Unit 4 · Topic 4.3 Beta

Soil Composition and Properties

Soil texture, the share of sand, silt and clay, sets how soil behaves.

Practice 2: Visual RepresentationsPractice 5: Data Analysis

Part 1 · Hook

Why this matters

Pour a glass of water on a sandy beach and it vanishes in seconds. Pour it on a clay ball field after a storm and it sits in a puddle for hours. The difference is the size of the particles, and it decides which crops a farm can grow, where a septic drain field can go and how much a field needs watering.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which soil horizon is topsoil?

  1. The A horizon
  2. The B horizon
  3. The C horizon
  4. Bedrock
Show the answer

The A horizon is topsoil: humus mixed with minerals.

  • Correct: The A horizon:
  • The B horizon:
  • The C horizon:
  • Bedrock:

2. What is humus?

  1. Decomposed organic matter in soil
  2. Weathered bedrock
  3. Water held in the soil
  4. Salt left by evaporation
Show the answer

Humus forms when decomposers break down dead organisms.

  • Correct: Decomposed organic matter in soil:
  • Weathered bedrock:
  • Water held in the soil:
  • Salt left by evaporation:

3. What is leaching?

  1. Water carrying dissolved substances down through the soil
  2. Wind lifting soil off a field
  3. Plants taking up nutrients
  4. Rock breaking into smaller pieces
Show the answer

Leaching moves dissolved nutrients and minerals downward with water.

  • Correct: Water carrying dissolved substances down through the soil:
  • Wind lifting soil off a field:
  • Plants taking up nutrients:
  • Rock breaking into smaller pieces:

Part 4 · See it

See it first

The USDA soil texture triangle. Each side is a percent scale from 0 to 100: clay on the left side, silt on the right side, sand along the bottom. Lines of equal clay run across the triangle; lines of equal silt slope down to the left; lines of equal sand slope down to the right. The triangle is divided into twelve texture classes: clay at the top; silty clay, sandy clay, silty clay loam, clay loam and sandy clay loam below it; loam in the middle, shaded; silt loam and silt toward the bottom right; sandy loam, loamy sand and sand toward the bottom left. Loam is a balanced mix of sand, silt and clay, roughly 40% sand, 40% silt and 20% clay.
The soil texture triangle. Read the percent clay, silt and sand along the three sides; where the three lines meet is the soil's texture class. Loam, shaded, is a balanced mix. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Sand grains are large and clay particles are tinysand leaves large pores, clay leaves very many tiny pores
  2. Water moves easily through large, connected poressandy soil has high permeability and drains fast
  3. Tiny pores hold water tightly and pass it slowlyclay soil has high water-holding capacity but drains slowly and can lack air
  4. Clay and humus hold nutrients that water would otherwise leach awaya loam with humus is fertile, holds water and still drains

Part 6 · Key ideas

Key ideas

  • Soil texture is the share of sand (largest), silt and clay (smallest) particles. The soil texture triangle turns the three percentages into a texture class.
  • Loam is a balanced mix (roughly 40% sand, 40% silt, 20% clay) that drains well yet holds water and nutrients.
  • Porosity is the share of soil volume that is pore space. Permeability is how easily water flows through: highest in sand, with large connected pores; lowest in clay, with tiny pores.
  • Water-holding capacity is how much water a soil can hold against gravity: highest in clay and soils rich in humus, lowest in sand.
  • Soil fertility is the soil's ability to supply nutrients. Clay and humus hold nutrients; pH controls whether roots can take them up (most crops prefer about 6 to 7).

Part 7 · Misconception

A common mistake

The wrong idea: Sand drains fast because it has more pore space than clay.

What actually happens: Clay usually has as much or more total pore space; its pores are just tiny. Sand drains fast because its pores are large and connected: that is permeability, not porosity.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. Why does water drain through a sandy soil faster than through a clay soil?

  1. Sand has large, connected pores that water flows through
  2. Sand holds more humus, which pulls water down through the soil
  3. Clay grains are larger, so they block the water from passing
  4. Clay particles pack so tightly that clay has almost no pores
Show the answer

Permeability depends on pore size. Large sand grains leave large, connected pores; tiny clay particles leave tiny pores that water moves through slowly.

