Soil formation (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.

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

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

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.

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).

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.

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.

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.

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.

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).

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.

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."

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.

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.