Soil and water degradation explained: HSC Agriculture Plant Production
“Analyse the processes of soil and water degradation, including erosion, structural decline, salinity, acidification and nutrient pollution, and the management practices that influence them”
The main degradation processes are erosion (sheet, rill, gully and wind), structural decline (compaction and surface crusting), salinity (dryland and irrigation), acidification and nutrient pollution of waterways. Each is driven by management: overgrazing and bare fallows expose soil to erosion, heavy machinery compacts it, clearing deep-rooted vegetation raises saline watertables, nitrate leaching and legumes acidify soil, and excess phosphorus runs off into rivers. Slowing degradation means keeping groundcover, controlling traffic, retaining perennials, liming and budgeting nutrients.
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What this dot point is asking
NESA wants you to identify the main processes that degrade the soil and water resource, explain the cause of each, and connect them to specific management practices that either accelerate or slow the damage. The verb "analyse" means you must break each process into cause, mechanism and effect, then show how a farm decision feeds into it. The strongest answers name a real Australian landscape and a real practice rather than talking in generalities.
The answer
Soil erosion
Erosion is the removal of topsoil by water or wind, and it strips the most fertile, nutrient-rich layer. Water erosion progresses from sheet erosion (a thin uniform layer lost across a slope), to rill erosion (small channels), to gully erosion (deep, unrecoverable channels such as those scarring parts of the NSW central west). Wind erosion lifts fine particles from bare paddocks, as seen in dust storms off the Mallee and western plains. The single biggest driver is loss of groundcover: overgrazing, over-cultivation and bare fallows leave soil exposed to raindrop impact and wind. Maintaining at least 70 percent groundcover, retaining stubble and using contour banks dramatically reduce erosion.
Structural decline
Structure degrades when aggregates collapse and pores close. Compaction from heavy machinery and from stock trampling wet soil (pugging) reduces infiltration and restricts roots. Surface crusting forms when raindrops shatter bare aggregates and the surface seals on drying, which then increases runoff and erosion. Working soil when too wet or too dry, and the loss of organic matter, both accelerate structural decline. Controlled traffic, reduced tillage and maintaining groundcover and organic matter protect structure.
Sodic soils, which cover large areas of inland NSW, are especially prone to structural decline because their clay disperses when wetted. The dispersed clay seals the surface, slashing infiltration and triggering both waterlogging and erosion. Applying gypsum (calcium sulfate) flocculates the clay and partly restores structure.
Salinity
Salinity is the accumulation of salts in the root zone or in waterways. Dryland salinity occurs when deep-rooted native vegetation is cleared and replaced with shallow-rooted annual crops and pastures: less water is used, the watertable rises and carries stored salt to the surface, killing plants and scalding land. This is widespread across the Murray-Darling Basin. Irrigation salinity occurs when irrigation adds water faster than it drains, raising watertables and concentrating salt. Reversing it means restoring deep-rooted perennials and trees, improving drainage and using water more efficiently.
Acidification
Australian agricultural soils acidify naturally over decades. Nitrate leaching below the root zone leaves acidity behind, ammonium and legume-based nitrogen add hydrogen ions, and product removal exports alkalinity. Below about pH 5 (calcium chloride), aluminium becomes toxic and roots are stunted. The granite-derived soils of the southern and central tablelands are strongly affected. The correction is regular liming, but because acidity stratifies down the profile, subsurface acidity can persist even under a limed topsoil.
Nutrient pollution of water
Nutrients lost off-farm degrade water resources. Phosphorus bound to eroded soil and dissolved nitrogen wash into rivers and dams, where they fuel eutrophication and toxic blue-green algal blooms, such as the major Darling River blooms. Over-application of fertiliser, poor effluent management at intensive piggeries and dairies, and erosion all contribute. Nutrient budgeting, buffer strips and fencing stock out of waterways reduce the load.
How to use this in the exam
Treat each process as cause, mechanism, effect, then management. For full marks, link a specific practice to the process: "removing deep-rooted vegetation reduced water use, so the watertable rose and brought salt to the surface." Anchor at least one process in a named landscape such as Murray-Darling Basin dryland salinity or central west gully erosion, and finish by judging whether the practice is preventing or merely treating the damage.
Students often confuse dryland and irrigation salinity, or blame salinity on adding salt. In dryland salinity no salt is added; clearing deep-rooted vegetation reduces water use, the watertable rises, and salt already stored in the subsoil is mobilised to the surface. Getting the water-balance mechanism right is what separates a top answer from a vague one.
Exam-style questions
Questions in the style of NESA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
2024 HSC-style2 marksIdentify TWO farming practices that have led to a decline in water quality.Show worked answer →
One mark for each correct practice (a brief identification is enough).
Two clear practices that degrade water quality are:
Excess fertiliser application, where nitrogen and phosphorus run off or leach into waterways and cause eutrophication (nutrient enrichment and algal blooms).
Cultivation or tillage close to waterways, which leaves bare soil that erodes and washes sediment into streams, raising turbidity.
Other acceptable answers include overgrazing of riparian zones, stock with direct creek access, and pesticide run-off. Only an identification is required, so two named practices score the 2 marks.
2024 HSC-style4 marksExplain how grassed waterways can improve water quality.Show worked answer →
Four marks needs the mechanism by which grassed waterways protect water quality, not just a definition.
A grassed waterway is a shaped, grassed channel that carries concentrated run-off across or off a paddock.
Slowing flow and binding soil. The grass cover slows the velocity of run-off and the dense root mass binds the soil, so far less soil is detached and carried away. This reduces gully and channel erosion.
Filtering run-off. As water passes through the grass, sediment and attached nutrients drop out and are trapped, so less sediment and fewer nutrients reach the stream. This lowers turbidity and reduces sedimentation and nutrient pollution downstream.
Full marks link both the reduced erosion and the filtering of sediment and nutrients to a measurable improvement in water quality.