Agriculture (0600) | Soil types, composition, texture and temperature

🌱 Welcome to the Foundation: Understanding Soil Structure!

Hello future agriculturalists! This chapter is absolutely critical because the soil is the engine of your farm. If you understand what soil is made of and how its physical properties work, you can manage it to grow bigger, healthier crops.
Don't worry if terms like 'capillary water' or 'soil pan' sound confusing—we'll break them down using simple analogies so you can master them easily!

1. The Soil Profile: Looking Beneath the Surface (2.2a)

Imagine slicing a cake vertically. The different layers you see are similar to the layers in the soil, known as the Soil Profile. We usually focus on three main sections:

Topsoil (A-Horizon)

• This is the dark, uppermost layer, typically 10–30 cm deep.
• It is rich in organic matter (humus), making it dark and fertile.
• It contains most of the plant roots and living soil organisms.
• This is the most important layer for crop production!

Subsoil (B-Horizon)

• Lighter in colour than the topsoil.
• Contains less organic matter.
• Often contains minerals that have been washed down (leached) from the topsoil.
• It is usually denser and has poorer aeration than the topsoil.

Underlying Materials (C/R Horizons)

• The lowest layer, consisting mainly of partially weathered rock fragments or unconsolidated sediment.
• This material is the Parent Material from which the soil above was originally formed through weathering.

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2. Soil Constituents: What is Soil Made Of? (2.2e)

Soil is a complex mixture. It has five essential ingredients (or constituents) that must be in balance for healthy plant growth:

(a) Mineral Matter (The Solid Base)

• Makes up about 45% of the soil volume.
• These are small rock particles (sand, silt, and clay) created by the weathering of parent material.
• They provide essential mineral nutrients (like Potassium, Calcium, etc.) for plants.

(b) Organic Matter (The Nutrient Sponge)

• Makes up about 5% of the soil volume (though ideally, it should be higher in the topsoil).
• Consists of decaying plants, animals, and microorganisms, forming a stable substance called Humus.
• Importance: Improves soil structure, holds water and nutrients, and provides food for living organisms.

(c) Soil Water (The Lifeblood)

Water makes up about 25% of the soil volume and is crucial for transporting nutrients into the plant. There are three types of water found in the soil pores:

1. Free or Gravitational Water: This water fills large pores immediately after heavy rain or irrigation. Gravity pulls it quickly down and out of the root zone (drainage). Plants cannot use this water easily; too much causes waterlogging.
2. Capillary Water: This water is held in the medium-sized pores (like tiny tubes) against the force of gravity. This is the main source of water used by plants.
3. Hygroscopic Water: This water is held tightly as a thin film around the individual soil particles. It is so tightly bound that roots cannot absorb it. It is generally unavailable to plants.

(d) Soil Air (The Breathing Space)

• Makes up about 25% of the soil volume (usually filling the spaces not occupied by water).
• Air is essential as it supplies Oxygen for root respiration and for soil organisms (like bacteria) to break down organic matter.
• Common mistake: If a soil is waterlogged, air is pushed out, leading to poor root growth and suffocation.

(e) Living Organisms (The Workers)

• Includes microscopic organisms like bacteria and fungi (which break down organic matter), and larger organisms like nematodes and earthworms.
• Earthworms are particularly important: they mix the soil, create channels for aeration and drainage, and produce nutrient-rich castings.

3. Soil Texture: Particle Size Matters (2.2b)

Soil Texture describes the relative proportion of the different sizes of mineral particles in the soil. Texture is a permanent property—you cannot easily change it.

Think of soil particles as balls:

Particle Type	Relative Size	Feel	Key Characteristics
Sand	Largest	Gritty, Rough	Large pores, fast drainage, poor water/nutrient retention.
Silt	Medium (like flour)	Smooth, Silky	Good water retention, easily washed away by erosion.
Clay	Smallest	Sticky when wet, hard when dry	Very small pores, slow drainage, high nutrient/water retention.

Memory Aid: Use the phrase Some Soil Can to remember the order of size: Sand > Silt > Clay.

4. Soil Structure: The Arrangement of Particles (2.2c)

While texture is *what* the soil is made of, Soil Structure is *how* those particles (sand, silt, clay) are grouped together into larger clusters called aggregates.

The Ideal Structure: Good Crumb Structure

The best structure for farming is the crumb structure.

