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.

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.

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 color.

• Layer 2 (8-25 cm): pH 4.0; residual quartz devoid of coatings; whitish ash-gray color; almost total absence of clays, iron, and organic matter.

• Layer 3 (25-50 cm): pH 4.9; massive enrichment in complexes of iron sesquioxides, aluminum, 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 aluminum 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.

It is estimated that under natural conditions about $1\ \text{cm}$ of soil forms every $100$-$500$ years, whereas erosion can wash away several centimeters in a single torrential event. Calculate how long nature would take to replenish $30\ \text{cm}$ of lost soil (use the most favorable rate) and conclude what this implies for agricultural management.