Welcome to Composition of Soils
Welcome to your study notes for Unit 1: Soils, Crops and Habitats! Soil is far more than just "dirt" under our boots. It is a living, breathing system that supports crops, grasslands, livestock, and diverse natural habitats across Northern Ireland and the wider UK. Understanding what soil is made of and how it works will give you the tools to succeed in your CCEA GCSE Agriculture and Land Use exam.
Don't worry if scientific terms feel daunting at first — we will break down every concept step-by-step with clear examples, simple calculations, and essential exam tips!
---1. What is Soil Made Of? The Four Key Components
A healthy, balanced agricultural topsoil is made up of four main ingredients. Imagine looking at a fresh scoop of good farmland soil; its volume is divided into solid materials and open pore spaces:
1. Mineral Particles (approx. 45%): These are tiny pieces of rock broken down from bedrock (parent material) through physical, chemical, and biological weathering over thousands of years. They consist of sand, silt, and clay.
2. Organic Matter / Humus (approx. 5%): This is the dark, rich material formed from decaying plant residues (like dead roots and leaf litter), animal wastes, and living micro-organisms. Humus acts like a sponge, helping soil stick together and store vital nutrients.
3. Soil Water / Moisture (approx. 25%): Held in the spaces (pores) between soil particles. Soil water is not just pure water; it is a nutrient solution carrying dissolved minerals that plant roots absorb through their root hair cells.
4. Soil Air / Gases (approx. 25%): Fills the remaining pore spaces. Soil air provides vital oxygen (\(O_2\)) for plant root respiration and beneficial aerobic soil organisms. It also contains carbon dioxide (\(CO_2\)) and nitrogen gas (\(N_2\)).
Memory Trick: Think of the 45 - 5 - 25 - 25 Rule. Half of the soil is solid stuff (45% rock minerals + 5% organic matter), and the other half is open pore space shared equally between air (25%) and water (25%)!
Key Takeaway
In ideal farm soil, half the volume is solid (mineral particles and organic matter) and half is pore space shared equally between air and water.
---2. Soil Particle Classification and Texture
Soil texture depends on the relative proportions of different-sized mineral particles. Scientists and farmers classify mineral particles into three main groups based on diameter:
Sand:
• Particle size: Largest particles, ranging from \(0.05\text{ mm}\) to \(2.0\text{ mm}\).
• Feel: Rough and gritty to touch.
• Characteristics: Sand particles do not fit tightly together, creating large open gaps called macro-pores. Water drains through very quickly, meaning sandy soils warm up fast in spring but dry out rapidly in summer.
Silt:
• Particle size: Medium-sized particles, ranging from \(0.002\text{ mm}\) to \(0.05\text{ mm}\).
• Feel: Smooth, silky, or soapy when wet (like flour or talcum powder when dry).
• Characteristics: Holds more water than sand and feels less gritty, but can pack down easily.
Clay:
• Particle size: Smallest particles, less than \(0.002\text{ mm}\) (\(< 0.002\text{ mm}\)).
• Feel: Sticky and plastic when wet; hard and cloddy when dry.
• Characteristics: Clay particles pack tightly together, creating tiny micro-pores. Clay holds large amounts of water and carries chemical charges on its surface that hold onto plant nutrients electrostatically.
What is Loam?
Loam is considered the ideal soil type for most agricultural crops. It is not a separate mineral, but a balanced mixture containing roughly equal influences of sand, silt, and clay, along with a good supply of organic matter. Loam combines the easy drainage and aeration of sand with the moisture-holding and nutrient-retaining powers of silt and clay.
Key Takeaway
Particle size controls soil texture: Sand (\(0.05 - 2.0\text{ mm}\)) is gritty, Silt (\(0.002 - 0.05\text{ mm}\)) is silky, and Clay (\(< 0.002\text{ mm}\)) is sticky. Loam is the ideal balanced mixture for farming.
---3. Soil Properties: Drainage, Aeration, and pH
Drainage and Permeability
Permeability is how easily water moves downward through the soil profile:
• Sandy soils have high permeability and rapid drainage. However, because water flows through so fast, they are prone to nutrient leaching (the washing away of soluble mineral nutrients, like nitrates, out of reach of plant roots).
• Clay soils have low permeability and slow drainage. They hold lots of water, which makes them prone to waterlogging and compaction (becoming squashed by heavy machinery or livestock).
• Loam soils provide moderate drainage, letting excess water escape while holding plenty of available moisture for plant roots.
Soil Aeration and Respiration
Plant roots are living organs that need oxygen to stay alive. Root cells undergo aerobic respiration to release the energy needed for active uptake of mineral ions:
• In well-aerated soils, pore spaces contain plenty of oxygen (\(O_2\)). Beneficial aerobic bacteria (such as nitrifying bacteria) thrive, breaking down organic matter and cycling nitrogen.
• In waterlogged soils, all the pore spaces fill with water, driving out the air. The soil becomes anaerobic (lacking oxygen). In these conditions, plant roots suffocate, nutrient uptake stops, and harmful processes like denitrification occur, causing valuable nitrogen to be lost into the atmosphere.
Soil pH and Liming
Soil pH measures how acidic or alkaline the soil is on a scale from \(1\) to \(14\):
• Optimum Agricultural pH: For most crops and grassland in Northern Ireland and the UK, the ideal soil pH is between \(6.0\) and \(6.5\) (slightly acidic to near neutral).
