Agriculture

 Agriculture is the production of food by growing crops and raising

livestock.
 It meets the most basic human needs by providing food.
 However, agriculture can harm the environment through:
o Displacement of native plants
o Destruction of wildlife habitats
o Soil erosion
o Pesticide pollution

Types of Agriculture

1. Crop Farming – Uses plant photosynthesis to produce grain, fruit,

and fiber.
2. Livestock Farming – Involves raising domesticated animals for
meat, milk, and other products.

Pesticides and Agriculture

 Pesticides, including insecticides and herbicides, are essential in

modern farming.
 They help control insects, weeds, and other pests.
 However, pesticides also pose risks because they are designed to
harm living organisms:
o They can negatively affect humans, animals, and the
environment.
 Despite the risks, pesticides benefit society by:
o Killing disease-causing organisms
o Protecting crops from harmful pests

SOIL
Definition & Importance:

 Soil is a thin layer of weathered minerals and organic matter that

supports plant growth.
 It is essential for agriculture and crucial for humans and most land
organisms.
 Despite being a thin layer compared to Earth’s total size, it produces
most of the food we consume.
  A nation’s most valuable asset is good soil combined with a suitable
climate for farming.

Role in Pollution & Environment:

 Soil absorbs pollutants like particulate matter from power plant

smokestacks.
 Fertilizers, pesticides, and other chemicals used in farming can lead
to water and air pollution.
 Soil plays a key role in environmental chemical cycles.

Formation & Structure:

 Soil forms through the weathering of rocks due to geological,

hydrological, and biological processes.
 It contains pores and is arranged in layers (horizons) due to water
movement and biological activity.
 Organic matter in soil grows and decays, contributing to its
continuous formation.
 Soil is an open system, constantly exchanging matter and energy
with the atmosphere, hydrosphere, and biosphere.

Nature and Composition of Soil

 Soil Composition: Soil is a mixture of minerals, organic matter,

water, and air, essential for plant growth.
 Microorganisms: It hosts a variety of bacteria, fungi, and small
animals like earthworms, which contribute to soil fertility.
 Air Spaces: Soil contains spaces filled with air and water, which are
crucial for plant roots and microbial life.

Figure 18.2 – Soil Profile

 Displays different soil horizons (layers):

o A horizon (topsoil): Rich in organic matter, where plants grow.
o B horizon (subsoil): Contains minerals and clay, supporting
root growth.
o C horizon (weathered parent rock): Forms the transition to
bedrock.
o Bedrock: The solid rock foundation beneath the soil.
 Illustrates how soil layers differ in texture, composition, and function.
 Soil texture
 The United Classification System (UCS)
 Gravels (2–60 mm)
 sands (0.06–2 mm)
 silts (0.06-0.006 mm)
 clays (less than 0.002 mm)

Water and Air in Soil

 Plants need a large amount of water for growth and production.

 Transpiration is the process where water moves from plant leaves
into the atmosphere.
 Water availability to plants is influenced by capillary forces (which
help water move through tiny pores) and gravitational forces (which
pull water downward).
 Nutrient availability in water depends on concentration gradients
(differences in nutrient levels) and electrical potential gradients
(differences in charge affecting nutrient movement).
 Water held in smaller soil pores or between clay layers is held
more strongly and is harder for plants to absorb.
 Soils rich in organic matter hold more water, but it is less available
to plants because organic matter physically and chemically binds
water, making it harder to extract.

Q: Discuss in detail the effects of soil water saturation (waterlogging)

on the physical, chemical, and biological properties of soil. Explain
how oxygen depletion, redox conditions, metal ion solubility, and
organic matter decay impact soil structure, plant growth, and nutrient
absorption. Support your answer with relevant chemical equations.

Answer:

When soil becomes waterlogged (saturated with water), it undergoes

significant physical, chemical, and biological changes that negatively
affect its structure, oxygen availability, redox conditions, and plant growth.
 1. Physical Effects:

 In waterlogged soil, excess water fills the soil pores, displacing air.
 The bonds that hold soil colloidal particles together are broken.
 This destroys soil structure, making it unsuitable for plant growth.
 As a result, crops (except rice) struggle to grow in waterlogged
conditions.

2. Oxygen Depletion & Its Effects:

 Oxygen is rapidly consumed by microorganisms that degrade

organic matter.
 In well-aerated soil, roughly 35% of the volume consists of air-filled
pores.
 Normally, the atmosphere at sea level contains 21% O₂ and 0.03%
CO₂ by volume.
 However, in waterlogged soil, due to microbial respiration, oxygen
can drop to as low as 15%, while CO₂ levels increase.

