Exercises: Pedogenesis

Define soil from a pedological point of view and reason why it is considered a virtually non-renewable resource on the scale of a human lifespan.

Observe the profile of a mature soil in the diagram. Name the horizons labelled with numbers 1 to 5 and indicate the main feature of each.

List the five factors of soil formation and explain in one sentence how each one operates.

Distinguish between parent rock, regolith and soil, and order their appearance during pedogenesis, indicating what role organisms play at the end of the process.

Explain what humus is, where it comes from (the decomposition of plant and animal remains) and why it is the key to soil fertility (nutrients, structure, water retention).

What is soil texture? Compare a sandy soil with a clayey one in terms of water retention, aeration and ease of tilling, and indicate which one warms up earlier in spring.

Name three soil degradation processes frequent in Spain (water erosion, salinisation of irrigated land, sealing by urbanisation, loss of organic matter) and explain briefly what each one consists of.

Soil stores more carbon than the atmosphere and vegetation together. Explain where this carbon resides (soil organic matter) and name two farming practices that increase it (plant cover, crop residues, rotational grazing).

Compare the soil that would develop on granite in a humid Atlantic climate with the one that forms on limestone in a dry Mediterranean climate, considering parent rock and climate as factors.

Compare a young alluvial floodplain soil with a highly developed soil from an ancient hillside: horizons, fertility, erosion risks, and reasoned agricultural use of each.

What is soil biodiversity (pedodiversity)? State three groups of soil organisms and explain the role they play in soil formation and fertility.

The pH of the soil depends largely on the parent rock. Compare the soils developed on limestones (basic) and on granites (acidic), and explain what calcicolous and silicicolous vegetation are and how the parent rock can be "read" by looking at the plants.

In the dehesa, the tree maintains the fertility of the soil. Explain what the holm oak contributes to the soil (leaf litter, shade that reduces evaporation, deep roots that recycle nutrients) and what happens to the soil and the pasture when the trees disappear.

Crop rotation with legumes and fallowing were pillars of traditional agriculture. Explain how they maintain the soil's organic matter and nitrogen and what has happened to these practices with industrial agriculture (and what is revaluing them).

Explain the difference between sheet erosion and gully erosion, which factors trigger them in hillside crops (slope, bare soil, torrential rains) and compare two control techniques (terraces, vegetation strips, mulching).

In Lanzarote, vines are grown on volcanic ash (the "enarenado" technique). Explain which physical principle this technique exploits (the porous ash lets dew and rain through but curbs evaporation) and what other traditional devices capture water in dry climates.

On a semi-arid hillside in the southeastern Iberian Peninsula with a 25 % slope on marls and schists, historical overgrazing has left active gullies ("badlands") and an organic matter content below 0,6 %. To restore the area, a municipal plan proposes terracing with heavy excavators and immediately planting Aleppo pine saplings irrigated with water bowsers.

1. a) Justify, from an edaphic and geomorphological point of view, why this massive earthmoving with heavy machinery will worsen soil loss and structural collapse in the medium term.
2. b) Design an alternative bioengineering proposal in three successive phases. Justify which soft physical measures (biologs/erosion control rolls, fascines, mulching) and biological measures (biological soil crusts, sowing native legumes/grasses, mycorrhization) you would apply in each phase to restart pedogenesis before introducing late woody species.

A soil profile developed on quartzitic sandstones under a humid Atlantic coniferous forest is analysed in the laboratory. The analytical data by layers reveal:

• Layer 1 (0-8 cm): pH 3,8; 15 % poorly decomposed organic matter (mor humus); very dark brown colour.

• Layer 2 (8-25 cm): pH 4,0; residual quartz devoid of coatings; whitish ash-grey colour; almost total absence of clays, iron, and organic matter.

• Layer 3 (25-50 cm): pH 4,9; massive enrichment in complexes of iron sesquioxides, aluminium, and colloidal humus that cement the pores, forming dense, reddish-blackish aggregates.

• Layer 4 (> 50 cm): Weathered sandstone with fissures.

1. a) Deduce the pedogenetic processes of eluviation (leaching) and illuviation that have formed this profile, assign the standard horizon nomenclature (A, E, Bhs, C...) to each layer, and identify the resulting soil type.
2. b) Explain the physicochemical mechanism by which iron and aluminium are solubilised in Layer 2 and what triggers their precipitation and immobilisation in Layer 3.
3. c) Predict what disturbances this soil would suffer if the coniferous forest were felled, heavily amended with crushed limestone (CaCO3), and subjected to deep tillage.

On a sloping olive grove estate (12 %) with a 25 cm thick surface horizon and a bulk density of 1,25 g/cm³, two agronomic strategies are monitored:

• Plot A (no-till with permanent grass and legume plant cover mechanically mown): recorded average erosion of 1,1 t/(ha·year).

• Plot B (traditional tillage with periodic cultivator passes and herbicide application to maintain bare soil): recorded average erosion of 22,5 t/(ha·year).

