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 soil 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 behavior compared with a loam soil;
3. c) propose an amendment to improve it.

Soils over limestone tend toward a neutral or alkaline pH, and those over granite, toward an acidic pH. Explain the influence of the parent rock on soil pH and which crops adapt best to 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 urbanization 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 in the Coast Range foothills of California used for olive orchard cultivation on vertisols with a slope of $12\,\%$, water erosion losses of $28\ \text{t}\times\text{ha}^{-1}\times\text{year}^{-1}$ have been recorded, while the estimated rate of pedogenesis in the area is only $0.8\ \text{t}\times\text{ha}^{-1}\times\text{year}^{-1}$.

1. a) Based on these data, justify why the soil on this farm must be managed as a nonrenewable resource and calculate how many years it would take to completely lose a surface 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 orchards to reduce the erosion rate below the rate of soil renewal.

In a traditional irrigated area of the San Joaquin Valley in California, 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 salinization 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 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 program of corrective measures for the district combining mechanical, chemical, and biological management practices aimed at simultaneously reversing compaction and loss of structure.