Organic Matter Decay (Respiration Reaction):

The decay of organic matter follows the reaction:
This process reduces oxygen levels (as low as 15%) and increases
CO₂ levels in soil air.
Higher CO₂ increases the equilibrium level of dissolved CO₂ in
groundwater, lowering soil pH and promoting weathering of carbonate
minerals like calcium carbonate (CaCO₃).
This also shifts the equilibrium process by which plant roots
absorb metal ions.
3. Redox Conditions & Chemical Effects:

 Waterlogging creates a more reducing environment, lowering pE

(electron activity) due to bacterial catalysts and organic reducing
agents.
 This shift causes certain metal ions, such as Fe²⁺ and Mn²⁺, to
dissolve in higher concentrations, which can be toxic to plants.
Reduction Reactions in Waterlogged Soil:

 These reactions illustrate how Mn⁴⁺ (from MnO₂) and Fe³⁺ (from
Fe₂O₃) are reduced to their more soluble Mn²⁺ and Fe²⁺ forms,
which can accumulate to toxic levels.

4. Impact on Plant Growth & Nutrient Absorption:

 The lack of oxygen and increased solubility of Fe²⁺ and Mn²⁺ can
damage plant roots.
 Additionally, the lower pH caused by increased CO₂ affects nutrient
availability and root ion absorption.
 The process disrupts nutrient uptake, making it harder for plants to
access essential minerals.

Conclusion:

Waterlogging severely affects soil structure, oxygen availability, redox

balance, and plant growth. The increased CO₂ levels, lower pH, and
higher solubility of toxic Fe²⁺ and Mn²⁺ make it difficult for most crops
(except rice) to survive. The breakdown of organic matter and redox shifts
further alter soil chemistry, making waterlogged conditions harmful to
agriculture.

Inorganic Components of Soil (Simplified

Explanation)
 Soil contains inorganic components that come from the weathering
(breakdown) of rocks and minerals.
 This process forms inorganic colloids, which store water and plant nutrients
and release them when needed.
 The Earth’s crust is mainly made up of oxygen, silicon, aluminum, iron,
calcium, sodium, potassium, and magnesium.
 Since silicon and oxygen are the most abundant, most soil minerals are made
up of these two elements.
 Organic Matter in Soil (Simplified Explanation)

 Soil contains less than 5% organic matter, but it plays a key role in
soil productivity.
 Organic matter acts as a food source for microorganisms,
supporting soil life.
 It undergoes chemical reactions like ion exchange, which helps in
nutrient availability.
 It also affects soil structure by improving water retention and
aeration.
 Some organic compounds contribute to mineral weathering, a
process that helps form soil.

Example of Mineral Weathering Reaction:

 This reaction shows how organic acids break down minerals,

releasing nutrients into the soil for plants.
 Factors Affecting Organic Matter Accumulation

1. Temperature:
o In colder climates, organic matter does not break down
quickly because microbial activity slows down.
o This leads to a buildup of organic matter in the soil.
2. Oxygen Availability:
o In waterlogged soils, decaying plants cannot access oxygen
easily.
o This causes organic matter to accumulate, sometimes
reaching 90% in areas with continuous plant growth and
decay in water-saturated soil.

Soil Humus – Simplified Explanation

What is Soil Humus?

 Humus is the most important organic component of soil.

 It consists of two parts:
1. Soluble fraction – Humic and Fulvic acids
2. Insoluble fraction – Humin

How is Humus Formed?

 It is the residue left after bacteria and fungi biodegrade plant

material.
 Most plant material is made up of:
o Cellulose (easily broken down)
o Lignin (harder to break down, contains more carbon)

Lignin in Humus

 Lignin is a complex polymer with:

o Aromatic rings (ring-shaped carbon structures)
o Alkyl chains (carbon chains)
o Methoxyl (-OCH₃) and hydroxyl (-OH) groups
 These structures remain in humus, giving it unique properties.
Humus and Its Properties

 Similar to lignin, but with more carboxylic acid (-COOH) groups.

 It has both:
o Nonpolar (hydrophobic) parts – repel water
o Polar (hydrophilic) parts – attract water
 Such molecules are called amphiphiles, meaning they can interact
with both water and oils.
 In soil, humus forms small colloidal particles, with the nonpolar
part inside and the polar groups outside.

Nitrogen/Carbon (N/C) Ratio and Humification

What is the N/C Ratio?

