**Soil** is a vital natural resource, serving as the foundation for plant life by providing anchorage, water, and nutrients. It's also a habitat for numerous organisms. Soil's importance extends to agriculture, which supplies us with food, clothing, and shelter, making soil an inseparable part of our lives. The refreshing scent of soil after rain is a familiar experience.

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Soil Teeming With Life

Soil is not just a collection of mineral particles; it is a dynamic environment filled with life.

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Activity 9.1

Observing different soil samples using a hand lens reveals that soil contains not only mineral particles but also various living organisms (like earthworms, ants, beetles) and parts of dead plants and animals (like roots, leaves). The composition and types of organisms found may vary depending on the source of the soil (garden, roadside, construction site).

Table 9.1: Observations of soil samples (Example Structure):

S. No.	Soil source	Plants (observed)	Animals (observed)	Any other observations (e.g., pebbles, plastic pieces)
1.	Garden soil	Grass, small roots	Ant, Earthworm	Dried leaves, small stones
2.	Soil from the roadside	Weeds	Insects	Pebbles, pieces of plastic, paper
3.	Soil from a construction area	Few weeds	None seen	Large stones, cement pieces, metal bits

Soil has numerous uses, from supporting plant growth and providing materials for construction (bricks, cement) to making pottery and toys.

Soil Pollution: Soil can be polluted by various substances. **Polythene bags and plastics** are major soil pollutants as they do not decompose and harm soil organisms. Other pollutants include waste products, chemicals, and excessive use of pesticides. It is crucial to treat waste products and minimise pesticide use to prevent soil pollution.

Soil Profile

Soil is not uniform throughout its depth; it is arranged in distinct horizontal layers called **horizons**. A vertical cross-section showing these layers is called the **soil profile**.

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Activity 9.2

Taking a soil sample, powdering it, and mixing it with water in a glass tumbler demonstrates the layering of soil particles based on size when allowed to settle. The heaviest particles (gravel) settle at the bottom, followed by sand, then clay, and finally silt. Any light, dead organic matter (leaves, animal remains) may float on the surface. This layered structure illustrates the components that make up soil particles.

The floating dead, rotting matter in the soil is called **humus**. Humus is derived from decomposed organic material and is important for soil fertility.

Soil is formed over long periods by the process of **weathering**, which involves the breaking down of rocks into smaller particles by wind, water, and climate.

Each horizon (layer) in the soil profile differs in terms of its texture (how it feels), colour, depth, and chemical composition. The main horizons are:

• **A-horizon (Topsoil):** The uppermost layer. It is usually dark in colour because it is rich in **humus** and minerals. Topsoil is generally soft, porous, and has good water-holding capacity. It provides nutrients and is the habitat for many soil organisms. The roots of small plants grow primarily in this layer.
• **B-horizon (Middle layer):** Located below the topsoil. It contains less humus but more minerals. This layer is generally harder and more compact than the topsoil.
• **C-horizon:** Lies below the B-horizon. It is made up of small lumps of weathered rocks with cracks and crevices.
• **Bedrock:** The layer below the C-horizon, consisting of unweathered, hard rock that is difficult to dig.

Soil profiles can be observed at the sides of recently dug ditches, wells, building foundations, or along roadsides in hilly areas or steep river banks.

Soil Types

Weathering breaks down rocks into small particles of various materials, primarily sand and clay. The proportion of these particle sizes determines the type of soil.

Soil is a mixture of rock particles and **humus**. Living organisms (bacteria, plant roots, earthworms) are also vital components of soil.

Soil is classified based on the relative proportions of different sized particles:

• **Sandy soil:** Contains a greater proportion of **large particles** (sand). These particles do not pack tightly, leaving large spaces filled with air. Sandy soil is well-aerated, light, and water drains through it quickly, making it dry.
• **Clayey soil:** Contains a higher proportion of **fine particles** (clay). These particles pack tightly, leaving little space for air. Clayey soil can hold water in the small gaps between particles, making it heavy as it retains more water than sandy soil, but it has less air. This type of soil is good for making pots and toys.
• **Loamy soil:** Contains a mixture of large particles (sand) and fine particles (clay) in roughly equal amounts, along with another particle type called **silt**. Silt particles are intermediate in size between sand and clay and are often found in riverbeds. Loamy soil also contains **humus**. Loamy soil is considered the **best topsoil for growing plants** because it has a good balance of air, water-holding capacity, and nutrients.

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Activity 9.3

This activity demonstrates how the different properties of soil types (sandy, clayey, loamy) affect their plasticity or workability. By adding water drop-by-drop to samples and trying to knead them, form a ball, roll it into a cylinder, and make a ring, it is observed that clayey soil is highly malleable and can be easily shaped into rings, while sandy soil is difficult to shape, and loamy soil has intermediate shaping ability. This helps identify the soil type based on its ability to be moulded, indicating clayey soil's suitability for pottery.

Properties Of Soil

Soils possess several properties that affect plant growth and other uses, including their interaction with water.

