Outputs from soil at Higher Level: The same ideas as SL with different examples: Sri Lanka's tea, Bangladesh's rice paddies, the Gulf of Mexico, the Loess Plateau and Mongolia's goats. At HL, expect a seven-mark answer on how outputs degrade productive soil.
Practise this as you read
- Sort every output by where it goes: air, plants, groundwater or away.
- Link each human activity to the output it speeds up.
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What leaves the soil: Soil is a system with inputs and outputs. Outputs are the matter and energy that leave it: to the air, to plants, to groundwater, or carried away across the land.
The points to remember
- An output is matter or energy leaving the soil system.
- Decomposition loses dead organic matter as carbon dioxide (and methane in waterlogged soil).
- Uptake by roots takes water and minerals; harvesting carries them away.
- Leaching washes dissolved nutrients out to groundwater; erosion carries soil away.
- Evaporation, denitrification and heat lose water, nitrogen and energy to the air.
- Do not give flows that stay inside the soil (nitrification, capillary rise): they are not outputs.
Remember it as: Up to the air, into plants, down to groundwater, away by erosion.
Real example: on a tea estate in the highlands of Sri Lanka, pickers take the top leaves every week, removing nitrogen and potassium. Monsoon rain leaches nutrients from the steep slopes, and on bare patches between the bushes it washes soil downhill.
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Some outputs go up into the air. Decomposers and roots respire, soil water evaporates, and denitrification turns nitrate into a gas. The gases diffuse out through the air spaces in the soil.
Outputs to the atmosphere
- Carbon dioxide: from respiration by decomposers and roots.
- Methane: from decomposition in waterlogged soil with no air.
- Water vapour: from evaporation of soil water.
- Nitrogen gas: from denitrification.
- Heat: energy lost from the warm soil; soil particles blown away by wind.
The output (the substance)
- Carbon dioxide
- Methane
- Water vapour
- Nitrogen gas
The process behind it
- Respiration
- Decomposition without air
- Evaporation
- Denitrification
Real example: Bangladesh's rice paddies are flooded for months, so the soil has no air. Decomposers there release methane instead of carbon dioxide; rice fields are one of the world's largest human sources of methane.
Name the substance: Asked for an output to the atmosphere, write 'carbon dioxide' or 'water vapour'. 'Respiration' or 'evaporation' is the process, not the output.
Other outputs go down or into plants. Water percolating through the soil carries dissolved nutrients out by leaching; roots take nutrients up, and the harvest carries them away.
Outputs downwards and into plants
- Leaching carries dissolved nutrients (nitrate, potassium) below the roots and into groundwater.
- Heavy rain soon after fertiliser is spread washes it off or down: fertilise in drier weeks.
- Uptake: roots take water and minerals out of the soil store into the plant.
- Harvesting removes those nutrients from the field for good; so does grazing.
- Mining or digging soil out is an output too. Drainage and percolation = leaching: count it once.
Leaching
- Nitrate and potassium washed below the roots.
- Ends up in groundwater and rivers.
Uptake
- Roots absorb water and minerals.
- They leave the soil store for the plant.
Harvest
- Grain, fruit or leaves are taken away.
- Their nutrients never return to the field.
Real example: nitrate leached and washed from the farms of the US Corn Belt flows down the Mississippi to the Gulf of Mexico. Each summer it feeds algae there, creating a dead zone of around 15,000 km².
Timing matters: Farmers add fertiliser when little rain is due, so it is not washed off or leached before the crop can absorb it.
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Erosion is the output that removes the soil itself. Water and wind carry topsoil away, and with it the organic matter and nutrients it holds.
The points to remember
- Erosion is soil carried away by water or wind.
- Steeper slopes lose more soil: water runs off faster and carries more.
- Other factors: heavy rain, strong wind, bare soil with no plant cover, ploughing.
- Forest protects soil: roots bind it, the canopy breaks the force of raindrops, litter covers it.
- Trees slow the wind and help water infiltrate, so less runs off.
- Total loss = rate x area: a big forest can lose the most in total at a low rate.
Rate and total are not the same. The rate is the loss from each hectare; the total is the rate multiplied by the area. A land use with a low rate can still lose the most soil if it covers most of the land.
Real example: on China's Loess Plateau, centuries of farming steep slopes of soft, wind-blown silt stripped the plant cover. Run-off carved deep gullies, and so much soil washed into the Yellow River that it was once the muddiest major river in the world.
Outputs are natural, and inputs normally replace them. When people speed the outputs up, the soil runs down: soil degradation.
The points to remember
- Outputs change the soil (fewer nutrients, less organic matter), even when the soil stays put.
- Erosion is different: the soil itself is lost.
- People speed up outputs: clearing forest, overgrazing, monoculture, ploughing, over-watering.
- Faster outputs than inputs lead to degradation of productive soil.
- Soil forms very slowly, so lost topsoil is not replaced in a human lifetime.
- Degraded soil means less food, muddy rivers, fewer soil species and less carbon stored.
- Building over soil seals it under concrete: it is lost to farming for good.
More goats
- High prices for cashmere wool have multiplied goat herds several times since 1990.
Overgrazed
- Goats pull grass up by the roots, and trampling leaves bare, hard ground.
Blown away
- Strong spring winds lift the bare topsoil in dust storms.
Poorer
- With its topsoil and nutrients gone, much of the pasture now feeds fewer animals.
Say how, not just what: 'Overgrazing' or 'deforestation' alone earns no mark. Say what it does: 'grazing removes the grass, so bare soil is washed away by rain'.
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How this comes up: Paper 2, Section B (b): explain how outputs from the soil system can degrade productive soil [7]. Seven outputs or effects, each linked to a loss of fertility.
In northern Burkina Faso, millet fields are cropped every year, cattle graze the stubble and summer storms are heavy.
Explain how outputs from the soil system can lead to the degradation of productive soil.
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