  • Correct: Sand has large, connected pores that water flows through: Right: big, connected pores make sand highly permeable.
  • Sand holds more humus, which pulls water down through the soil: Humus does not pull water down; sand usually holds little humus.
  • Clay grains are larger, so they block the water from passing: Clay particles are the smallest, not the largest.
  • Clay particles pack so tightly that clay has almost no pores: Clay has plenty of pore space, but its pores are tiny, so water moves slowly through them.

2. A gardener wants a soil that holds enough water for plants but still drains so roots get air. Which soil texture fits best?

  1. Loam
  2. Sand
  3. Clay
  4. Silty clay
Show the answer

Loam mixes sand, silt and clay, so it combines enough drainage and air space with enough water-holding capacity and nutrients.

  • Correct: Loam: Right: loam balances drainage and water-holding capacity.
  • Sand: Sand drains so fast that it holds little water for plants.
  • Clay: Clay holds water but drains slowly, so roots can run short of air.
  • Silty clay: Silty clay is mostly fine particles, so it drains slowly too.

Model or diagram

Four soil samples on the soil texture triangle

The USDA soil texture triangle with four soil samples marked. Sample A: 70% sand, 8% silt, 22% clay, in the sandy clay loam class. Sample B: 40% sand, 40% silt, 20% clay, in the loam class. Sample C: 10% sand, 30% silt, 60% clay, in the clay class. Sample D: 20% sand, 74% silt, 6% clay, in the silt loam class. Invented samples for practice.
LevlPrep original diagram.

3. On the triangle, which sample is a loam?

  1. Sample B
  2. Sample A
  3. Sample C
  4. Sample D
Show the answer

Sample B (40% sand, 40% silt, 20% clay) sits inside the shaded loam region in the middle of the triangle.

  • Correct: Sample B: Right: B is in the loam region.
  • Sample A: Sample A is in the sandy clay loam region.
  • Sample C: Sample C is in the clay region at the top.
  • Sample D: Sample D is in the silt loam region.

4. Sample D is 6% clay and 20% sand. What percent of sample D is silt?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

100%−(6%+20%)=74%100\% - (6\% + 20\%) = 74\% silt, which is why D sits in the silt loam region.

  • Answer: 74 %

5. Which sample would hold the most water but let water pass through it most slowly?

  1. Sample C
  2. Sample A
  3. Sample B
  4. Sample D
Show the answer

Sample C is 60% clay. Clay's tiny pores hold water tightly and pass it slowly, so C has high water-holding capacity and low permeability.

  • Correct: Sample C: Right: the clay sample holds water and drains slowly.
  • Sample A: Sample A is 70% sand, so it drains quickly.
  • Sample B: Sample B is a loam, in between.
  • Sample D: Sample D is mostly silt with little clay, so it holds less water than C.

Data

A class tests how water moves through three soils

Each soil: a 10 cm deep column, 200 mL of water poured on top (invented data for practice)
SoilTime for the water to pass through the column (minutes)Water that drained out (mL)Water held by the soil (mL)
Sand217030
Loam1012080
Clay5090110

6. The water in the loam column traveled 10 cm in 10 minutes. Find the percolation rate in centimeters per hour. Write the setup with units, then the answer. (1 hour = 60 minutes)

Type the setup as on the exam, with brackets and units, then the answer.

Show the answer

10cm10min×60min1h\frac{10\,\text{cm}}{10\,\text{min}} \times \frac{60\,\text{min}}{1\,\text{h}}=60cm/h= 60\,\text{cm/h}. The minutes cancel.

  • Setup: 10cm10min⋅60min1h\frac{\frac{10\,\text{cm}}{10\,\text{min}} \cdot 60\,\text{min}}{1\,\text{h}}; answer 60 cm/h

Part 9 · Summary

Summary

Soil texture, the share of sand, silt and clay, sets how soil behaves. Sand drains fast (high permeability) but holds little water or nutrients; clay holds water and nutrients but drains slowly; loam balances the two. The texture triangle reads a sample's class from its three percentages. Humus and a pH near 6 to 7 make soil more fertile.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

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