 The Nitrogen to Carbon (N/C) ratio shows how much nitrogen is

present compared to carbon in organic material.
 This ratio changes during the transformation of plant biomass into
humus.

How the N/C Ratio Changes During Humification

 Fresh plant material starts with an N/C ratio of about 1:100.

 Microorganisms break down organic carbon to produce CO₂ for
energy.
 At the same time, bacteria add nitrogen to the decaying material.
 By the end of the process, the N/C ratio becomes 1:10.

Why is This Important?

 The decrease in the N/C ratio means that humus contains more
nitrogen than fresh plant material.
 This makes humus nutrient-rich and beneficial for soil fertility.
 Importance of Humus – Simplified Explanation

Humus plays a crucial role in improving soil quality. Here’s why it is

important:

1. Helps Retain Nutrients

o Humic substances bind metal ions, keeping essential
micronutrients available for plants.
2. Acts as a Soil Buffer
o Because humus has acid-base properties, it helps maintain a
stable soil pH.
3. Increases Water Holding Capacity
o Soil with more humus can hold more water, keeping it moist
for plants.
4. Improves Soil Structure
o Humus helps bind soil particles together, making the soil
more stable and preventing erosion.
5. Enhances Sorption
o Humic substances help the soil absorb organic compounds,
improving nutrient retention.
o They also trap heavy metal ions (polyvalent cations), reducing
their toxicity.

The Soil Solution – Simplified Explanation

What is the Soil Solution?

 The soil solution is the liquid part of the soil.

 It contains dissolved chemicals from soil processes and interactions
with water (hydrosphere) and living organisms (biosphere).
 It plays a key role in:
o Transporting nutrients to and from soil particles.
o Providing water for plant growth.
o Helping plants absorb essential nutrients through their
roots.
 Acid-Base and Ion Exchange Reactions in Soil
Cation Exchange Capacity (CEC)

 CEC (Cation Exchange Capacity) is the soil’s ability to exchange

cations (positively charged ions).
 It is measured in milliequivalents (meq) per 100g of soil (dry
weight).
 Why is CEC important?
o It helps supply potassium (K⁺), calcium (Ca²⁺), magnesium
(Mg²⁺), and trace metals to plants.
o When plant roots absorb these nutrients, hydrogen ions (H⁺)
replace them in the soil.
o Over time, this process makes the soil more acidic.

Soil Acidity Increase

 Leaching (washing away) of calcium and magnesium by water

containing carbonic acid (H₂CO₃) makes the soil more acidic:

Formation of Acid Sulfate Soils ("Cat Clays")

 Acid sulfate soils form when pyrite (FeS₂) oxidizes in the presence
of air and water:

This reaction produces highly acidic soils (pH as low as 3.0) and
toxic hydrogen sulfide (H₂S).

 Toxic effects:
o H₂S is harmful to citrus plant roots.
o Aluminum ions (Al³⁺) released in acidic soil are very toxic to
plants.
 How to Adjust Soil Acidity?
If Soil is Too Acidic

 Solution: Add lime (CaCO₃ - calcium carbonate) to neutralize

excess acid:

If Soil is Too Alkaline (Basic)

 Problem: In dry areas, soils may become too alkaline due to basic
salts like Na₂CO₃.
 Solution: Add aluminum sulfate (Al₂(SO₄)₃) or iron sulfate
(Fe₂(SO₄)₃), which release acid when mixed with water:

Another method: Add sulfur (S), which bacteria convert into

sulfuric acid (H₂SO₄) to lower pH:

By using lime for acidic soils and sulfur for alkaline soils, farmers can
maintain the best soil conditions for plant growth

Nature of Solids in the Geosphere

 The Earth has different layers: the solid, iron-rich inner core, molten outer core,
mantle, and crust.
 Environmental chemistry focuses mainly on the lithosphere, which is the outer
part of the mantle and the crust.
 The crust is the Earth's outermost layer, which is thin compared to the Earth's
total diameter, ranging from 5 to 40 km thick.
 Rocks are made of minerals. A mineral is a naturally occurring, inorganic solid
with a specific internal crystal structure and chemical composition.
 A rock is a solid, cohesive mass of one mineral or an aggregate of two or more
minerals.
ROCK CYCLE
 1. Formation of Sedimentary Rock

 Igneous or metamorphic rock breaks down due to weathering and

erosion.
 Small rock particles settle through sedimentation (in rivers, lakes, or
oceans).
 Over time, these layers compact and harden through lithification,
forming sedimentary rock.

