Properties of Soil: Physical, Chemical and Biological Characteristics

Introduction: Soil as a Three-Phase, Multi-Property System

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Soil is the upper weathered layer of the Earth’s crust, shaped by the combined action of plants and animals. A vertical section through this zone constitutes a soil profile; within each profile there are usually several distinguishable layers, or horizons, each exhibiting a different type of soil material that allows soil scientists to recognise and classify it.

Crucially, soil contains matter in all three physical states — solid, liquid, and gaseous. The solid portion is partly organic and partly inorganic:

• The inorganic, or mineral, part of the soil is made up of particles derived from the parent material — the rocks which weather to form the soil.
• The organic portion consists of living and decayed plant and animal materials, such as roots and worms. The end-product of decay is humus, a black, amorphous organic matter.
• Soil water is a dilute but chemically complex solution, derived from direct precipitation and from run-off, seepage, and groundwater.
• The soil atmosphere fills the pore spaces of the soil whenever these are not occupied by water.

Several diagnostic properties allow soils to be described and told apart, each introduced briefly here before being developed in full below:

• Texture refers to the sizes of the solid particles composing the soil, ranging from gravel to clay. Proportions of different particle sizes vary from soil to soil and from layer to layer, and texture largely determines the water-retention properties of the soil: in sandy soil, pore spaces are large and water drains rapidly, while in clay soil, individual pore spaces are too small for adequate drainage. Generally speaking, loam textures are best for plant growth.
• Acidity is a property related to the proportion of exchangeable hydrogen ions present in the soil relative to other elements. Degree of acidity is measured on the logarithmic pH scale, ranging from 0 (extreme acidity) to 14 (extreme alkalinity). Few soils reach these extremes; a pH of about 6.5 is normally regarded as most favourable for the growth of cereal crops.
• Colour varies considerably in soils and reveals much about how a soil was formed and what it is made of. In recently formed soils, colour will largely reflect that of the parent material, but in many other cases the colour differs markedly from the underlying rock. Soils can range from white to black, usually depending on the amount of humus present.

In cool, humid areas, most soils contain relatively high humus content and are generally black or dark brown, whereas in desert or semi-desert areas, little humus is present and soils are light brown or grey. Reddish colours in soils are associated with the presence of ferric compounds — particularly oxides and hydroxides of iron — and usually indicate that the soil is well-drained, although locally the colour may instead be derived from a red-coloured parent material.

Regional Colour Pattern

All soils, in short, contain mineral particles, organic matter, water, and air, and the combination of these four components determines the soil’s core properties — its texture, structure, porosity, chemistry, and colour. Knowing a soil’s water, mineral, and organic proportions helps determine its productivity and the best use for that land; several readily testable soil properties are conventionally used to describe and differentiate soil types.

Physical Properties of Soil

Possible framing: “Discuss the physical properties of soil and their significance for plant growth and land-use planning.”

Physical properties depend upon the amount, size, shape, arrangement, and mineral composition of soil particles. They also depend on the organic matter content and pore spaces present within the soil body.

1. Soil Texture

Soil texture defines the proportion in which the soil separates into the mineral components of the soil. These separates are classified as sand, silt, and clay. Sand and silt are of relatively minor importance to soil fertility, as they contribute little to the soil’s ability to retain water or nutrients. Clay, by contrast, is the active part of soil texture — clay particles are of small size and possess a large amount of surface area per unit mass, which helps in storing ions and water.

Texture refers, more precisely, to the coarseness or fineness of the mineral matter in the soil, and is determined by the relative proportion of sand, silt, and clay particles. The equal proportion of all three is known as loam. Soil texture affects water-holding capacity, nutrient retention, nutrient fixation, drainage, compressibility, and aeration of the soil.

Loamy Soil: Loamy soil is the one in which none of the three fractions (sand/silt/clay) dominates the other. In particular, loamy soil has about 40% sand, 40% silt, and 20% clay — and is, for this reason, generally regarded as the ideal agricultural texture.

2. Soil Structure

Soil structure is the arrangement of soil particles into certain patterns — such as plate-like structure, block-like structure, or prism-like structure. It describes the way in which sand, silt, and clay particles are clumped together into larger units, or peds. Organic matter (decaying plants and animals) and soil organisms such as earthworms and bacteria all influence soil structure.

Clays, organic matter, and materials excreted by soil organisms bind soil particles together to form aggregates. Soil structure is important for plant growth because it regulates the movement of air and water, influences root development, and affects nutrient availability. Good-quality soils are friable (crumbly) and have fine aggregates, so the soil breaks up easily when squeezed; poor soil structure is coarse, forms very firm clods, or has no structure at all.

Two structural characteristics of soil deserve particular attention:

• Permeability — the ease with which liquids/gases can pass through rock or a layer of soil. It depends on the size, shape, and packing of particles, and is usually greatest in sandy soils and poor in clayey soils.
• Porosity — the volume of water which can be held within the soil, expressed as a ratio of the volume of voids (pores) to the total volume of the material.

