AP Environmental Science

## Learning Outcomes

- Understand why soil testing begins with representative sampling rather than random digging
- Identify common methods for testing soil pH, texture, bulk density, porosity, and infiltration
- Explain the difference between infiltration and permeability
- Carry out simple APES-style calculations for bulk density, porosity, and infiltration rate
- Interpret how sand, silt, clay, organic matter, and compaction affect test results
- Avoid common mistakes, such as confusing active pH with buffer pH or texture with structure

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand methods for soil testing, with a focus on the following syllabus points:

- Know that soil can be tested for chemical, physical, and biological properties to judge suitability for crops and land use
- Identify common tests for soil pH, texture, water-holding capacity, permeability, and percent humus
- Explain how USDA soil particle sizes define sand, silt, and clay
- Understand how the USDA soil texture triangle is used to classify soils
- Explain why representative sampling and sampling depth matter when collecting soil
- Distinguish infiltration at the surface from permeability or hydraulic conductivity within soil
- Understand how bulk density and porosity relate to compaction, root growth, and water movement
- Interpret how acidic conditions can increase the solubility of heavy metals in soil

## 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. Why is a composite soil sample usually better than one random scoop of soil?
2. What is the difference between soil pH and buffer pH?
3. How does the jar settling test help estimate soil texture?
4. Why can clay soil have high porosity but still poor drainage?
5. How would you calculate soil porosity from bulk density?

## Introduction

Soil testing matters because soil is not just “dirt.” It is a mixture of minerals, water, air, and organic matter, and small differences in its properties can change whether land is good for crops, whether water infiltrates or runs off, and whether nutrients remain available to plants. In AP Environmental Science, soil testing is often used as a practical way to connect abiotic factors with ecological productivity and land management.

Many exam questions do not ask for advanced lab chemistry. Instead, they test whether you understand what a given method measures, how the test is done in simple terms, and what the results mean. That means you need to know both the methods and the reasoning behind them.

> **Key Term: soil testing**
> The use of field or laboratory methods to measure soil properties such as pH, texture, water movement, and organic matter.

### Representative sampling: getting a useful soil sample

Before any test is done, the sample has to represent the soil being studied. This is easy to overlook, but it is often the biggest source of error. Soil changes from place to place and from one horizon to another. A sample taken from a wet depression, a fertilized patch, or near a compost pile may not reflect the rest of the site at all.

> **Key Term: representative soil sampling**
> Collecting soil in a way that reflects the average condition of the area being studied.

A good field approach is to take multiple subsamples from the same area and the same depth, then combine them. This creates a **composite sample**. The reason this works is simple: one scoop may be unusual, but several mixed together reduce the effect of local variation.

> **Key Term: composite sample**
> A sample made by combining several subsamples from the same area and depth to reduce bias.

Depth matters too. If you mix topsoil from the A horizon with deeper subsoil, you distort pH, texture, and nutrient results. For APES purposes, remember this rule: composite samples are suitable for tests like pH and texture, but not for tests that require undisturbed structure, such as bulk density.

Why not? Because mixing destroys the natural arrangement of particles and pores. That makes the sample useless for measuring compaction as it exists in the field.

> **Test Tip:** If a question asks why a soil result may be unreliable, look first at sampling errors: too few subsamples, mixed depths, or collection from unusual spots.

### Chemical testing: pH and nutrient-related interpretation

Soil pH is one of the most important chemical tests because it affects nutrient availability. Even if nitrogen, phosphorus, or potassium are present, plants may not be able to absorb them efficiently if the pH is too high or too low.

> **Key Term: soil pH**
> A measure of how acidic or basic the soil solution is, based on the concentration of hydrogen ions.

Most soils fall between pH 4 and pH 8. APES often links pH to plant health and pollution. Acidic soils can reduce availability of some nutrients and increase the solubility of metals such as aluminum and mercury. Once dissolved, these metals can move into groundwater or streams and harm organisms.

Common ways to test soil pH include:

- pH paper or indicator kits for quick field estimates
- portable probes for faster readings
- laboratory pH meters using soil-water mixtures for greater accuracy

In a lab, soil is usually suspended in water or a neutral salt solution, then measured. This is more standardized than simply pushing a probe into dry ground.

A major distinction students miss is the difference between **active pH** and **buffer pH**. Active pH is the soil’s current acidity in the soil solution. Buffer pH estimates reserve acidity and helps determine lime requirement. In other words, a quick pH strip tells you the present pH, but not how much lime would be needed to change it.

> **Key Term: buffer pH**
> A laboratory measure used to estimate reserve acidity and how much lime is needed to raise soil pH.

Suppose two soils both test at pH 5.5. One is sandy, one is clay-rich. The clay-rich soil usually resists change more strongly because it has more sites that hold acidic ions. So even though the pH values match, the clay soil may require more lime to raise the pH.

This is why the source material notes that clay soil requires more lime or alum to alter pH than sandy or loam soil.

> **Test Tip:** If a question asks which soil needs more lime, do not just pick the most acidic soil. Consider texture too. Clay-rich soils usually need more amendment to shift pH.

### Physical testing: texture, structure, and water-holding clues

Soil texture means the relative proportion of sand, silt, and clay.

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

The USDA particle-size classes are:

- **sand**: 2.0 to 0.05 mm
- **silt**: 0.05 to 0.002 mm
- **clay**: less than 0.002 mm

Texture matters because it affects pore size, permeability, water-holding capacity, and root growth. Sandy soils have larger particles and larger pores, so water enters and moves through quickly, but retention is low. Clay soils have very small particles and many tiny pores, so water may be held strongly and movement may be slow.