2. Formation of Metamorphic Rock

 When sedimentary rock (or igneous rock) is buried deep

underground, it experiences heat and pressure.
 This causes the rock to deform and recrystallize, turning into
metamorphic rock.

3. Formation of Igneous Rock

 If metamorphic rock melts due to extreme heat, it forms magma.

 When this magma cools and solidifies, it forms igneous rock.
 This cooling can happen inside the Earth (intrusive igneous rock)
or on the surface from volcanic eruptions (extrusive igneous rock).

4. Rock Cycle Continuation

 Igneous rock can break down into sediments, starting the cycle
again.
 Sedimentary and igneous rock can transform into metamorphic
rock under heat and pressure.
 Any rock type can melt and crystallize, forming new igneous rock.

This continuous cycle reshapes Earth’s rocks over millions of years!

 Geochemistry and Weathering
Geochemistry

✔ Geochemistry studies chemical species, reactions, and processes in

the lithosphere and their interactions with the atmosphere and
hydrosphere.
✔ Environmental geochemistry focuses on how rock, water, air, and life
systems influence the chemical characteristics of the Earth's surface.

Physical Aspects of Weathering

✔ Rocks weather faster when there are extreme physical changes, such
as:

 Freezing and thawing cycles

 Alternating wet and dry conditions
✔ Mechanical processes that contribute to weathering:
 Swelling and shrinking of minerals due to hydration and
dehydration
 Plant roots growing through cracks in rocks, causing breakage
✔ Higher temperatures increase the rate of chemical weathering.

Chemical Weathering

✔ Chemical weathering happens as rock, water, and minerals try to reach

equilibrium.
✔ It occurs through chemical reactions like:

 Dissolution and precipitation

 Acid-base reactions
 Complexation
 Hydrolysis
 Oxidation-reduction
✔ Weathering is very slow in dry air but happens much faster in water.
✔ Water is a key chemical agent because it:
 Helps dissolve and transport weathering agents
 Brings these agents into contact with rock surfaces at the molecular and
ionic level
 Step-by-Step Process of Groundwater Recharge

1️⃣ Precipitation (Rainfall) 🌧️

 Water from rain, snow, or other sources falls to the ground.

 Some water is absorbed by plants, while some evaporates back into
the atmosphere.

2️⃣ Soil Zone 🌱

 The upper layer of the soil where plants absorb water through their
roots.
 Some water remains in the soil, while the rest moves downward.

3️⃣ Unsaturated Zone 🌍

 Below the soil zone, this layer has both air and water in the pores.
 Water continues moving downward due to gravity.

4️⃣ Capillary Fringe 💧

 A thin layer just above the water table where water is pulled up by
capillary action.
 This helps keep some moisture available for plants.

5️⃣ Water Table 📏

 The boundary between the unsaturated zone and the saturated

zone.
 The water table level changes depending on rainfall and water usage.

6️⃣ Saturated Zone (Groundwater) 🚰

 The lower layer where all spaces in the soil are completely filled
with water.
 This stored water is called groundwater, which can be used for
drinking, irrigation, and other purposes.

7️⃣ Recharge to Water Table ⬇️

  Rainwater seeps down and replenishes the groundwater supply,
maintaining the water cycle.

This process explains how rainwater moves through the soil and adds to
groundwater storage!

This image represents different zones of water movement in soil and

underground layers.
1️⃣ Land Surface 🌿

 The topmost layer where plants grow.

 Water enters through rainfall or other sources.

2️⃣ Zone of Aeration (Unsaturated Zone) 🌍

 This area contains air and water in the spaces between soil
particles.
 It is divided into different parts:
o Belt of Soil Water – Water that is available for plants.
o Intermediate Vadose Water – Water moving downward toward
the water table.
o Capillary Fringe – A thin layer just above the water table
where water moves upward due to capillary action.

3️⃣ Water Table 📏

 The boundary between the unsaturated zone and the saturated

zone.
 Below this level, all the spaces in the ground are completely filled with
water.

4️⃣ Zone of Saturation (Groundwater or Phreatic Water) 💧

 This zone contains groundwater, which is fully stored in rock and soil
pores.
 It is an important source of drinking water, irrigation, and
underground water flow.

5️⃣ Internal Water and Zone of Rock Fracture ⛰️

 The deepest zone where water is stored in rock fractures and

underground layers.
 Water here moves very slowly and may not be easily accessible.

Summary

This diagram explains how water moves from the surface through
different underground layers before reaching groundwater storage. This
process is essential for maintaining the water cycle and recharging underground water sources.