Exercises: Soil

What is soil and what five factors are involved in its formation? Briefly describe the role of each.

Name the main horizons of a mature soil, from the surface to the bottom, and state one characteristic of each.

What is soil texture and what three particle-size fractions define it?

What is the O horizon of the soil and which humification process occurs in it? What does it provide to the lower horizons?

What is the parent rock and how does its composition influence the properties of the soil that forms over it (pH, nutrients, texture)?

Soil hosts bacteria, fungi, earthworms and arthropods. Explain what role that community plays in soil fertility (decomposition, structure, aeration).

What differences are there between a young, poorly developed soil and a mature soil with differentiated horizons? Give an example of terrain where you would expect to find each one.

What is the water retention capacity of a soil and which grain-size fraction increases it? Why does a very sandy soil require more frequent watering?

The diagram shows the profile of a mature soil with six numbered levels. Assign to each number its horizon (O, A, E, B, C, R) and describe the dominant process (decomposition, leaching or eluviation, precipitation) in levels 1, 3 and 4.

A soil with $40\ \%$ sand, $40\ \%$ silt and $20\ \%$ clay is classified as 'loam' on the texture triangle. Explain what agricultural advantages this intermediate texture has compared to a very sandy or very clayey soil.

Water erosion washes away large amounts of soil every year on cultivated slopes. Explain why the A horizon is the first to be lost and what consequences this loss has for soil fertility.

A soil contains $60\ \%$ sand, $30\ \%$ silt and $10\ \%$ clay.

1. a) Classify it approximately with the texture triangle;
2. b) predict its drainage and its agricultural behaviour compared with a loam soil;
3. c) propose an amendment to improve it.

Soils over limestones tend towards a neutral or alkaline pH and those over granite, towards an acidic pH. Explain the influence of the parent rock on soil pH and which crops adapt best to each case.

Compare the processes of laterisation, typical of humid tropical climates, and podsolisation, typical of cold humid climates: which elements are leached, which horizons form and what fertility results in each case.

Soils near old smelters accumulate lead and arsenic. Explain the sources of that pollution, why heavy metals persist for centuries in the soil and two decontamination techniques.

What is soil sealing by urbanisation and which ecosystem functions (infiltration, agricultural production, habitat, water regulation) are lost with it? Relate it to the increase in urban flooding.

On a slope of the Betic piedmont dedicated to olive grove cultivation on vertisols with a slope of $12\,\%$, water erosion losses of $28\ \text{t}\cdot\text{ha}^{-1}\cdot\text{year}^{-1}$ have been recorded, while the estimated rate of pedogenesis in the area is only $0,8\ \text{t}\cdot\text{ha}^{-1}\cdot\text{year}^{-1}$.

1. a) Based on these data, justify why the soil in this holding must be managed as a non-renewable resource and calculate how many years it would take to completely lose a superficial A horizon of $15\ \text{cm}$ in thickness (average bulk density: $1,3\ \text{g/cm}^3$; $1\ \text{ha} = 10\,000\ \text{m}^2$) if no intervention takes place.
2. b) The farmer plans to carry out continuous tilling in the direction of maximum slope to 'rapidly drain water from autumn storms and avoid waterlogging'. Rebut his proposal technically by predicting what hydrogeomorphological processes it will trigger, and design a combined strategy of three agronomic or structural measures adapted to olive groves to reduce the erosion rate below the rate of soil renewal.

In a traditional irrigated sector of the south-eastern Iberian Peninsula under a semi-arid climate (potential evapotranspiration of $1100\ \text{mm/year}$ and precipitation of $300\ \text{mm/year}$), the saline water table is located at only $1,1\ \text{m}$ depth with water whose electrical conductivity is $4,2\ \text{dS/m}$. The farmer observes the appearance of whitish crusts on the surface and a drastic drop in the yield of his citrus trees, so he decides to increase the frequency of drip irrigations with that same water to 'keep the wet bulb moist and continuously dissolve the salts'.