• The Problem with Acid Soils (\(\text{pH} < 6.0\)): High acidity locks up essential macronutrients — especially Nitrogen (N), Phosphorus (P), and Potassium (K) — making them insoluble and unavailable for crop uptake.
• Liming: Farmers correct acidic soils by spreading ground agricultural limestone (calcium carbonate, \(CaCO_3\)). Liming raises the pH back to the optimal \(6.0 - 6.5\) range, neutralises soil acidity, frees up essential nutrients, and promotes flocculation (causing tiny clay particles to clump together into crumbly structures that improve drainage and aeration).
Key Takeaway
Good soil must balance moisture and air. An optimum soil pH of \(6.0\) to \(6.5\) maximizes nutrient availability (N, P, K). Acidic soils are treated with agricultural lime (\(CaCO_3\)).
---4. Required Practicals & Mathematical Calculations
In your exam, you will likely be asked to describe laboratory experiments or calculate percentages from practical data. Follow these simple steps and formulas carefully.
Practical 1: Measuring Soil Moisture (Water Content)
Step-by-step method:
1. Weigh an empty evaporating dish, add fresh soil, and record the mass of the fresh soil sample (\(m_1\)).
2. Place the sample in a drying oven set at approximately \(105^\circ\text{C}\). Heat until it reaches a constant mass (weigh, heat again, and reweigh until the mass no longer changes, showing all water has evaporated).
3. Record the final mass of the oven-dried soil (\(m_2\)).
Formula:
\(\text{Percentage Moisture (\%)} = \frac{m_1 - m_2}{m_1} \times 100\)
Worked Example:
A fresh soil sample has a mass of \(50\text{ g}\) (\(m_1 = 50\text{ g}\)). After drying in an oven to constant mass, the soil weighs \(41\text{ g}\) (\(m_2 = 41\text{ g}\)).
Mass of water lost = \(50\text{ g} - 41\text{ g} = 9\text{ g}\)
\(\text{Percentage Moisture} = \frac{9}{50} \times 100 = 18\%\)
Practical 2: Measuring Soil Organic Matter Content
Step-by-step method:
1. Take the completely dry soil sample from Practical 1 and note its dry mass (\(m_2\)).
2. Place the dry soil in a crucible and heat strongly using a roaring blue Bunsen burner flame (or in a muffle furnace) for several minutes. The organic matter burns away as smoke and carbon dioxide.
3. Allow the crucible to cool and reweigh the remaining mineral ash (\(m_3\)). Reheat and reweigh until a constant mass is achieved.
Formula:
\(\text{Percentage Organic Matter (\%)} = \frac{m_2 - m_3}{m_2} \times 100\)
Worked Example:
The dry soil from above weighs \(41\text{ g}\) (\(m_2 = 41\text{ g}\)). After strong heating, the remaining ash weighs \(38.94\text{ g}\) (\(m_3 = 38.94\text{ g}\)).
Mass of organic matter lost = \(41\text{ g} - 38.94\text{ g} = 2.06\text{ g}\)
\(\text{Percentage Organic Matter} = \frac{2.06}{41} \times 100 = 5.02\%\)
Practical 3: Measuring Soil Drainage / Infiltration Rate
Step-by-step method:
1. Set up a filter funnel lined with filter paper (or wire gauze) inside a measuring cylinder.
2. Place a fixed mass of dry soil into the funnel.
3. Pour a known volume of water (e.g., \(100\text{ cm}^3\)) onto the soil and start a stopwatch.
4. Record the volume of water collected in the cylinder at set time intervals, or measure the total time taken for all water to drain through.
Key Takeaway
Always check your numbers! For moisture, divide the mass of water lost by the fresh mass (\(m_1\)). For organic matter, divide the mass lost on burning by the dry mass (\(m_2\)).
---5. Examiner Tips & Common Traps to Avoid
1. Avoid Calculation Traps:
• When calculating Percentage Organic Matter, students often mistakenly divide by the original fresh mass (\(m_1\)) or the final ash mass (\(m_3\)). Always divide by the dry mass (\(m_2\))!
2. Precise Scientific Vocabulary:
• Drainage vs. Leaching: Drainage is the physical downward movement of water through the soil. Leaching is the chemical process where dissolved minerals (nutrients) are washed away with that draining water.
• Permeability vs. Porosity: Porosity is the total volume of pore space in the soil. Permeability is how connected those pores are to let water flow through. (Clay has high porosity but low permeability; sand has lower total porosity but very high permeability).
3. Soil Sampling in the Field (Avoiding Bias):
• In extended writing questions on soil testing, never write "throw a quadrat or auger randomly". Examiners give zero marks for throwing equipment.
• Instead, describe systematic random sampling: "Lay out two tape measures at right angles to create a grid, use a random number generator to create coordinate pairs (X, Y), and take soil core samples at those exact points." This ensures an unbiased, representative sample of the field.
Quick Chapter Summary Checklist
Can you answer these key revision questions? If yes, you are exam ready!
• What are the four components of a balanced soil and their proportions (\(45\%\), \(5\%\), \(25\%\), \(25\%\))?
• Which mineral particle has the smallest size: sand, silt, or clay?
• What is the optimum soil pH for grassland and crops in the UK (\(6.0 - 6.5\))?
• Why do farmers apply agricultural lime (\(CaCO_3\)) to fields?
• How do you calculate percentage moisture and percentage organic matter from experimental data?