• In a crumb structure, soil particles are clumped together into small, rounded aggregates (like bread crumbs).
• These crumbs create both large and small spaces (pores) in the soil.
• Why it's important: It ensures excellent drainage (via large pores) and good water holding capacity (via small pores), as well as aeration for roots.

Maintaining Good Structure (The Role of Humus)

To maintain a good crumb structure, you must manage organic matter (humus). Humus acts like a glue, binding the mineral particles into stable crumbs. Adding manure, compost, or crop residues helps maintain this structure.

Dangers to Soil Structure

Poor farming practices or weather can destroy the crumb structure, leading to problems:

1. Oxidation of Organic Matter: When soil is excessively cultivated (ploughed/dug too much), oxygen mixes in rapidly. This speeds up the decay of organic matter by microorganisms (oxidation). This loss of 'glue' (humus) causes the soil structure to collapse.
2. Capping (Surface Sealing): If the crumb structure at the surface is destroyed (often by heavy rain hitting bare soil), the fine particles (silt and clay) settle together, forming a hard, dense layer or "cap" when the soil dries. This prevents seedlings from emerging and stops water infiltration.
3. Soil Pans (Compaction): These are dense, impermeable layers that form deeper down in the subsoil due to heavy machinery or repeated ploughing at the same depth. Soil pans restrict root growth and prevent water from draining away, leading to waterlogging.

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5. Describing Different Soil Types (2.2d)

Farmers classify soils based on their texture, as this determines the soil's major properties—especially its water-holding capacity and drainage.

(a) Sandy Soils

• Texture: Predominantly sand particles (large).
• Drainage: Excellent (water moves through very quickly).
• Water-Holding Capacity: Poor. Water and nutrients are easily leached out.
• Temperature: Warms up quickly in spring (due to low water content).
• Nickname: "Light" soil (easy to cultivate).

(b) Clay Soils

• Texture: Predominantly clay particles (tiny).
• Drainage: Very poor (small pores restrict water movement). Easily waterlogged.
• Water-Holding Capacity: Very high. Can hold a lot of water, but much of it is held too tightly for plants to use.
• Temperature: Slow to warm up in spring (due to high water content).
• Nickname: "Heavy" soil (difficult to cultivate when wet or dry).

(c) Loam Soils

• Texture: An ideal mixture of sand, silt, and clay (usually about 40% sand, 40% silt, 20% clay).
• Properties: Excellent balance of large and small pores.
• Drainage & Capacity: Drains well, but still retains enough capillary water for plant needs.
• Loam soil is considered the most productive soil type for general agriculture.

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6. Soil Temperature (2.2f)

Soil temperature is a crucial environmental factor. It directly affects the rate at which plants grow and take up nutrients.

Influence on Plant Growth

• Rate of Growth: Chemical reactions (including photosynthesis and respiration in roots) happen faster at warmer temperatures (up to an optimal limit). Warmer soil means faster nutrient absorption and root development.
• Microbial Activity: The beneficial bacteria and fungi that release nutrients from organic matter work faster when the soil is warm.

Dangers of Extreme Temperatures

Temperature extremes can cause major problems:

• Danger of Excessive Heat: Young, tender seedlings have shallow roots and stems. Extremely hot soil (especially dark topsoil exposed to direct sun) can damage or kill these seedlings instantly.
• Danger of Frost: Low temperatures and frost can damage or kill susceptible crops by freezing the water inside their tissues, causing cell rupture. Frost can also slow or halt seed germination.

Methods for Reducing Effects of Extreme Temperatures

Farmers use physical barriers to protect crops from rapid temperature changes:

1. Mulching of Seedbeds: Applying a layer of material (like straw, grass cuttings, or plastic sheets) to the soil surface.
   • Effect: Mulch acts as an insulator. It prevents rapid heating during the day (protecting against excessive heat) and slows heat loss at night (protecting against frost/cold). It also conserves water.
2. Shading of Transplanted Seedlings: Providing shade (using shade nets, palm leaves, or temporary structures) for newly transplanted seedlings.
   • Effect: Shading reduces the direct intensity of the sun, preventing the soil and the plant tissues from overheating during the establishment phase, reducing transplant shock.

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You have now mastered the physical aspects of soil! Remember, a farmer who understands the structure and composition of their soil is equipped to make the best decisions about tillage, water management, and planting. Keep up the great work!