1. a) Interpret the water and salt balance of the soil profile and predict the consequences of the farmer's decision, justifying why salinisation will increase and what osmotic and sodification effects it will cause on soil structure and vegetation.
2. b) Design an agronomic and engineering action plan to restore the fertility of this soil, detailing the role of artificial subsurface drainage, the calculation of a leaching fraction with low-salinity water, and the justified use of gypsum ($\text{CaSO}_4$) amendments.

In a pedological survey, two profiles with marked contrasts are analysed:

• Profile X: Developed on loess in a temperate-cold continental climate with steppe grassland. It features an Ah horizon $70\ \text{cm}$ thick with $7,5\,\%$ humified organic matter, a very stable crumbly structure, high cation exchange capacity saturated in $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$, and secondary accumulation of carbonates in the deep Ck horizon.

• Profile Y: Developed on limestones in a Mediterranean climate with pronounced summer drought. It features a thin A horizon ($<10\ \text{cm}$) with scarce organic matter ($<1,5\,\%$) over an intensely red (rubefaction) Bt horizon, clayey and highly enriched in unhydrated iron sesquioxides (hematite), in sharp contact with the underlying bedrock.

1. a) Reasonably identify which major soil type each profile corresponds to (chernozem and Mediterranean red soil / terra rossa) and analyse how the interaction between the bioclimatic regime (water and thermal seasonality) and lithology has determined the horizons observed in each case.
2. b) Predict how each profile will respond to a scenario of intensive cultivation under conventional tillage: which one will experience more accelerated structural degradation and loss of fertility? Justify your answer by analysing the dynamics of humus mineralisation and susceptibility to crusting and leaching.

In an intensive agricultural irrigated district in an alluvial plain, four problems are recorded simultaneously:

1) formation of persistent puddles after moderate rainfall that previously infiltrated rapidly, and difficulty for root elongation below $25\ \text{cm}$;

2) continuous decline of soil organic carbon, dropping from $2,3\,\%$ to $0,7\,\%$ in fifteen years;

3) increase in the electrical conductivity of the saturation extract up to $5,8\ \text{dS/m}$;

4) development of rills and gullies along the edges of the plots after autumn storms.

1. a) Diagnose the specific physical and chemical degradation processes that are operating and explain the positive feedback relationships between the continuous use of heavy machinery, the decline in organic matter, and the increase in surface runoff.
2. b) To establish a monitoring network, select a quantifiable indicator for each of the four observed problems, indicating the method or unit of measurement and what critical threshold would warn of functional soil collapse.
3. c) Design an integrated programme of corrective measures for the district combining mechanical, chemical, and biological management practices aimed at simultaneously reversing compaction and loss of structure.

A dryland cereal cooperative in the Northern Plateau is considering certifying carbon credits by transitioning from conventional moldboard tillage with straw removal to a no-till farming model with stubble cover. In the initial state, the loam soil has $1,1\,\%$ organic matter in the first $30\ \text{cm}$ and a bulk density of $1,42\ \text{g/cm}^3$.

1. a) A consulting firm promises the farmers that 'with this change, the soil will triple its carbon stock in five years and will act as a permanent and unlimited sink for atmospheric $\text{CO}_2$'. Technically refute this statement by analyzing the biogeochemical factors that limit the stabilization of organic carbon in Mediterranean soils (seasonal microbial kinetics, saturation of the clay mineral fraction, and C/N ratio of crop residues).
2. b) Explain with reasoned arguments how the progressive increase in organic matter will modify the moisture retention curve, the cation exchange capacity (CEC), and the formation of water-stable aggregates in that soil.
3. c) If after ten years the percentage of organic matter rises to $1,8\,\%$ in the first $20\ \text{cm}$ (with a bulk density stabilized at $1,30\ \text{g/cm}^3$), calculate the net amount of organic carbon fixed per hectare (considering that carbon represents $58\,\%$ of the organic matter mass; $1\ \text{ha} = 10\,000\ \text{m}^2$) and discuss the fragility of this storage pool in the event of a possible return to traditional tillage.

