AQA A-Level Environmental Science (7447)

## Learning Outcomes

- Define **soil texture**, **soil structure**, **soil fertility** and **water-holding capacity**
- Explain how the proportions of sand, silt and clay affect drainage, aeration and nutrient retention
- Compare sandy, clay and loam soils in terms of fertility and plant growth
- Understand the roles of humus, soil organisms, pH, air and water in fertile soil
- Calculate soil water content using a simple gravimetric method
- Apply soil science to exam questions on fertility, erosion and agricultural productivity

## AQA A-Level Environmental Science (7447) Syllabus

For the AQA A-Level Environmental Science (7447), you are expected to understand soils within the physical environment, with a focus on the following syllabus points:

![Soil texture, structure and fertility, including effects of sand, silt, clay, humus, air and water on plant growth.](https://static.pastpaperhero.com/images/aqa-a-level-environmental-science-7447-physical-environment-soils_00.png)

_Soil properties are linked to drainage, aeration, nutrient retention and water availability in sandy, clay and loam soils._

- Know that **soil texture** refers to the relative proportions of **sand, silt and clay**
- Identify how texture affects **drainage**, **permeability**, **aeration**, **water content** and **nutrient retention**
- Explain the difference between **soil texture** and **soil structure**
- Understand how **humus**, soil organisms, pH and water availability affect **soil fertility**
- Compare typical properties of **sandy**, **clay** and **loam** soils
- Understand that high total water-holding capacity does not always mean the highest water availability to plants
- Use simple practical methods to investigate **soil texture** and **soil water content**
- Understand how poor soil management can increase **leaching** and **soil erosion**

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is meant by soil texture?
2. Why do sandy soils usually lose nutrients more quickly than clay soils?
3. How is soil structure different from soil texture?
4. Why can a clay soil hold a lot of water but still not always support the best plant growth?
5. How can humus improve both soil fertility and resistance to erosion?

## Introduction

Soil is much more than crushed rock. It is a mixture of mineral particles, water, air, dead organic matter and living organisms. In Environmental Science, soils matter because they control plant growth, food production, water movement and nutrient cycling.

In exam questions, soils are often tested through links between physical properties and real outcomes. You may need to explain why one soil is more fertile than another, why crops grow better in loam than pure sand, or how farming increases erosion risk. Clear definitions are vital because students often mix up texture, structure and water retention.

> **Key Term: soil fertility**
> The ability of a soil to support plant growth through suitable nutrient availability, water supply, aeration and other soil conditions.

### Soil components and texture

The mineral part of soil comes from weathered rock. The main particle-size groups are **sand**, **silt** and **clay**. Their relative proportions make up the soil’s texture.

> **Key Term: soil texture**
> The relative proportions of sand, silt and clay particles in a soil.

Sand has the largest particles, silt is intermediate, and clay has the smallest particles. This matters because particle size controls pore size. Large pores allow water to move quickly through soil. Small pores slow water movement and hold water more strongly.

A soil with a high proportion of sand usually has:

- rapid drainage
- high aeration
- low water-holding capacity
- greater nutrient loss by leaching
- easier root penetration

A soil with a high proportion of clay usually has:

- slow drainage
- poorer aeration if waterlogged
- high total water-holding capacity
- higher nutrient retention
- more resistance to root penetration

A **loam** is a balanced mixture of sand, silt and clay. It is often best for plant growth because it combines good drainage with reasonable water and nutrient retention.

> **Key Term: loam**
> A soil with a balanced mix of sand, silt and clay, giving a good balance of drainage, aeration and water retention.

A common exam error is to define texture by saying “how much water a soil holds”. That is not the definition. Water-holding capacity is an effect of texture, not texture itself.

> **Test Tip:** If a question asks about **texture**, always mention **sand, silt and clay proportions** or named textural types such as sandy, clay or loam. Do not drift into general comments about “good soil”.

### Structure, humus and living components

Texture is not the same as structure. Structure describes how particles are arranged into aggregates, called peds.

> **Key Term: soil structure**
> The arrangement of soil particles into aggregates or peds.

Crumb structure is usually best for fertility. Small rounded peds allow good drainage, aeration and root penetration. Platy structure slows drainage and reduces air spaces.

Dead organic matter is another major soil component. As it is broken down, it forms humus.

> **Key Term: humus**
> The complex organic material formed by decomposition of dead matter, which helps bind soil, retain water and store nutrients.

Humus improves soil in several ways:

- binds particles together, reducing erosion
- helps retain water
- improves drainage in some soils
- provides nutrients during decomposition
- insulates soil, reducing temperature swings

Soil organisms are also essential. Detritivores such as earthworms break dead material into smaller pieces. Decomposers such as bacteria and fungi complete breakdown and release nutrients. Soil organisms also create channels that improve infiltration and aeration.

> **Key Term: decomposers**
> Micro-organisms, mainly bacteria and fungi, that break down dead organic matter and release nutrients.
>
> **Key Term: detritivores**
> Organisms that feed on dead organic matter, breaking it into smaller pieces for decomposers.

