Agricultural Science · Lesson 16

Practical 3: rocks, soil texture, capillarity, water-holding capacity, porosity and pH

WAECNECO

What the syllabus asks

WAEC: rocks (igneous, sedimentary, metamorphic), soil samples (sand, loam, clay), soil properties and experiments: texture (sedimentation), capillarity, water-holding capacity and drainage, porosity, air, water and organic matter, pH, soil profile.

NECO: rocks (igneous, sedimentary, metamorphic), soil samples (sand, loam, clay), soil properties and experiments: texture (sedimentation), capillarity, water-holding capacity and drainage, porosity, air, water and organic matter, pH, soil profile.

What you will learn

Identify rock specimens, classify them and state the soils they form.
Identify sand, loam and clay samples and compare their properties.
Describe and draw the sedimentation, capillarity and water-holding experiments.
Calculate texture percentages, water retained, drainage rate, porosity and bulk density.
Determine the air, water and organic matter content of soil.
Write up a soil experiment with results, conclusion and precautions.

Key points

Rocks: three classes; parent–product pairs; acid test for carbonates.
Sand, loam, clay: feel, colour, drainage, water-holding, capillarity, fertility.
Sedimentation: layers by size; percentages from heights or volumes.
Capillarity: clay highest but slowest; sand fastest but lowest.
Water retained = added − drained; drainage rate = volume ÷ time.
Porosity = (1 − BD ÷ 2.65) × 100; BD = dry mass ÷ core volume.
Moisture and organic matter by oven drying and ignition; state the basis.
Write-ups: aim, apparatus, labelled drawing, procedure, table, conclusion, precautions.

Start here: Rocks as practical specimens

The ecology part of the WAEC practical syllabus begins with rocks. A typical question places three or four rock specimens on the bench (labelled A, B, C, D) and asks you to (a) name each rock, (b) state the class (igneous, sedimentary or metamorphic) to which it belongs, (c) state two observable features you used, and (d) name the soil each tends to form. Nigeria is a good place to learn rocks: about half the country sits on the ancient Basement Complex of granites, gneisses and schists, while the rest is covered by younger sedimentary basins (the Niger Delta, Benue Trough, Sokoto and Chad basins) and a little volcanic basalt.

The three classes of rock

Classes of rock
Class	How formed	Typical features	Nigerian examples
Igneous	cooling and hardening of molten magma or lava	crystalline; no layers; no fossils; usually hard	granite (Olumo Rock, Abeokuta), granodiorite and gabbro (Zuma Rock, Niger State), basalt (Jos Plateau)
Sedimentary	deposition and cementing of particles, shells or plant remains in layers	layered (strata); may contain fossils; often softer; grains visible	sandstone, shale (Enugu escarpment), limestone (Ewekoro, Ogun State; Obajana, Kogi State)
Metamorphic	existing rock changed by great heat and/or pressure	crystals often aligned or banded; very hard	marble (Igbeti, Oyo State; Jakura, Kogi State), gneiss and schist (Basement Complex)

Remember the parent–product pairs, which examiners love: limestone → marble, sandstone → quartzite, shale → slate, granite → gneiss.

Model answer: rock specimens

Question: Specimens A, B and C are granite, limestone and marble. (a) Classify each. (b) State one observable feature of each. (c) Name a test to distinguish B from A. Model answer: (a) A igneous; B sedimentary; C metamorphic. (b) A: coarse interlocking crystals of pink/white feldspar, glassy quartz and dark mica; B: grey, fine-grained, may contain fossil shells; C: white or banded, sugary crystalline texture. (c) Add a drop of dilute hydrochloric acid: B fizzes (carbon dioxide), A does not.

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Practice questions

1. Granite is a

igneous rock
sedimentary rock
metamorphic rock
soil

Show answer

A: igneous rock
Formed from magma.

2. Sandstone is a

igneous rock
metamorphic rock
mineral
sedimentary rock

Show answer

D: sedimentary rock
Cemented sand.

3. Marble is formed by metamorphism of

sandstone
clay
limestone
granite

Show answer

C: limestone
Heat and pressure.

4. Sandy soil has

small particles
large particles and good drainage
high water-holding capacity
sticky texture

Show answer

B: large particles and good drainage
Gritty feel.

5. Clay soil has

high water-holding capacity
very large particles
poor nutrient retention
gritty feel

Show answer

A: high water-holding capacity
Sticky and plastic when wet.

Key words

Igneous rock: Rock formed by cooling of molten magma or lava.
Sedimentary rock: Rock formed from layers of deposited and cemented particles.
Metamorphic rock: Rock changed by heat and pressure.
Texture: Relative proportions of sand, silt and clay in a soil.
Structure: Arrangement of soil particles into aggregates.
Capillarity: Upward movement of water through fine soil pores.
Water-holding capacity: Amount of water a soil retains after free drainage.
Porosity: Percentage of soil volume occupied by pores.
Bulk density: Oven-dry mass of soil per unit total volume.
Particle density: Density of the solid particles alone (about 2.65 g/cm³).
Soil profile: Vertical section of soil showing its horizons.

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