AP Syllabus focus:
‘Methods to reduce erosion include contour plowing, terracing, strip cropping, and using windbreaks to slow wind.’
Soil erosion reduces farm productivity and degrades water quality. Field-shape and field-cover strategies work by slowing water and wind, keeping topsoil in place, and improving infiltration during storms and irrigation.
Core idea: reduce erosion by reducing transport energy
Erosion happens when moving water or wind has enough energy to detach and carry soil particles. Soil conservation methods in this subtopic focus on changing field shape and maintaining cover to:
decrease flow speed (less detachment and transport)
increase infiltration (less runoff)
trap sediment before it leaves the field
Contour plowing (field shape)
Contour plowing aligns rows and furrows perpendicular to the slope, following elevation contours. This creates small barriers that interrupt downhill water movement.
Contour plowing: Plowing and planting along lines of equal elevation to slow runoff and reduce soil erosion on slopes.
Key APES mechanisms and outcomes:
furrows act like miniature dams, reducing runoff velocity
more time for water to soak in, reducing rill formation
works best on gentle to moderate slopes
can be paired with grassed waterways where water naturally concentrates (to prevent gully formation)
Limitations to know:
less effective on steep or highly irregular terrain
requires careful layout; mistakes can channel water and worsen erosion
Terracing (field shape)
Terracing transforms a steep slope into a sequence of flatter “steps,” shortening slope length and reducing the gravitational force driving runoff.
Terracing: Reshaping sloped land into a series of level or nearly level platforms that slow runoff and reduce erosion.
How terraces conserve soil:
water slows and spreads across each level bench
sediment settles instead of being carried downslope
reduces formation of gullies by breaking long slopes into shorter segments
Trade-offs students should recognise:
high installation and maintenance costs (labour, machinery)
failure or poor drainage can concentrate flow and cause severe local erosion
may reduce usable planting area due to terrace walls and access paths
Strip cropping (field cover + layout)
Strip cropping alternates bands of erosion-prone crops (e.g., row crops) with bands of erosion-resistant cover (e.g., grasses/legumes), ideally aligned along contours on sloped fields.
This aerial photograph shows contour-aligned buffer/strip bands that follow lines of equal elevation across sloping farmland. The image helps students connect the idea of “rows/strips perpendicular to the slope” with the mechanism: breaking up downslope flow paths so water slows, infiltrates, and drops sediment before leaving the field. Source
Strip cropping: Planting alternating strips of different crops or vegetation to reduce erosion by slowing runoff and trapping sediment.
Why it works:
dense strips provide surface roughness, slowing water and wind near the ground
cover strips trap soil particles moving downslope (natural sediment filters)
can reduce nutrient loss by keeping sediment (and attached phosphorus) on-site
Design features that matter:
strips should be sized based on slope steepness and local rainfall intensity
greatest benefit when strips follow contours rather than running downhill
Windbreaks (field cover for wind erosion)
Wind erosion is most severe in dry periods, in open landscapes, and where soils are bare and fine-textured.
Windbreaks reduce wind speed across the field, lowering the force available to lift and move particles.
Windbreak: A line of trees, shrubs, or fencing placed to reduce wind speed and thus wind erosion across farmland.
What windbreaks do:
reduce wind velocity on the leeward side, decreasing soil particle detachment
trap moving sediment and protect seedlings from abrasion
can reduce evaporation by lowering near-surface airflow, helping retain soil moisture
Important implementation points:
effectiveness depends on height, density (porosity), and orientation to prevailing winds
may compete with crops for light and water near the barrier; spacing and species selection reduce this impact
FAQ
Windbreak performance depends on height, porosity, and prevailing wind direction.
Guidelines often aim for moderate porosity and spacing that protects a set distance downwind; local extension services tailor designs to crops and climate.
Yes. Terraces can concentrate water at outlets.
Planning typically includes stable drainage channels, protected outlets, and inspections after heavy rainfall to prevent breaches and gully initiation.
Yes, but it requires layout trade-offs.
Orienting strips along contours targets water erosion.
Orienting strips perpendicular to prevailing winds targets wind erosion. Hybrid designs may be used where both risks are high.
Dense, fast-establishing plants with strong root systems tend to work best.
Perennial grasses and legumes are commonly chosen because they create continuous ground cover and improve soil structure over time.
Common approaches include:
measuring sediment accumulation at field edges or in traps
checking for rills/gullies after storm events
tracking topsoil depth and organic matter over multiple seasons
Practice Questions
Explain how contour ploughing reduces soil erosion on sloped farmland. (2 marks)
Ploughing along contour lines creates ridges/furrows that slow downhill runoff (1).
Reduced runoff velocity increases infiltration and/or reduces soil particle detachment/transport (1).
A farm on a windy plain also has one steep hillside field. Describe how strip cropping and terracing would reduce erosion in their most suitable locations, and give one limitation of each method. (6 marks)
Strip cropping: alternating strips of crops/cover slow wind near the surface and trap soil particles (1).
Strip cropping is most suitable on the windy plain/open fields (1).
Limitation of strip cropping: requires careful strip orientation/width or may reduce usable cropping area/management complexity (1).
Terracing: converts a long steep slope into shorter level steps, reducing runoff speed and trapping sediment (1).
Terracing is most suitable on the steep hillside (1).
Limitation of terracing: expensive to build/maintain or failure can cause concentrated flow and severe erosion (1).