There are five basic types of structural units, each associated with characteristic horizons and climatic conditions:

1. Platy: Plate-like aggregates that form parallel to the horizons, like pages in a book. This structure may reduce air, water, and root movement, and is a common structure in an E horizon, usually not seen in other horizons. (Thin, flat plates lying horizontally; usually found in compacted soil.)
2. Blocky: Two sub-types — angular blocky and subangular blocky. These structures are commonly seen in the B horizon. Angular blocky is cube-like with sharp corners, while subangular blocky has rounded corners. (Irregular blocks, usually 1.5–5.0 cm in diameter.)
3. Prismatic: Vertical axis is longer than the horizontal axis. If the top is flat, it is referred to as prismatic; if the top is rounded, it is called columnar. Prismatic units are vertical columns of soil, possibly several cm long, usually found in lower horizons; columnar units have a “salt cap” at the top and are found in soils of arid climates.
4. Granular: Peds are round and porous, spheroidal — usually less than 0.5 cm in diameter, and commonly found in surface horizons where roots have been growing. This is usually the structure of A horizons.
5. Structureless: No observable aggregation or structural units — sub-divided into single grain (soil broken into individual particles that do not stick together, always accompanying a loose consistence, commonly found in sandy soils) and massive (a continuous, unconsolidated mass without aggregates).

3. Soil Colour

Soil colour (brown, yellow, red) basically depends on oxidised or ferric iron compounds. The darker the colour of the soil, the more organic content it contains; the higher the organic content, the higher the soil temperature tends to be, as darker soils absorb more heat.

• Soils rich in humus tend to be dark because decomposed organic matter is black or brown. Soils with high humus content are usually very fertile, so dark brown or black soils are often referred to as “rich.”
• Red or yellow soils typically indicate the presence of iron.

Soil colour is described using three parameters — hue, value, and chroma. Hue represents the dominant wavelength, or colour, of the light; value refers to the lightness of the colour; and chroma refers to the relative purity, or strength, of the colour. The colour of a soil, in terms of these parameters, can be quickly determined by comparing a sample with a standard set of colour chips mounted in a notebook called the Munsell Soil Colour Charts. In these charts, the right-hand top corner represents the hue; the vertical axis, the value; and the horizontal axis, the chroma.

4. Soil Permeability

Soil permeability is a broad term used to define the ability of the soil to transmit water. Understanding water dynamics and water balance in the soil is important, and permeability must be known for accurate management of irrigation. It is determined partly by texture — sandy soils have high permeability compared to clay soils — and it can be altered through soil management practices.

• Most porous rocks are permeable, with the exception of clay, whose pore spaces are so small that they are often sealed with groundwater held by surface tension. Another exception is granite, which is non-porous but permeable: being a crystalline rock, it is hence non-porous, and its individual crystals absorb little or no water — but the rock may have numerous joints or cracks through which water can pass, rendering it permeable overall.
• A soil with high organic content also tends to have high porosity.

5. Soil Horizons

The soil is divided vertically into different horizons from top to bottom, namely:

• A-Horizon: This is the uppermost layer of soil, also called topsoil. This layer is rich in humus and minerals and holds most of the water compared to other layers. It consists of sand, silt, and clay, and is also home to many living organisms, such as snakes and earthworms.
• B-Horizon: This is the second layer from the top, a little richer in humus, and it supports moisture. This layer consists of silt, clay, weathered rocks, and some nutrients — minerals are more concentrated in this layer than in the topsoil.
• C-Horizon: This layer consists of small pieces of rock broken down due to weathering.
• Bedrock: This is the last layer and consists of layers of solid, unweathered rock.

Chemical Properties of Soil

Possible framing: “Explain the role of pH and colloids in determining soil fertility.”

Chemical properties of soil depend on the following factors:

• Inorganic matter present in the soil: the mineral content of the soil is the major factor differentiating various types of soil, precisely because of its abundance in the soil.
• Organic matter present in the soil: though present in very small quantities, organic matter plays an important role in deciding the fertility of the soil.
• Colloidal properties of soil: colloids are mainly of two types —
  • Clay colloids: important for the adsorption of a large quantity of water.
  • Organic colloids: help increase the moisture and nutrient-retention capacity of the soil.
• The pH of soil: the measure of the chemical reaction a soil shows is expressed by its pH value; the pH value of soil determines its acidic or basic nature.

1. Acidity and Alkalinity

An important aspect of soil chemistry is acidity, alkalinity (baseness), or neutrality. Low pH values indicate acidic soil, and a high pH indicates alkaline conditions. Most complex plants grow only in soils with levels between pH 4 and pH 10, but the optimum pH varies with the plant species.

