Two kinds of input: Matter enters a soil as dead organic matter (from living things) and as inorganic minerals (from rock, water and air). This page starts with the dead organic matter.
Dead organic matter in
- Plant litter: fallen leaves, twigs, fruit, dead roots.
- Dead animals: bodies and body parts, from earthworms to deer.
- Manure: animal dung and urine (excretion).
- Decomposers turn this dead matter into humus and release minerals such as nitrates.
- Most of it comes from within the ecosystem: the plants and animals living on the soil.
Remember it as: Leaves, bodies, dung: the dead feed the soil.
Real example: in Tanzania's Serengeti, more than a million wildebeest leave dung on the grassland. Dung beetles roll it into balls and bury it, so the dung's nutrients go into the soil instead of drying out on the surface.
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The inorganic inputs come from the rock below, from the air and from the water falling on the soil. Energy comes in as sunlight.
Inorganic inputs
- Weathering of the parent rock releases minerals (from within the ecosystem).
- Precipitation: rain carries dissolved minerals and adds water.
- Deposition: dust, ash, silt and mud laid down by wind or water.
- Gases and air: oxygen, carbon dioxide and nitrogen; bacteria fix nitrogen into nitrates.
- Humidity: water vapour from the air.
- Solar energy: sunlight warms the soil; this is an input of energy, not matter.
Real example: in 2010 the Eyjafjallajökull volcano covered farms in southern Iceland with ash. It smothered the grass that year, but ash is made of minerals, and as it weathers it adds them to the soil.
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Inputs from far away: Most inputs come from within the ecosystem: its plants, its animals, the rock under it. Some are carried in from other ecosystems by wind, water or animals.
From other ecosystems
- Some inputs come from other ecosystems, carried in by wind, water or animals.
- Wind-blown deposition: dust from deserts, ash from volcanoes.
- Waterborne deposition: silt and mud left by floods and rivers.
- Guano: seabird and bat droppings, rich in nitrogen and phosphorus, brought from the sea.
From within the ecosystem
- Weathering of the parent rock
- Leaf litter from the plants above
- Decomposition of dead matter
From other ecosystems
- Dust blown from deserts
- Silt left by river floods
- Guano from seabirds that feed at sea
Real example: for thousands of years the Nile flooded every summer and left a layer of fertile silt on the fields of Egypt. Since the Aswan High Dam was finished in 1970, the silt is trapped behind the dam, and farmers must buy fertiliser instead.
In a managed soil, many inputs are added by people. They are called anthropogenic inputs.
Inputs people add
- Compost, manure and other organic matter dug in.
- Fertiliser: inorganic nitrates, phosphates and potassium.
- Other agrochemicals: pesticides, and lime to make soil less acid.
- Irrigation water, pumped from rivers and wells.
- Salt: irrigation water leaves salt behind as it evaporates (salinization).
- Biochar: charcoal mixed into soil to hold nutrients and carbon.
Remember it as: Feed it, spray it, water it, salt it.
Real example: in India's Punjab, the Green Revolution brought heavy fertiliser use and irrigation from millions of wells. Wheat and rice harvests rose fast, but in places salt now builds up in the soil.
Two traps: Pesticides are an input, but they add no nutrients: do not give them as an input to a nutrient store. And 'fertiliser' alone is vague: say 'inorganic fertiliser such as nitrates' or 'compost'.
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Some questions ask about the inputs to one store, such as the minerals in the A horizon. Then an input is anything that adds to that store, even from another horizon of the same soil.
Inputs to one store
- First ask: which store is the question about? An input must cross into that store.
- Into the mineral store of the A horizon: decomposition of litter, leaching down from the O horizon.
- ... nitrogen fixation, inorganic fertiliser, capillary rise of water with minerals from the B horizon.
- Changes inside one store are not inputs to it: nitrification turns one nutrient into another.
- Pesticides add no nutrients; plain 'water' is not a mineral input.
Worked example: A wheat field's A horizon: decomposition of the stubble adds minerals. Other inputs: inorganic nitrate fertiliser spread by the farmer, and nitrogen fixed by bacteria.
For the whole soil, movement between horizons is a flow inside the system, not an input: always check which boundary the matter crosses.
Inputs and outputs together: Some questions ask for two inputs AND two outputs. Outputs have their own page; here are the ones to know for these questions.
Outputs from a soil
- Nutrients and water taken up by plant roots.
- Soluble nutrients leached down to the groundwater.
- Soil and nutrients carried off by erosion in wind or water.
- Water lost by evaporation and run-off; nitrogen gas by denitrification.
- Carbon dioxide from the respiration of soil organisms; methane where there is no air.
Just naming
- 'Water'
- 'Nutrients'
- 'Organic matter'
Outlining
- Water is added by rain soaking in
- Nitrates are lost by leaching to groundwater
- Dead leaves fall and add organic matter
Real example: on a Punjab wheat farm, inputs are irrigation water and nitrate fertiliser; outputs are nitrates leached into the groundwater and water lost by evaporation in the heat.
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How this comes up: Paper 2, Section A: short State and Identify parts about the inputs to a soil.
A market garden in Kenya grows vegetables all year. The grower spreads goat manure, waters the beds from a well and adds a little lime.
(a) State two anthropogenic inputs to this managed soil.
(b) Identify two natural inputs to the soil's store of inorganic nutrients.
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