Agricultural Science · Lesson 4

Land clearing, tillage, irrigation, drainage, farm surveying and planning

JAMBWAECNECO

What the syllabus asks

JAMB: B2c soil and water conservation, B4 irrigation and drainage, E farm surveying and farmstead planning

WAEC: B10 Land preparation (clearing, tillage, minimum and zero tillage); B11 Irrigation and drainage (meaning, methods, advantages, problems); C7 Farm surveying (instruments, methods, area calculations); C8 Farm planning and layout; C9 Farmstead planning (factors, layout, buildings)

NECO: B10 Land preparation (clearing, tillage, minimum and zero tillage); B11 Irrigation and drainage (meaning, methods, advantages, problems); C7 Farm surveying (instruments, methods, area calculations); C8 Farm planning and layout; C9 Farmstead planning (factors, layout, buildings)

What you will learn

Describe methods of land clearing and their merits.
Explain primary and secondary tillage, minimum and zero tillage.
Define irrigation and describe its methods.
State the advantages and problems of irrigation.
Define drainage and describe its methods and importance.
Name and use farm survey instruments.
Calculate areas of farm plots and convert to hectares.
Explain the principles of farm planning and layout.
State factors to consider in siting a farmstead.
Draw a simple farmstead layout.

Key points

Clearing: manual, mechanical, chemical, burning; then stumping.
Tillage aims: loosen soil, control weeds, mix residues, prepare seedbed, form ridges.
Minimum and zero tillage reduce erosion, conserve moisture and organic matter, save cost.
Irrigation: artificial water supply; allows dry-season and all-year farming.
Methods: surface (flood, basin, furrow, border), sprinkler, drip/sub-surface.
Irrigation problems: salinity, waterlogging, schistosomiasis and malaria, erosion, high cost.
Nigerian schemes: Kano River, Hadejia Valley, Bakolori, South Chad; Fadama projects for small farmers.
Drainage removes excess water; methods: open ditches, tile and mole drains, ridges and beds.
Waterlogging: no air for roots, cold soil, denitrification, toxins.
Survey instruments: ranging poles, tape/chain, arrows, pegs, offset staff, cross staff, prismatic compass, level, theodolite, GPS.
Area: rectangle l × b; triangle ½bh; trapezium ½(a + b)h; 1 ha = 10 000 m².
Farm planning considers objectives, land, climate, capital, labour, market, risk and policy.
Farmstead siting: water, drainage, access, central position, wind direction, security, expansion.
Livestock houses downwind; chemical stores away from water and homes.
Conservation: contour ridging, terraces, bunds, cover crops, mulch, strip cropping, windbreaks, tied ridges.

Start here: Land clearing methods

Before crops can be planted on a forest or bush fallow plot near Ondo or Ikom, the vegetation must be removed. The method chosen depends on the size of the farm, the type of vegetation, money available and the need to protect the soil.

Method	How it is done	Advantages	Disadvantages
Manual	Cutlass, axe, hoe; slash and pile	Cheap; little soil disturbance; suits small farms	Slow, tiring; limits farm size
Mechanical	Bulldozers, tree pushers, root rakes, chain between two crawlers	Fast; large areas; removes stumps	Expensive; compacts and scrapes away topsoil; erosion
Chemical	Herbicides (e.g. glyphosate; arboricides for trees; paraquat is now banned in Nigeria)	Fast; less labour; kills perennial weeds	Costly; pollution; hazardous; skill needed
Burning	Setting fire to dried slash	Cheap; quick; ash adds potash; kills pests	Destroys organic matter and soil organisms; exposes soil to erosion; air pollution; uncontrolled bush fires

Stumping

Stumps and roots obstruct tractors and implements and can harbour pests such as termites. They are removed by digging and cutting, pulling with a tractor or winch, or are left to rot on small manual farms.

Good clearing practice

Clear only what you will use, and leave trees on steep slopes and stream banks.
Use slash as mulch or compost rather than burning where possible.
Mechanical clearing should use root rakes rather than blades that push away topsoil.
Leave shade trees on cocoa land; leave windbreaks in the north.

Worked example 4.1

State three advantages and three disadvantages of bush burning as a method of land clearing.

Advantages: cheap and quick; ash supplies potash and raises pH temporarily; kills some pests, weed seeds and disease organisms.
Disadvantages: destroys humus and soil organisms; leaves soil bare to erosion; fires may spread to neighbouring farms and forests; air pollution (accept: loss of nitrogen and sulphur as gases).

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Practice questions

1. Which land-clearing method is most likely to remove the fertile topsoil?

clearing with a bulldozer blade
manual clearing with cutlass
slashing
chemical clearing

Show answer

A: clearing with a bulldozer blade
The blade scrapes away topsoil.

2. Planting into untilled soil with weeds controlled by herbicides is

conventional tillage
puddling
subsoiling
zero tillage

Show answer

D: zero tillage
It reduces erosion and saves fuel.

3. The irrigation method that uses water most efficiently is

furrow irrigation
basin irrigation
drip (trickle) irrigation
flood irrigation

Show answer

C: drip (trickle) irrigation
Water goes straight to the roots.

4. A traditional counterweighted pole for lifting water is the

drip line
shaduf
sprinkler
theodolite

Show answer

B: shaduf
It is used in parts of northern Nigeria and Egypt.

5. A problem of poorly managed irrigation is

salinisation of the soil
increase in soil fertility always
less need for drainage
lower water tables in all cases

Show answer

A: salinisation of the soil
Evaporation leaves salts at the surface.

Key words

Land clearing: Removing vegetation (trees, shrubs, grass) to prepare land for cultivation.
Stumping: Removing tree stumps and large roots after clearing.
Tillage: Mechanical manipulation of soil to make it suitable for crop growth.
Tilth: The physical condition of the soil after tillage; a fine tilth has small crumbs.
Minimum tillage: Reducing tillage operations to the least needed for crop production.
Zero tillage: Planting directly into undisturbed soil without ploughing or harrowing.
Irrigation: The artificial application of water to the soil to supply crop needs.
Drainage: The removal of excess water from the soil surface or root zone.
Waterlogging: Saturation of soil with water, driving out air from pore spaces.
Salinisation: Build-up of salts in soil, often from irrigation water evaporating in hot dry areas.
Furrow irrigation: Water flows in small channels between crop ridges.
Sprinkler irrigation: Water is sprayed into the air and falls like rain.

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