In a Mediterranean mountain valley with average slopes of $30\,\%$, a stepped system of agricultural benches or terraces supported by dry stone masonry walls was completely abandoned four decades ago. Following an extreme Mediterranean event of $210\ \text{mm}$ of precipitation in $24\ \text{hours}$, the hillside suffered catastrophic collapses that blocked the valley bottom with debris and mud flows.

1. a) Analyze the hydrodynamics and slope mechanics of this system: why can an abandoned and unmaintained set of terraces become, in the long term, much more unstable and prone to generating catastrophic mass movements ('domino effect') than a natural slope with its dynamic equilibrium plant profile?
2. b) On the platforms of the terraces that are still standing, the soil has a thickness of more than $1\ \text{m}$, whereas on adjacent deforested slopes the rocky substrate almost outcrops. Justify the ecological role of terraces as edaphic and moisture reservoirs in the context of global change.
3. c) As an environmental technician, you must evaluate two intervention options for the slope: 1) remove the deteriorated walls using heavy machinery, reprofiling the slope to a uniform gradient, and sow herbaceous species; or 2) restore the collapsed dry stone walls by incorporating weepholes and runoff spillways, combined with revegetation using native shrubs. Justify your technical choice based on criteria of sediment balance, life-cycle costs, and preservation of edaphic function.

In late August, a high-severity forest fire scorches $800\ \text{hectares}$ of pine forest on a siliceous hillside with slopes of $32\,\%$. During the post-fire inspection, it is verified that in the top $4\ \text{cm}$ of soil, applied water droplets do not infiltrate for more than four minutes (WDPT test), leaving a layer of ash and completely dry mineral particles underneath after scraping the surface. Weather models warn of the imminent arrival of an autumn cold drop (DANA).

1. a) Interpret the physicochemical origin of the detected hydrophobicity and predict how this repellent horizon will alter the water balance partitioning (infiltration rate vs. runoff coefficient) and the probability of generating debris flows during a high-intensity downpour.
2. b) A logging cooperative requests authorization to enter with heavy tracked machinery to urgently fell and extract the burned timber. Write a technical justification advising against immediate entry, arguing the effects that traffic and dragging would cause on surface compaction, the disruption of the hydrophobic layer, and the net rate of soil loss.
3. c) Design an emergency action plan to be executed in the three weeks prior to the DANA: compare the protective efficacy and technical feasibility of straw mulching versus the construction of contour log barriers (fajinas) or check dams using felling debris, determining on which sectors of the slope you would apply each one.

In a drainage basin feeding into an ephemeral stream (rambla) in the southeastern Iberian Peninsula (mean annual precipitation of $280\ \text{mm}$, potential evapotranspiration of $1200\ \text{mm}$), severe ecosystem regression is recorded, linked to three pressures: clearing of native scrubland for woody crop plantations on $15\,\%$ slopes through intensive tillage, massive pumping that has lowered the water table by $70\ \text{m}$, and abandonment of traditional water conservation practices (boqueras and albarradas).

1. a) A development company argues that 'the area is a natural desert and therefore one cannot speak of desertification or soil loss attributable to human activities'. Technically debunk this statement by distinguishing a climatomorphic desert from anthropogenic desertification based on ecosystem functional variables (biomass, nutrient cycling, infiltration capacity, and edaphic resilience).
2. b) Analyze the cascading mechanism by which the combination of aquifer overexploitation and slope tillage leads to the salinization of low fertile floodplains (vegas) and headward erosion in the ephemeral gully network.
3. c) Design a comprehensive restoration plan for the bed and banks of the rambla that combines bioengineering measures (gabion check dams or permeable masonry dams for flood attenuation and sediment capture) with reforestation techniques based on phytostabilizing species with mycorrhization, justifying how they will act to restore the hydrological cycle and soil formation.