Three balances decide how well plants grow: Soil texture affects primary productivity. Each kind of particle, and the humus, changes how the soil holds nutrients, water and air.
The points to remember
- Primary productivity = the rate at which plants make new biomass; the soil sets how fast they can.
- Balance 1: holding nutrients v losing them by leaching (washing down out of reach).
- Balance 2: holding water v draining it away.
- Balance 3: aeration (air for roots) v compaction or waterlogging.
- Sand, silt, clay and humus each push these balances a different way; the best soil balances all three.
| Nutrients | Water | Air | |
|---|---|---|---|
| Sand | Few held; easily leached | Drains fast; dries out | Well aerated |
| Silt | Some held | Holds water well | Can pack down (compact) |
| Clay | Many held on its charged surfaces | Holds water; drains slowly | Poor air; waterlogs |
| Humus | Holds and releases them | Holds water like a sponge | Makes crumbs with air spaces |
Remember it as: Hold the food, hold the water, let the air in.
Real example: the Mekong Delta in Vietnam has deep soils of river silt and clay that hold water and nutrients. Farmers there can grow two or three rice crops a year.
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Loam is the most productive texture for most crops. Each reason compares it with sand or with clay.
The points to remember
- A good balance of sand, silt and clay, so it avoids the worst of each.
- Drains well, so it does not waterlog (better than clay).
- Aerated: roots get oxygen (better than clay).
- Easy for roots to push through and easy to dig (better than clay).
- Holds water and nutrients (better than sand).
- Stable: less likely to blow away (better than sand).
Remember it as: Loam takes the best of sand and the best of clay.
Real example: much of California's Central Valley, which grows a large share of the USA's fruit, nuts and vegetables, has deep loam soils.
Say what it is better than: 'Loam drains well' is half a reason. 'Loam drains better than clay, so the roots are not waterlogged' makes the comparison and the link to growth.
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Humus: a small amount, a big effect: Humus is the dark brown or black material just below the leaf litter. It is loose and crumbly, made by the partial decay of dead plants. Although it is often only about 5% of a soil, it changes how the whole soil behaves.
The points to remember
- Nutrients: holds them and releases them as it decomposes; it feeds the soil's microorganisms.
- Water: holds water in its organic material, like a sponge.
- Structure and air: binds particles into crumbs with air spaces between them.
- Infiltration: raises porosity, so rain soaks in instead of running off.
- pH: acts as a buffer, keeping the soil's pH steady.
- Temperature: releases heat as it decomposes and insulates the soil.
Remember it as: Humus holds, binds, buffers and warms.
Real example: in the Amazon, people made terra preta ('black earth') centuries ago by adding charcoal, food waste and pottery to the soil. These dark, humus-rich soils are still far more fertile than the soils around them.
Link humus to a property: 'Humus is good for soil' scores nothing. Name the property it changes and how: 'humus binds particles into crumbs, leaving air spaces, so roots get oxygen'.
A question may ask for the characteristics of a highly productive soil. Each point is a property of the soil itself, linked to plant growth. One teaspoon of such soil holds billions of living organisms.
The points to remember
- Enough nutrients, the usual limiting factors: nitrates, phosphates, potassium; also calcium, magnesium, sulfates, iron.
- Plenty of humus / organic matter: holds water and nutrients.
- A loamy, balanced texture: good drainage, porosity, no waterlogging.
- A suitable pH, about 6 to 7 for most crops (accept 5.5 to 7.0).
- A deep topsoil, so roots find more water and nutrients and plants are well anchored.
- Rich soil life: decomposers, nitrogen-fixing bacteria (in legume roots), mycorrhizal fungi, earthworms.
- No toxic substances (and low salt).
Real example: Ukraine's black earth (chernozem) has a deep topsoil rich in humus and a near-neutral pH. It is one of the most productive soils on Earth, and Ukraine is one of the world's biggest grain exporters.
What does not count: Soil colour on its own is not a characteristic that scores. Ground cover and soil conservation methods are things people do, not properties of the soil: they do not count in a 'soil properties' question.
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People change soils, for better and worse: Soil fertility depends on nutrients, water, pH, low salt, texture, air and soil life. Changing any of them changes productivity.
The points to remember
- Adding sand to a clay-rich soil: better drainage and infiltration, more air, easier root growth.
- But more sand also means less water held, more leaching and less anchorage for roots.
- Adding organic matter (compost, manure) raises humus: more water and nutrients held, better structure.
- Lowering fertility: over-irrigation (salt), overcropping, monoculture, overgrazing (nutrients lost).
- Also: tilling, overgrazing, land clearance (erosion, compaction), acid rain (lower pH).
- Global warming raises evaporation, so soils hold less water and plants grow less.
- Raising fertility: fertilizers, polyculture, drip irrigation, terracing, windbreaks, contour ploughing.
Lowers fertility
- Over-irrigation: salt builds up
- Overcropping, monoculture: nutrients used up
- Tilling, overgrazing: erosion, compaction
- Acid rain: pH falls
Raises fertility
- Fertilizer, manure, compost
- Polyculture, crop rotation with legumes
- Drip irrigation: less salt, less erosion
- Terraces, windbreaks, contour ploughing
Real example: before the Aswan High Dam was finished in 1970, the Nile flooded each year and left fresh silt on Egypt's fields. Now the silt is trapped behind the dam, so farmers rely on fertilizer instead.
Link all three: In a 'fertility, productivity and human activity' question, each human activity must be linked to a change in fertility AND to its effect on productivity, or the answer is capped.
Some questions give a line graph for three soils and ask you to describe a trend and calculate a change. The graph below is the same kind.
The points to remember
- Describe a trend: say what happens to one variable as the other rises, with values from the graph.
- As one rises and the other falls, it is a negative correlation; both rising is a positive one.
- Calculate a change: read both values, then subtract, and show the working.
- Give the unit (here %, or mm) with your answer.
- Read the right line: check the key before you read a value.
Describe the loam line
As vegetation cover rises from 20% to 100%, water soaked in rises from 28 to 40 mm: a positive correlation.
Calculate the clay change
From 50% to 100% cover: 14 - 9 = 5 mm more water soaked in.
Explain it
Roots and humus open up pores, so more rain can soak in.
Real example: on the Mekong's fields, farmers leave rice stubble on the soil after harvest: the cover protects the surface and helps the next rains soak in.
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How this comes up: Paper 2, Section B (a): four soil properties, each linked to how it supports productivity [4].
A farmer in Kenya compares her best field, where maize grows tall, with a poor field where maize grows slowly.
Referring to four soil properties, outline how each can contribute to high primary productivity.
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