Natural weathering of the bedrock in this basin generates new soil at a rate of 0,15 mm/year.

1. a) Convert the natural soil formation rate to units of t/(ha·year) and determine the net annual soil balance (t/ha) in each of the plots.
2. b) Calculate how many years it will take for Plot B to completely lose its 25 cm of fertile soil, exposing the sterile substrate.
3. c) Based on the results, refute the widespread idea that "soil regenerates on its own and continuous tillage aerates the land without long-term consequences."

To expand a 40-hectare technology park, a city council is debating between two locations:

• Site 1: Alluvial floodplain with deep Class I soil (Fluvisol), high water retention capacity, 3,2 % organic matter, and moderate permeability (infiltration capacity: 25 mm/h).

• Site 2: Capped landfill and disused industrial esplanade on degraded flysch without structured soil.

The planning team argues that Site 1 is flat and reduces foundation costs by 25 %.

1. a) Assess the hydrological impact: during a torrential storm (cut-off low / DANA) of 80 mm falling in 2 hours, calculate the approximate volume of surface runoff generated at Site 1 if paved (runoff coefficient = 0,90) compared to the volume that infiltrated in its initial natural state. Deduce the consequences for downstream flooding.
2. b) Analyse two irreplaceable ecosystem services that would be irreversibly lost at Site 1 and that would barely be degraded at Site 2.
3. c) Justify from the thermodynamics of edaphic systems why artificial sealing by concrete and asphalt is a much more irreversible form of degradation than severe erosion.

Following a high-intensity wildfire on a siliceous hillside with a pine forest, the top centimetres of the soil are analysed before the arrival of autumn rains:

1. A 1,5 cm ash layer is observed on the surface.

2. Between 2 and 5 cm deep, a waxy water-repellent layer is detected (water drop penetration time WDPT > 360 seconds).

3. The structural aggregation of the surface horizon has almost completely broken down.

1. a) Explain at a molecular and physical level how this hydrophobic layer forms within the profile during the fire, and predict what type of extreme water erosion will occur during the first autumn storm with intensities of 35 mm/h.
2. b) A forestry contractor proposes immediately removing all burned timber using tracked log skidders. Argue edaphologically why authorising that extraction at this critical moment would be disastrous.
3. c) Design two low-impact emergency treatments (analysing materials, hillside arrangement, and mode of action) that a restoration team should implement during the first four weeks following the fire to retain the soil.

In Priorat, vines grow on steep slopes of "llicorella" (crumbly, weathered Paleozoic slates). A tourist brochure claims: "The roots of the vines feed directly on the slate sheets, transferring the pure minerals of the rock to the grape must, which gives the wine its unmistakable taste of wet stone and graphite."

1. a) Refute the brochure's claim from the perspective of plant physiology and soil chemistry: explain how plant roots actually absorb nutrients and why the direct "transfer of stone flavour" from the bedrock is biologically impossible.
2. b) Analyse how the edaphic properties of this skeletal soil (dark colour and low albedo, abundance of coarse fragments, presence of subvertical foliation fissures) determine the thermal regime of the vine and the deep rooting strategy to cope with summer water stress.
3. c) Conclude whether the edaphic concept of "terroir" has a measurable scientific basis or if it is a commercial myth, supporting your answer with at least two variables of the soil-plant relationship.

On a slope with an 18 % gradient dedicated to almond cultivation, an average erosion rate of 16 t/(ha·year) is quantified due to maintaining the soil without plant cover. The bulk density of the soil is 1,3 g/cm³ and the A horizon presents an average residual thickness of 15 cm. The average rate of pedogenesis in that semiarid calcareous environment is 0,2 mm/year.

1. a) Determine whether the system is above or below the tolerable soil loss rate (T-factor). To do so, calculate the natural soil formation in t/(ha·year) and obtain the net annual balance.
2. b) Calculate in how many years the fertile A horizon will completely disappear if the current management is maintained.
3. c) Design an agroecological management proposal combining a mechanical technique and a biological technique to reduce erosion below the natural renewal rate, justifying how each interacts with the kinetic energy of raindrops and the velocity of runoff flow.

During the legislative process of the European Soil Directive, several sectors argued that «legislating on soil quality violates the private property rights of farmers and developers, since soil is an immobile private good that does not move between countries, unlike air or river water».

1. a) Refute this argument by demonstrating how the biological and chemical degradation of private soil generates negative cross-border impacts on two global public goods: the carbon cycle and the hydrogeological quality of shared aquifers.
2. b) As a scientific advisor, design a diagnostic battery with three objective indicators (one physical, one chemical, and one biological) to assess whether a European agricultural soil is in a «degraded» or «healthy» state. Justify what specific functional alteration each indicator measures.
3. c) Justify why the principle of «land degradation neutrality» (which requires equivalent de-sealing for any new urban development) is essential for European food sovereignty on a timescale of centuries.