Air in soil fills pore spaces not occupied by water. This air provides oxygen for aerobic organisms. Good aeration supports rapid decomposition and nutrient cycling. If oxygen is lacking, anaerobic conditions develop. Then decomposition is incomplete, methane may be produced, and soils can become acidic and less suitable for nutrient uptake.

### Water, nutrients and plant-available water

Plants need soil water for transport, support and photosynthesis. Roots can only absorb most mineral nutrients when they are dissolved in water. This is why completely dry soils are infertile, even if nutrients are present.

> **Key Term: leaching**
> The downward removal of soluble nutrients from soil by percolating water.

Sandy soils are especially vulnerable to leaching because water drains through them quickly. Clay soils retain more nutrients because clay particles have surface charges that attract nutrient ions.

However, more water is not always better. Waterlogged soils have low oxygen content, which harms roots and slows aerobic decomposition. Also, some water in clay soils is held so tightly that plants cannot easily extract it.

Useful water terms:

> **Key Term: field capacity**
> The amount of water remaining in soil after excess gravitational water has drained away.
>
> **Key Term: permanent wilting point**
> The soil moisture level at which plants can no longer extract enough water and remain wilted.

Plant-available water is the difference between field capacity and permanent wilting point. This explains why a clay soil can hold a lot of total water but not necessarily provide the most water to plants. Loam often gives a better balance.

Soil pH also affects fertility. Very acidic or very alkaline conditions can damage root proteins. Many plants grow best in slightly acidic soils, often around **pH 5.5 to 6.5**. Low pH may increase nutrient solubility, but can also reduce uptake by roots.

> **Test Tip:** In “explain fertility” questions, link every point to plant growth. For example: “low water causes stomata to close, so carbon dioxide uptake falls, so photosynthesis and growth decrease.”

### Comparing soils and applying the science

A good exam answer often compares soil types directly.

| Property               | Sandy soil | Clay soil         | Loam             |
| ---------------------- | ---------- | ----------------- | ---------------- |
| Particle size          | Large      | Very small        | Mixed            |
| Drainage               | Rapid      | Slow              | Moderate         |
| Aeration               | Good       | Often poor if wet | Good             |
| Water-holding capacity | Low        | High              | Moderate to high |
| Nutrient retention     | Low        | High              | Good             |
| Leaching risk          | High       | Lower             | Moderate         |
| Root penetration       | Easy       | More difficult    | Good             |

Loam is often the most productive agricultural soil because it avoids the extremes. Sandy soil may dry out and lose nutrients. Clay soil may become compacted or waterlogged.

Soil management can change these properties. Loss of vegetation, ploughing, falling humus levels and compaction from machinery all increase erosion risk. Heavy rain can dislodge particles; runoff may then remove them, especially on steep slopes. Wind erosion is also worse where soil is dry, loose and exposed.

Long-term crops such as grass reduce erosion because soil is disturbed less often and roots hold the soil together. This links soil science to wider Environmental Science topics such as agriculture and pollution.

### Worked example: soil water content

A common practical method is the gravimetric measurement of soil water content.

Formula:

$$
\text{Soil water content (\%)} = \frac{\text{mass of water lost}}{\text{dry soil mass}} \times 100
$$

Where:

$$
\text{mass of water lost} = \text{wet soil mass} - \text{dry soil mass}
$$

Worked example:

- Mass of empty container = **20.0 g**
- Mass of container + wet soil = **68.0 g**
- Mass of container + dry soil = **60.0 g**

Step 1: Find wet soil mass  
Wet soil mass = 68.0 - 20.0 = **48.0 g**

Step 2: Find dry soil mass  
Dry soil mass = 60.0 - 20.0 = **40.0 g**

Step 3: Find mass of water lost  
Mass of water lost = 48.0 - 40.0 = **8.0 g**

Step 4: Calculate percentage water content

$$
\frac{8.0}{40.0} \times 100 = 20\%
$$

So the soil water content is **20%**.

Common student errors:

- using wet mass instead of dry mass in the denominator
- forgetting to subtract the container mass
- not giving the final answer as a percentage

If the exam gives data from two soils, link the result back to texture. A higher water content often suggests more clay or more organic matter, but not always.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Soil contains mineral particles, water, air, organic matter and living organisms
- Soil texture means the proportions of sand, silt and clay
- Sandy soils have large pores, rapid drainage, high aeration and high leaching risk
- Clay soils have small pores, slow drainage, high total water-holding capacity and good nutrient retention
- Loam gives a useful balance of drainage, aeration and retention
- Soil structure describes the arrangement of particles into peds and is different from texture
- Humus improves water retention, soil binding, nutrient supply and erosion resistance
- Soil air is needed for aerobic decomposition and nutrient cycling
- Water is needed for nutrient uptake because roots absorb most nutrients in solution
- High total water-holding capacity is not the same as highest plant-available water
- Slightly acidic soils are often best for plant growth, commonly around pH 5.5 to 6.5
- Soil water content can be measured by drying a soil sample and calculating mass loss

## Key Terms and Concepts

- soil fertility
- soil texture
- loam
- soil structure
- humus
- decomposers
- detritivores
- leaching
- field capacity
- permanent wilting point


Physical environment - Soils