time) and varies with depth and time of the day. It depends on the amount of heat reaching the surface, soil mulch (insulation), vegetation, angle of sun rays to surface, soil colour, soil water, etc. Soil temperature affects plant activity and distribution and the speed of soil processes.

Soil properties management

Soil is the basis of most crop production. Only 11% of the earth's soils are suitable for agricultural production (Dimensions of Need, FAO, 1995). Managing soil with adequate conservation practices ensures farms are based on a stable and viable production system.

Soil offers physical support for plants, a complex structure of mineral and organic elements, and diversified living organisms which are all responsible for the overall soil condition. The agricultural health of a soil refers to the consistency of yields over a number of years under different weather conditions. The maintenance of stable soil properties allow producers to lower their input costs and to ensure a sustainable agricultural landbase for the years to come.