Soils

Lessons

Senior 1 · Geography

Soils

4 lessons

Lesson 1 of 4

What Soil Is Made Of

22 min

A handful of hill

Pick up a handful of damp soil from a garden and squeeze it. Some of it crumbles like biscuit, some is gritty, some is smooth and sticky. Put it in a clear bottle of water, shake it and wait: a layer of sand sinks first, then finer silt, and last the clay; bits of dead leaves float on top, and air bubbles rise. That bottle holds the whole answer to the question "what is soil?"

Definition

Soil: the thin, loose top layer of the Earth's land surface, made of mineral particles from weathered rock mixed with organic matter, water and air, in which plants can grow. Example: the dark, crumbly soil of a banana garden near Musanze.

The four parts of soil

Figure 1. What a healthy loam soil is made of, by volume. The water and air shares change after rain and in dry spells.

Definition

Humus: dark, decayed organic matter in the soil, formed from rotted leaves, roots, dung and dead organisms. It holds water and nutrients like a sponge and gives topsoil its dark colour. Example: the black upper layer of soil under a forest floor in Nyungwe.

Component	Where it comes from	Why plants need it
Mineral particles	Weathering of the parent rock: sand, silt and clay	Give the soil its body; release nutrients such as potassium and phosphorus
Organic matter	Dead leaves, roots, animal dung and dead organisms, rotted into humus	Holds water and nutrients; feeds soil life; binds grains into crumbs
Water	Rain, held in the small pores between particles	Plants take it up with dissolved nutrients
Air	The atmosphere, filling the larger pores	Roots and soil organisms need oxygen
Living organisms	Earthworms, termites, fungi, bacteria	Break down dead matter, mix the soil, make tunnels for air and water

Worked example 1: drawing the pie chart

A soil scientist finds that a soil sample, by volume, is 45% minerals, 25% water, 25% air and 5% organic matter. How many degrees does each part take in a pie chart?

\text{angle} = \frac{\text{percentage}}{100} \times 360^\circ

\text{minerals} = \frac{45}{100} \times 360^\circ = 162^\circ

\text{water} = \text{air} = \frac{25}{100} \times 360^\circ = 90^\circ

\text{organic matter} = \frac{5}{100} \times 360^\circ = 18^\circ

Check: $162 + 90 + 90 + 18 = 360^\circ$ . After heavy rain the water share rises and the air share falls; a waterlogged soil has almost no air, which is why most crops die in flooded fields.

How soil forms

Figure 2. Soil forms slowly as rock weathers and living things add organic matter.

Five factors control what kind of soil forms in a place:

Parent rock: the rock below gives the minerals. Volcanic rock around Musanze makes dark, fertile soil; quartzite makes sandy, poor soil.
Climate: warm, wet conditions speed up weathering and rotting; Rwanda's rains make deep, weathered soils.
Living organisms: plants add organic matter; earthworms and termites mix it in.
Relief: soils are thin on steep slopes, where they are washed away, and thick at the foot of slopes and in valleys, where material collects.
Time: the longer the processes act, the deeper and more developed the soil.

Definition

Weathering: the breaking down of rock where it lies, by the weather and living things: by heating and cooling, by water and chemicals dissolving minerals, and by roots. Weathering makes the mineral part of soil.

Worked example 2: why lost soil is lost for good

Soil scientists estimate that in many places it takes a few hundred years, and sometimes about a thousand, to form just 1 cm of soil. Suppose a bare hillside field loses 5 mm of soil in one very wet rainy season. If new soil forms at 1 cm per 500 years, how long would it take to replace?

\text{formation rate} = \frac{10\ \text{mm}}{500\ \text{years}} = 0.02\ \text{mm per year}

\text{time} = \frac{5\ \text{mm}}{0.02\ \text{mm per year}} = 250\ \text{years}

One season of erosion can remove what took about 250 years to build. That is why terraces and ground cover matter so much on Rwanda's hills.

Important

Case study: three Rwandan soils from three parent materials

Volcanic soils (Musanze, Nyabihu, Rubavu): formed from young lava and ash; dark, deep and rich in plant nutrients. They grow potatoes, beans, maize and pyrethrum, and support some of the densest farming populations in the country.
Soils on old rocks (central plateau and the ridges): formed over a very long time from schist, quartzite and granite; often red or yellow-brown, deeply weathered, leached of nutrients and acidic, so they need manure, compost and lime.
Valley-bottom soils (marshlands): built from mud and organic matter washed down from the hills; heavy, dark or grey and often waterlogged. Drained and irrigated, they grow rice and vegetables in the dry season.

What Is Soil Made Of?, by Earth Science Classroom (6.5 min). As you watch, check each component against Figure 1 and note one job it does for plants.

Discuss

Two farmers own fields of the same size, one near Musanze and one on a quartzite ridge in the Southern Province. Using the five factors of soil formation, explain why one of them needs to buy much more fertiliser than the other.

Tip

Soil = "MOWA": Minerals, Organic matter, Water, Air. For formation, remember "Please Can Little Rabbits Talk?": Parent rock, Climate, Living things, Relief, Time.

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Practice

Define soil.
Name the four main components of soil and give their approximate percentages in a good loam.
What is humus, and give two ways it helps crops?
List the five factors that control soil formation.
In a pie chart of soil composition, air makes up 20%. How many degrees is its sector?
Explain why soils are thin on steep slopes but thick in valley bottoms.
If soil forms at 1 cm per 400 years, how long would it take to replace 2 cm lost by erosion?

Show answersHide answers

The loose top layer of the land surface, made of mineral particles from weathered rock, organic matter, water and air, in which plants grow.
Mineral particles about 45%, water about 25%, air about 25%, organic matter about 5%.
Dark, decayed organic matter. It holds water, stores and supplies nutrients, binds particles into crumbs and feeds soil organisms (any two).
Parent rock, climate, living organisms, relief and time.
$\frac{20}{100} \times 360^\circ = 72^\circ$ .
On steep slopes rain washes soil downhill faster than it forms; the washed material collects at the foot of slopes and in valleys, making deep soils there.
800 years.

Remember

Soil is weathered mineral particles plus organic matter, water and air (about 45 : 5 : 25 : 25 by volume).
Humus is the dark, decayed organic part that holds water and nutrients.
Soil forms slowly from parent rock under the influence of climate, living things, relief and time, so it must be protected.

Sources and credits

Rwanda - dense agriculture: photo by "not not phil" from San Francisco, Wikimedia Commons, CC BY-SA 2.0. Reproduced without changes.
Video: What Is Soil Made Of?, Earth Science Classroom, embedded from YouTube.

The lesson text, pie chart, soil-formation diagram, table, worked examples and practice are written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A handful of hill

2The four parts of soil

3How soil forms

4Discuss

A handful of hill

The four parts of soil

How soil forms

Discuss