One common field method is the **texture-by-feel** or ribbon test. The soil is moistened, squeezed into a ball, then pressed between thumb and finger into a ribbon. Students then judge whether it feels gritty, smooth, or sticky and note ribbon length. Longer ribbons usually indicate more clay.

Another classroom-friendly method is the **jar settling test**, also called a sedimentation test. Soil is shaken in water and then allowed to settle. Sand settles first because its particles are largest and heaviest. Silt settles next. Clay stays suspended the longest. You estimate each fraction by measuring layer height.

Formula:

$$
\% \text{component} = \frac{\text{layer height}}{\text{total soil height}} \times 100
$$

Worked example:

A jar test shows:

- sand layer = 3 cm
- silt layer = 2 cm
- clay layer = 1 cm

Total soil height:

$$
3 + 2 + 1 = 6 \text{ cm}
$$

Percent sand:

$$
\frac{3}{6} \times 100 = 50\%
$$

Percent silt:

$$
\frac{2}{6} \times 100 = 33.3\%
$$

Percent clay:

$$
\frac{1}{6} \times 100 = 16.7\%
$$

These percentages can then be placed on the USDA soil texture triangle to estimate textural class.

> **Key Term: USDA soil texture triangle**
> A classification diagram used to identify soil type from the percentages of sand, silt, and clay.

A common APES misunderstanding is to confuse **texture** with **structure**. Texture is about particle size proportions. Structure is about how those particles group into aggregates. Two soils with the same texture can behave differently if one is well aggregated and the other is compacted.

Humus improves many physical properties because it helps soil crumble, improves aeration, and allows water to move through spaces between aggregates while still being retained within them.

### Bulk density, porosity, infiltration, and permeability

Bulk density is a measurement of soil compaction.

> **Key Term: bulk density**
> The dry mass of soil per unit total volume, including the space occupied by pores.

To measure it properly, you need an undisturbed core sample of known volume. Then the soil is dried and weighed.

Formula:

$$
\text{Bulk density} = \frac{\text{dry mass}}{\text{total volume}}
$$

Usually the units are g/cm³.

Worked example:

A cylindrical core has:

- radius = 2 cm
- height = 5 cm
- oven-dry mass = 125 g

Step 1: Calculate volume

$$
V = \pi r^2 h
$$

$$
V = 3.14 \times (2)^2 \times 5
$$

$$
V = 3.14 \times 4 \times 5 = 62.8 \text{ cm}^3
$$

Step 2: Calculate bulk density

$$
\text{Bulk density} = \frac{125}{62.8} = 1.99 \text{ g/cm}^3
$$

That is high for many mineral soils and suggests strong compaction.

Porosity is the proportion of soil volume that consists of pore space.

> **Key Term: porosity**
> The percentage of a soil’s total volume that is made up of air- or water-filled pore spaces.

For mineral soils, porosity is often estimated from bulk density using particle density of 2.65 g/cm³:

$$
\text{Porosity (\%)} = \left(1 - \frac{\text{bulk density}}{2.65}\right)\times 100
$$

Using the bulk density above:

$$
\text{Porosity} = \left(1 - \frac{1.99}{2.65}\right)\times 100
$$

$$
= (1 - 0.751)\times 100
$$

$$
= 24.9\%
$$

So the soil has about 25% pore space.

This shows the inverse relationship: higher bulk density means lower porosity.

Now the key distinction:

- **infiltration** = water entering the soil surface
- **permeability** = water moving through soil once inside
  > **Key Term: infiltration rate**
  > The speed at which water enters the soil from the surface.
  >
  > **Key Term: permeability**
  > The ease with which water or air moves through soil after entering it.

These are related, but not identical. A compacted crust at the surface can reduce infiltration even if deeper soil is relatively permeable. Likewise, cracks or root channels can make infiltration rapid even in clay-rich soils.

Field infiltration is often measured with a ring infiltrometer. A double-ring infiltrometer is better because the outer ring reduces sideways movement, so the inner ring better reflects vertical entry.

Worked example:

If 2.4 cm of water infiltrates in 30 minutes:

$$
\text{Infiltration rate} = \frac{2.4 \text{ cm}}{0.5 \text{ hr}} = 4.8 \text{ cm/hr}
$$

A fast rate suggests coarse texture, good aggregation, or macropores. A slow rate may suggest compaction, crusting, or clay-rich surface soil.

Why does this matter ecologically? Lower infiltration means more runoff, more erosion, and less groundwater recharge. High compaction also restricts roots and lowers oxygen availability.

Common student errors here include:

- using a mixed sample for bulk density
- saying clay always has low porosity
- treating infiltration and permeability as the same thing

Clay often has high total porosity, but most pores are tiny, so water movement is slow. Sandy soil may have lower total porosity than clay yet drain faster because its pores are larger.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Soil testing begins with representative sampling, not one random scoop
- Composite samples improve pH and texture testing when all subsamples come from the same depth
- Soil pH measures active acidity, while buffer pH estimates reserve acidity and lime requirement
- Sand, silt, and clay differ by particle size and affect water movement differently
- Texture-by-feel and jar settling tests are common ways to estimate soil texture
- The USDA soil texture triangle classifies soils from sand, silt, and clay percentages
- Bulk density measures dry mass per total volume and indicates compaction
- Porosity usually decreases as bulk density increases
- Infiltration is water entering the soil surface; permeability is water moving through soil
- Clay soils may have high porosity but poor drainage because pore size is very small
- Compaction reduces porosity, infiltration, and root growth
- Acidic soils can increase the solubility of metals such as aluminum and mercury

## Key Terms and Concepts

- soil testing
- representative soil sampling
- composite sample
- soil pH
- buffer pH
- soil texture
- USDA soil texture triangle
- bulk density
- porosity
- infiltration rate
- permeability


Soil Composition and Properties - Methods for soil testing