• In arid and semi-arid regions, soils tend to be alkaline, while soils in humid regions tend to be acidic.
• To correct soil alkalinity and make the soil more productive, the soil can be flushed with irrigation water.
• Strongly acidic soils are also detrimental to plant growth, but soil acidity can generally be corrected by adding lime to the soil.

The most important effect of pH in the soil is on ion solubility, which in turn affects microbial and plant growth. A pH range of 6.0 to 6.8 is ideal for most crops, because it coincides with optimum solubility of the most important plant nutrients. Some minor elements (e.g., iron) and most heavy metals are more soluble at lower pH — which makes pH management important for controlling the movement of heavy metals (and potential groundwater contamination) in soil.

Lime requirement — the amount of liming material needed to raise soil pH to a certain level — increases with cation-exchange capacity (CEC). To decrease soil pH, sulfur can be added, which produces sulfuric acid within the soil.

2. Soil Colloids

Soil colloids are the most active constituent of the soil, and they are important because their surfaces attract soil nutrients dissolved in soil water as positively charged mineral ions, or cations.

Some cations are needed for plant growth, including calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), and sodium (Na⁺). These need to be dissolved in a soil-water solution to be available to plants when they are in close contact with root membranes.

The fertility of the soil-water solution for plants is based on the soil’s capability to hold and exchange cations; this is referred to as the cation-exchange capacity (CEC). Without soil colloids, most vital nutrients would be leached out of the soil by percolating water and carried away in streams.

Biological Properties of Soil

Possible framing: “Discuss the role of organic matter and soil organisms in soil formation and fertility, with reference to India.”

Organic matter in the soil improves soil structure and increases the nutrient- and water-holding capacity of the soil. Organic matter also provides a food supply for soil biology. Soils with low organic matter can have a ‘poor’ structure, hold little water, and erode or leach nutrients easily — the exception being cracking clay soils, where clay minerals have the main effect on structure rather than organic content. Soils with high organic matter levels have a ‘good’ structure, good water-holding capacity, and reduced erosion and nutrient leaching.

Biological properties include:

• organic matter
• soil organisms
• the presence of disease-causing organisms

The total role of biotic processes in soil formation includes the presence and activities of living plants and animals, as well as their non-living organic products. Living plants contribute to soil formation in two basic ways:

1. Biomass: the production of organic matter — biomass both above the soil, as stems and leaves, and within the soil, as roots. It provides the raw material of organic matter in the O horizon and in lower horizons. Decomposer organisms process this raw material, reducing it to humus and ultimately to its initial components, carbon dioxide and water.
2. Nutrient Recycling: involves the cycling of nutrients from dead plant tissues back into the soil. Nutrient recycling is a mechanism by which nutrients are prevented from escaping through the leaching action of surplus soil water moving downward through the soil.

Animals living in the soil play an important role in the biological processes of soil formation — for example, earthworms rework the soil not only by burrowing but also by passing soil through their intestinal tracts.

Some of the important factors which decide the biological behaviour of soil are:

• Respiration rate: CO₂ evolution under standard laboratory conditions or in the field.
• Potential N/C mineralisation: increase in mineral nitrogen or carbon content under standard laboratory conditions.
• Earthworms: density of earthworms present in the soil.
• Bacterial biomass: total bacterial biomass for a given soil mass.
• Bacterial diversity: determined by functional groups, or by describing genetic diversity.
• Presence of pathogens: assessed by different pathology techniques, ranging from cultures to DNA profiling.

📊 Indian Case Evidence — Applying These Properties

• Regur (black cotton) soils of the Deccan Trap region derive their dark colour from high organic/humus content combined with basaltic parent material — illustrating the colour–organic-matter link directly.
• Laterite soils of the Western Ghats show reddish colouration from ferric oxide/hydroxide accumulation under intense wet-dry leaching — a textbook case of the “reddish colour indicates iron and good drainage” principle.
• Arid soils of Rajasthan tend toward alkalinity (consistent with the arid-region pH pattern described above), while high-rainfall soils of the Western Ghats and Northeast India tend toward acidity — directly matching the humid-vs-arid pH generalisation.
• Soil Health Card Scheme (Government of India) operationalises exactly these chemical properties — pH, organic carbon, and macro/micro-nutrient levels — as the diagnostic basis for farmer-level fertiliser and lime recommendations.

Conclusion

Soil properties — physical, chemical, and biological — are not independent checklists but a tightly interlinked system: texture governs permeability and structure; structure and colour signal organic-matter and drainage status; pH governs nutrient solubility and, through colloids and cation-exchange capacity, links directly to fertility; and biological activity (earthworms, bacteria, decomposers) both creates and sustains the humus that underlies nearly every favourable physical and chemical property described above. A strong 20-mark answer should treat these three property classes as one continuous causal chain rather than three disconnected lists.