Soils Academy: Composition Of The Soil

Mineral soils are broadly divided into solid materials and pore spaces. The solid materials are further divided into mineral matter and organic matter, while the pore spaces are made up of air and water. Therefore the four major components of the soil are:

— Mineral matter

— Organic matter

— Soil air

— Soil water

These four major components exist in an intimately mixed condition, allowing for both simple and complex reactions to occur within and between each component.

The volume composition of each of the soil component varies in different soils, due to factors such as the texture of the soil, the climatic region of the area sampled, the land use of the area sampled, and whether the sample is obtained from the topsoil or subsoil. However, at optimum condition for plant growth, the ideal topsoil of a well-watered, well-drained representative mineral soil is made up of 45% mineral matter, 5% organic matter, 25% water and 25% air, making up 50% pore spaces and 50% solid materials; the proportion of air and water fluctuates greatly under natural conditions.

The ideal volume composition of a representative mineral soil

Available plant nutrients are contained in both the mineral and organic fractions of the soil. The organic fraction mostly supplies Nitrogen (N), Phosphorus (P), and Sulphur (S), while the mineral fraction supplies more of Potassium (K), Calcium (Ca), Magnesium (Mg), and Iron (Fe). Therefore, plants obtain nutrients from the solid materials, moisture from soil water and oxygen from soil air.

Solid materials:

The solid materials are the part of the soil that is mostly referred to when the term ‘soil’ is being used. By their nature, properties and constitution, they strongly influence to a large extent, the nature and properties of soils e.g. texture, structure, pH, base saturation, tilt, consistency, etc.

Solid materials form the body of soils, thus giving it its unique ability to provide support. They are the products of the formation of soil, and as such, they originate from soil forming rocks. Solid materials are made up of mineral matter (gotten from weathering) and organic matter (gotten from the deposition of organic residues in soils). The organic fraction of the solid materials gives soil its dark colour while the mineral fraction imparts the brighter colours.

Pore Spaces:

Pore spaces are an integral part of the soil. They not only provide the atmosphere under which various soil reactions occur, they also allow for plant roots, micro organisms, burrowing animals and mechanical implements to easily penetrate the soil system. The other two components of the soil – air and water – become part of the soil system through the availability of pore spaces, thereby completing and making the soil functional. Without pore spaces, the soil cannot play certain roles such as supporting plant growth, hydraulic conductivity and solute removal.

Pore spaces mostly result from the low packing density of the soil particles; they are the spaces that exist between the solid materials in the soil system. Therefore, higher packing densities of soil particles will result in lower volumes of pore spaces in the soil. Other factors also influence the volume of pore spaces example, the size and roughness of soil particles. There are 3 types of pore spaces:

— Large-sized pores also known as Macro-pores

— Medium-sized pores also known as Meso-pores and

— Small-sized pores also known as Micro-pores

Pore spaces often contain air, which is gotten from the atmosphere and water, which is gotten mostly from precipitation and irrigation.

Pore spaces are more functional if they connect to form vertical or near vertical tunnels.

Further Studies in this theme: