Soil is the surface layer of the Earth's crust that can be shaped and on which plants grow.
Without the presence of soil, life as we know it on Earth would not be possible. Soil makes agriculture possible, holds rainwater, and provides a habitat for many animal species. Furthermore, it provides support for living beings to move around and feed themselves, as well as for the foundations of buildings and roads. Since each type of soil can serve a particular purpose, it is important to define its characteristics and its evolution.
Two processes lead to the formation of a soil type: the alteration of the bedrock and the enrichment of organic matter. The ground is thus formed of materials from mineral and organic origin which mix and arrange into different layers.
Soil is first formed out of a hard rock that makes up the Earth's crust. This rock is called the "bedrock." Soil corresponds precisely to the layer which is found on top of this bedrock. Initially, various physical, chemical, or biological processes partially or totally transform the elements that make up the bedrock. For example, runoff, wind, freezing, thawing, and chemical changes slowly break up bedrock to form debris and smaller mineral particles. Thus, a soil builds up slowly, through the accumulation of rock fragments and minerals.
Then, the soil is gradually enriched with materials of organic origin as a result of the action of decomposers (bacteria, fungi and invertebrates) which degrade debris of plant origin (dead roots, leaves, bark, fruits, etc.) and animal origin (excrement, feathers, carcasses, etc.) constantly found in the soil. These transformations of organic matter are very important because they enrich the soil with an essential material called humus, thus making the soil rich and suitable for plant growth and agriculture.
Soil formation is a slow process; a very young soil is very thin, while an older soil is thicker.
Soil contains four components: water, air, minerals, and organic matter. It is the proportion and the organization of these different components that determine the properties of the soil and the usage that can be made of it.
The water content of a soil determines its ability to hold water. Thus, the soil is said to be more or less drained or to have a high or low water retention. Water can be found in soil in solid form (ice) or in liquid form.
The air content of a soil determines its level of aeration. A loosely compacted soil is more likely to let air in than a crushed soil. Thus, air will be more or less present.
Organic materials can be diverse. We can find living organisms: plant roots, insects, burrowing mammals, etc. They can also be debris from plants or animals such as dead tree branches, excrement, animal corpses, etc.
The mineral portion of the soil comes from the degradation of the bedrock.
The type of soil is determined by the size of the particles composing it and by their arrangement.
Soils vary in texture and structure. The texture of a type of soil depends on the size of the particles that compose it. To determine the texture of a soil, a simple touch test or the jar of water test can be used. Indeed, a soil composed mainly of clay will form a ball which can be held in the hand, whereas a soil made of sand will instead slip between the fingers. The texture of the soil has a direct effect on its structure, nutrient content, moisture, and on its ability to drain water.
Soils can be grouped into four main types.
Texture: Contains mostly sand; very porous soil that does not retain water; dries and heats up in no time.
Structure: Slips between the fingers due to the lack of cohesion between particles; very sensitive to wind erosion and leaching.
Culture: Not suitable for plants with high water requirements; suitable for growing asparagus, carrots, potatoes, cacti, etc.
Advantages: Soil that is easily workable and which drains water well; can be warmed up very quickly.
Disadvantages: Poor fertile soil which does not retain water; very prone to erosion.
Texture: Contains mostly silt; partial adhesion of soil particles, which disintegrate into small pieces under the effect of the environment; medium porosity.
Structure: Relatively tightly bound particles allowing air and water to circulate more or less easily; prone to the superficial formation of a dry crust, which limits water infiltration, while promoting runoff; very sensitive to water erosion, especially where the slopes are steep.
Cultivation: Suitable for growing wheat, corn, beets, etc.
Advantages: Very workable and fertile soil.
Disadvantages: Very fragile soil which erodes easily.
Texture: Contains mostly clay; heavy and compact soil with particles remaining stuck together like dough; risk of soil compaction if heavy masses (such as trucks) run over the surface; compacted soil is then difficult to drain; often referred to as clay.
Structure: Little space for water and air circulation; retains water and nutrients well; prone to wind erosion, because the topsoil can be blown away if the surface is dry, this surface layer can literally be pulverized; tendency to be alkaline.
Culture: Very fertile soil, as it is rich in nutrients; it should not be waterlogged so as not to affect plant growth; suitable for growing tomatoes, barley, soybeans, etc.
Advantages: Soil which retains water well and is very fertile.
Disadvantages: Poorly workable and reacts badly to temperature variations (freezes easily).
Texture: Contains mostly organic matter; relatively loose dark particles that slip between the fingers; allows water to drain easily.
Structure: Can hold a large amount of water without becoming sticky like clay soil; retains fertilizers well; may be prone to wind erosion.
Culture: Often used for growing vegetables.
Advantages: Very fertile soil, which retains water well and warms up easily.
Disadvantage: Soil is sometimes very acidic which can affect plant growth.
No type of soil is better than another. Each has its advantages and disadvantages. In fact, the quality of a soil depends on its intended use and its geographical location, and therefore on the climate. The best soil for agriculture is a well-balanced soil: it should contain a good proportion of clay, silt, and sand, and a good amount of organic matter.