Exercises: Soil formation

The diagram shows a soil profile with its horizons numbered 1 to 5.

1. a) Name the horizons labelled with numbers 1 and 5.
2. b) Which number indicates the horizon where organic matter mixes with minerals, and where most roots and earthworms live?
3. c) Which number corresponds to fragments of slightly weathered bedrock?

Order the following stages of soil formation: appearance of distinct horizons, fragmentation of bedrock, accumulation of organic remains on the surface, and mature soil.

State two functions of soil that are important for living things, and briefly explain one of them.

Name the horizons of a mature soil from top to bottom and cite one feature of each.

Define what humus is and explain why it is so important for soil fertility.

Name two organisms that live in the soil and help to form it, and explain what one of them does.

Classify these soil materials according to whether they come from the bedrock or from living things: sand, fallen leaves, clay, earthworm remains, gravel, and humus.

Explain in your own words what bedrock is and what happens to it during soil formation.

Explain the difference between soil horizon A and horizon B: what each contains and which process predominates in them (accumulation of organic matter or leaching of substances by infiltrating water).

Forming just one centimetre of soil can take several centuries. Reason why soil is considered a non-renewable resource on a human time scale and cite an activity that degrades it.

How does climate influence soil formation? Compare the expected differences between soil in a cold, wet region and soil in a warm, dry region in terms of thickness and amount of organic matter.

A market gardener adds compost (already decomposed plant remains) to their vegetable plot. Explain which soil horizon improves with this addition and why fertility increases.

The B horizon of many soils has reddish or yellowish tones. Relate this colour to the leaching of substances from the A horizon by infiltrating water.

The soil on a hillside is usually thinner than the soil at the bottom of the valley. Explain which process causes this and what happens to the materials that the water carries down the slope.

Earthworms turn over the soil, bury leaves, and open tunnels. Explain two ways in which they improve the soil for plants.

In very rainy areas, water washes the soil and carries nutrients down to deeper layers. Reason why the crops in these areas need more fertiliser and which horizon is most affected.

After a forest fire on a pine hillside, the soil has lost all its leaf litter (O horizon) and the high temperatures have turned the surface of the A horizon into a compacted, hydrophobic (water-repellent) layer of ash.

1. a) Predict how this situation will affect the relationship between infiltration and surface runoff when the first torrential autumn rains arrive, and what impact this will have on the lower soil horizons.
2. b) Design an emergency action plan with two complementary measures (one physical and one biological) to halt immediate erosion and promote the retention of the remaining soil.

Two agricultural soils are analysed for a reforestation project:

• Soil X: total thickness of 75 cm, heavily clayey and compact texture, 10 cm A horizon with 1,8 % organic matter and low permeability.

• Soil Y: total thickness of 30 cm over fissured limestone rock, loamy texture, 20 cm A horizon with 7,2 % organic matter and abundant earthworm activity.

A technician states: "Soil X is undoubtedly more fertile than Soil Y because it has more than twice the total thickness".

1. a) Refute the technician's argument by justifying which properties reflect the real fertility of a soil and why total thickness alone is not a sufficient indicator.
2. b) Predict what rooting and aeration limitations Soil X will present compared to Soil Y during a prolonged period of rain.

In an optimal agricultural soil, the volume is approximately distributed as: 45% mineral matter (geosphere), 5% organic matter (biosphere), 25% water (hydrosphere), and 25% air (atmosphere).

A farmer drives very heavy machinery over the waterlogged ground and then applies prolonged flood irrigation, causing severe soil compaction.

1. a) Predict how the percentages of air and water in the soil pores will change after these two actions, and what will happen to the infiltration rate of rainwater.
2. b) Justify the direct biological consequences of this disturbance on root cellular respiration and on the aerobic microorganisms that convert organic matter into mineral nutrients.

The thickness of the A horizon is measured in three nearby places: a grassland (25 cm), a forest (15 cm), and an intensively farmed field (6 cm). Interpret the data, relate them to the input of organic matter and to ploughing, and propose one measure to restore the soil of the farmland.

At the bottom of an abandoned quarry, completely smooth and bare limestone bedrock has been left exposed. To restore the ecosystem, a proposal suggests directly planting trees and shrubs watered with liquid fertilizers, claiming that this will make the soil form much faster.

1. a) Justify why this initial proposal will fail from the perspective of pedogenesis, and what physical and nutritional limitations these plants will face.
2. b) Design the actual biological sequence that should take place on the rock: indicate which pioneer organisms must colonize it first, how they act on the rock to weather it, and what conditions will ultimately allow vascular plants to establish.

Two soils have developed over the same bedrock (granite) and over the same geological time period on a mountain slope, but they display very different profiles:

• Soil 1 (upper slope with a 35% gradient exposed to dry winds): barely 10 cm thick, lacking a B horizon, and with scarce organic matter.

• Soil 2 (nearly flat and moist valley bottom, sheltered by a deciduous forest): over 90 cm thick, with an A horizon rich in humus and a well-developed B horizon.

Justify how the interaction of relief, local climate, and living organisms has caused such marked differences between both soils despite sharing the same bedrock and formation time.

Two neighbouring hillsides with the same bedrock and the same rainfall have different uses: one is covered with forest and the other has an olive grove with bare soil between the trees. Compare erosion and soil formation on both hillsides and justify the differences.

In a Mediterranean agricultural area, pedogenesis generates an average of 0,1 mm of soil per year. After an autumn storm over a plowed field lacking plant cover, a loss of 1,2 cm in fertile soil thickness due to erosion is recorded in a single afternoon.

1. a) Calculate how many years of natural soil formation were lost during that storm.
2. b) Justify, based on this calculation, why soil must be considered a non-renewable resource on a human timescale, and predict two direct consequences of this loss for agricultural productivity and water retention in the plot.