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Percolation Rate Of Water In Soil

**Percolation rate** is the speed at which water passes down through the soil. Different soil types have different percolation rates.

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Activity 9.4

This activity measures the percolation rate of water in a soil sample. A hollow cylinder (pipe) is inserted into the ground, and a measured volume of water (e.g., 200 mL) is poured into it. The time taken for all the water to percolate down through the soil is recorded. The percolation rate is calculated using the formula:

$\textsf{Percolation rate (mL/min)} = \frac{\textsf{Amount of water (mL)}}{\textsf{Percolation time (min)}}$

Comparing percolation rates for different soil samples reveals that **sandy soil** has the **highest percolation rate** (water drains very quickly) and **clayey soil** has the **lowest percolation rate** (water drains very slowly).

The rate of percolation is important for plant growth; too fast (sandy) means roots don't get enough water, too slow (clayey) can lead to waterlogging and lack of air for roots. Loamy soil has a moderate percolation rate suitable for many crops.

Moisture In Soil

Soil contains water, known as **soil moisture**. The amount of moisture in soil can vary.

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Activity 9.5

Heating a sample of soil in a boiling tube demonstrates the presence of moisture. Upon heating, the water present in the soil evaporates and condenses on the cooler upper parts of the boiling tube as water droplets. This indicates that soil contains water.

During hot summer days, the evaporation of moisture from the soil surface causes the air just above the land to shimmer due to the refraction of sunlight through the water vapour. This visible shimmering effect indicates the presence of moisture evaporating from the soil.

Absorption Of Water By Soil

Different soil types have varying capacities to **absorb and retain water**. This property is crucial for plant growth as it determines how much water is available to plant roots.

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Activity 9.6

Measuring the amount of water absorbed by a specific weight of dry soil sample. A known weight of dry soil (e.g., 50g) is placed in a funnel lined with filter paper. Water is slowly added drop by drop using a measuring cylinder until the soil is saturated and water starts dripping from the bottom of the funnel. The volume of water absorbed by the soil is calculated by subtracting the final volume of water remaining in the measuring cylinder from the initial volume.

Volume of water absorbed = (Initial volume - Final volume) mL

Since 1 mL of water weighs approximately 1 g, the weight of water absorbed is equal to the volume in mL.

Percentage of water absorbed can be calculated as:

$\textsf{Percentage water absorbed} = \frac{\textsf{Volume of water absorbed (mL)}}{\textsf{Weight of soil (g)}} \times 100$

Comparing results for different soil types shows that **clayey soil** has the **highest water absorption and retention capacity**, while **sandy soil** has the **least**. Loamy soil has moderate water absorption capacity.

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Water that percolates down through the soil eventually contributes to **groundwater**. Soils with higher percolation rates (like sandy soil) allow water to reach groundwater sources faster, but they don't retain much water in the top layers for immediate plant use. Soils with lower percolation rates (like clayey soil) retain more water, which is good for plants needing a lot of water (like paddy), but can lead to waterlogging if drainage is poor. Loamy soil provides a balance.

To increase the amount of rainwater that percolates into the ground and recharges groundwater, methods like rainwater harvesting (Chapter 14) or increasing vegetative cover can be used, as plants and their roots help water infiltrate the soil.

Soil And Crops

The type of soil in a region, along with climatic factors (wind, rainfall, temperature, light, humidity), determines the type of vegetation and crops that can be grown there. Different crops thrive best in specific soil types that provide the optimal balance of water retention, drainage, and aeration.

• **Clayey and loamy soils** are suitable for cereals like wheat and gram due to their good water retention.
• **Clayey soil** rich in organic matter and good at retaining water is ideal for **paddy (rice)**.
• **Loamy soils** that drain water easily are required for pulses like **lentils**.
• **Sandy-loam or loam soils** that drain easily but can still hold plenty of air are more suitable for **cotton**.

Fine clayey soils rich in humus are very fertile and good for crops like wheat.

Table 9.2: Suitability of soil types for different crops (Example Structure):

S. No.	Type of soil	Crop grown
1.	Clayey	Paddy, Wheat
2.	Loamy	Wheat, Gram, Lentils
3.	Sandy-loam / Loam	Cotton

Case Study: Making Earthen Pots: The traditional craft of making earthen pots (matkas, surahis) uses specific types of soil (often black soil, rich in clay). The soil is cleaned, soaked, and kneaded with materials like horse dung. Horse dung creates pores when burnt during baking, allowing water to percolate and evaporate from the pot's surface, keeping the stored water cool. This demonstrates how soil properties are utilised in crafts.

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Soil Erosion

**Soil erosion** is the process of the land surface being removed by natural agents like water, wind, or ice. Plant roots act as natural binders, holding the soil particles together. In areas with little or no vegetation (like deserts or bare lands), the soil is loose and easily carried away by wind or flowing water, leading to severe erosion.

To prevent soil erosion, it is essential to stop the cutting of trees (**deforestation**) and actively work to increase green areas through planting trees and promoting vegetation cover. Protecting vegetation helps bind the soil and reduces its susceptibility to erosion.

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