Soils
Senior 1 · Geography
Soils
Lesson 2 of 4
25 min
Take a pinch of moist soil and roll it between your thumb and finger into a ribbon. Sandy soil falls apart at once and feels gritty. Silty soil feels smooth like flour. Clay soil makes a long, shiny ribbon that bends without breaking. In thirty seconds you have measured the most important property of a soil: its texture.
Definition
Soil texture: the relative amounts of sand, silt and clay particles in a soil. Example: a soil with 40% sand, 40% silt and 20% clay has a loam texture.
Figure 1. The mineral particles in soil, from largest to smallest.
| Property | Sandy soil | Loam | Clay soil |
|---|---|---|---|
| Feel | Gritty, falls apart | Soft, crumbly | Sticky when wet, hard and cracked when dry |
| Drainage | Very fast | Good | Very slow; waterlogs easily |
| Water held | Little; dries out quickly | Plenty | A lot, but much is held too tightly for plants |
| Nutrients | Poor; washed out | Good | Rich but hard to reach |
| Ease of digging | Easy | Easy | Heavy, hard to hoe |
| Rwandan example | Parts of Bugesera and the east | Well-farmed hillside gardens | Valley-bottom marshlands used for rice |
Definition
Loam: a soil with a balanced mixture of sand, silt and clay (and usually good humus), which drains well but holds enough water and nutrients. It is the best texture for most crops.
Soil scientists name a soil by plotting its percentages of sand, silt and clay on a texture triangle (Figure 2). Read the clay percentage across from the left edge, the silt percentage down from the right edge, and the sand percentage along the bottom; the three lines meet at one point.
Figure 2. The soil texture triangle (USDA classes). Point X is the soil in Worked example 1.
Worked example 1: using the triangle
A soil contains 40% sand, 40% silt and 20% clay. Name its texture.
Step 1. Check the total: .
Step 2. Clay: find 20 on the left edge and follow the line across (parallel to the base).
Step 3. Silt: find 40 on the right edge and follow the line down towards the bottom left.
Step 4 The lines meet at point X, which lies in the loam area. (The sand line from 40 on the base passes through the same point, which is a useful check.)
Worked example 2: a jar test
A student shakes soil in a jar of water and lets it settle for two days. The layers measure: sand 4.0 cm at the bottom, silt 3.0 cm above it and clay 1.0 cm at the top. Find the percentage of each.
On the triangle, 12.5% clay and 37.5% silt fall in the loam class, close to the edge of sandy loam. (The jar test is only approximate: fine clay may take days to settle.)
Texture is about the size of the particles; structure is about how they clump together into lumps, called peds.
| Structure | What the peds look like | Where it is found | Effect |
|---|---|---|---|
| Crumb (granular) | Small, rounded crumbs like breadcrumbs | Topsoil rich in humus and earthworms | Best for crops: air and water move easily |
| Blocky | Cube-like lumps with sharp or rounded corners | Subsoil | Moderate drainage |
| Platy | Thin, flat plates lying on top of each other | Soils compacted by trampling or machines | Water and roots cannot pass easily |
| Prismatic (columnar) | Tall columns | Clay subsoils that crack in the dry season | Water runs down the cracks |
| Single-grain | No peds; loose grains | Sandy soils | Drains fast, holds little |
Definition
Soil structure: the way soil particles are grouped together into lumps (peds) of different shapes, such as crumbs, blocks or plates. Example: a well-manured garden has a crumb structure.
Colour is a quick clue to what is happening in a soil:
Important
Case study: matching crops to texture in Rwanda
In the marshlands of the valley bottoms, heavy clay soils hold water, which suits rice grown in flooded plots. On the well-drained red loams of the hillsides, farmers grow beans, maize, cassava and bananas, adding manure to improve the crumb structure. In the sandier soils of Bugesera and parts of the east, water drains away fast, so farmers mulch the ground and choose drought-tolerant crops, and irrigation schemes bring extra water.
The Jar Test: Decoding your Soil's Texture, by Milkwood (5.5 min). Watch how the layers are measured after settling, then compare with Worked example 2.
A farmer says: "My soil is clay, so it must be the most fertile, because clay holds the most nutrients." Is she right? Use the table of sandy, loam and clay soils to give a balanced answer.
Tip
Texture is size, structure is shape. Texture = how much sand, silt and clay; structure = how the grains clump. And "Sand is Gritty, Silt is Silky, Clay is Clingy."
Practice
Remember
Sources and credits
The lesson text, texture triangle, particle diagram, tables, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
