Soil formation (pedogenesis)
In the Mixteca region of Oaxaca there are hillsides where only tepetate is left, a hard, whitish rock, and others where the milpa still yields ears of corn. Soil is not simply "dirt": it is a living system that takes centuries to form and that a few torrential rains can wash away. How does it form, why does it change so much from one place to another, and why do we need to take care of it?
What soil is and how it forms
When we dig in a pine-oak forest we find leaf litter on top, a dark layer with earthworms, a clayier zone below, and, at the bottom, fragmented rock. How does a compact rock become a living system organized in layers?
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.
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.
List the five factors of soil formation and explain in one sentence how each one operates.
List the five factors of soil formation and explain in one sentence how each one operates.
The soil profile and humus
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.
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.
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…
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.
- 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.
- 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.
- 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.
The factors that make soils diverse
In Yucatán, on limestone, the soil is thin and sometimes reddish, and rock sticks out everywhere. On the slopes of Pico de Orizaba, on volcanic material, there are deep, dark soils. Why do different rocks, climates, and reliefs produce such different soils?
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…
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.
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.
Soil life and soil diversity
A spoonful of forest soil contains more microorganisms than there are people in Mexico. What do all those living things do?
What is soil biodiversity (pedodiversity)? State three groups of soil organisms and explain the role they play in soil formation and fertility.
What is soil biodiversity (pedodiversity)? State three groups of soil organisms and explain the role they play in soil formation and fertility.
Soil degradation
After a downpour in the sierra, the river runs chocolate-brown, loaded with mud, next to a deforested hillside; in the neighboring forest the stream runs almost clear. What processes make the fertile layer disappear?
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…
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.
- 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.
- 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.
Soil as a resource: a matter of rates
Saying that soil is "nonrenewable" does not mean it does not form, but that it is lost much faster than it forms. To check this, we have to compare rates in the same units.
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…
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.