Elemental Science 103: Soils, Agriculture, and Environmental Impacts

The Importance and Function of Soils

• Plant Growth Medium: Soils serve as the primary medium for terrestrial plant growth, making them indispensable for global agriculture and human food production.

• Ecological Contribution: Soils support terrestrial ecosystems through elemental cycling and the maintenance of biodiversity.

• Terrestrial Biodiversity: Most terrestrial biodiversity resides within the soil. This includes a wide range of vertebrates and invertebrates, such as mice, earthworms, and various insects.

• Microscopic Biodiversity: On a microscopic scale, soils contain fungi, bacteria, and protozoa. Viruses are also present, though most do not cause human diseases and instead act as vectors for genetic information.

• Nutrient Cycling (Nitrogen Fixation): A majority of nitrogen flowing through ecosystems enters via bacteria found in root nodules, which fix atmospheric nitrogen into a usable form for plants.

• Construction Material: Soil is one of the cheapest construction materials globally. Most dams on Earth are constructed from soil, necessitating a deep understanding of soil physical and structural properties.

Soil Structure: Horizons and Profiles

• Soil Horizon: A horizontal layer of soil characterized by specific physical properties and chemical composition.

• Soil Profile: The vertical arrangement of these horizontal layers from the surface to the underlying bedrock.

• Standard Horizon Sequence in Temperate Forest Soils:

  • O Horizon: This layer is found at the very top and contains more than $17\,\%$ organic content. It consists of decaying organic materials.

  • A Horizon: Located near the surface, this contains humus, which is the byproduct of the decomposition of organic material. It is typically dark in color.

  • B Horizon: A mineral-rich layer influenced by the horizons above. It contains precipitates such as iron oxides and aluminum oxides. These compounds are leached from the A horizon by downward water flow.

  • C Horizon: The deepest layer of the soil profile. It is primarily mineral in nature and retains the properties of the underlying parent bedrock, such as granite.

• Visual Characteristics: Iron oxides in the B horizon often produce a characteristic ochre or orange color, acting as a diagnostic feature for the mineral content of that layer.

• Anthropogenic Disturbance: Human activities such as plowing for agriculture mix the O, A, and B horizons. This disruption of the natural soil profile diminishes the specific functional roles each horizon plays in the ecosystem.

Topsoil Formation and Organic Decay

• Topsoil Composition: Topsoil is the result of interactions between decaying organic matter (leaves, stems, flowers, seeds) and mineral particles.

• Bioturbation: The process by which soil organisms (invertebrates, protozoa, fungi, bacteria) mix mineral particles with decaying organic matter. They also release stored chemical elements like phosphorus ($P$) and nitrogen ($N$).

• Environmental Factors for Decay: Temperature, water availability, and the diversity of the decomposer food web determine the rate of organic matter decomposition.

  • Boral Forest: Decay takes approximately $353\,\text{years}$ due to cold temperatures and slow bacterial/fungal activity.

  • Tropical Rainforest: Decay takes only $5\,\text{months}$ due to warm, wet conditions and high biological activity.

  • Temperate Forest: Decay typically ranges from $4\,\text{to}\,17\,\text{years}$.

• Functional Benefits of Topsoil:

  • Improved water-holding capacity.

  • Enhanced nutrient retention.

  • Better water infiltration and aeration.

  • Structural stability (the "glue" of the soil).

Soil Texture and Physical Properties

• Soil Texture Definition: The relative composition of mineral particles in the soil based on their diameters.

• Particle Size Categories (Diameters):

  • Sand: Largest particles ($0.05\,\text{to}\,2.0\,\text{mm}$). Categories include coarse ($0.5\,\text{to}\,1.0\,\text{mm}$), medium ($0.25\,\text{to}\,0.5\,\text{mm}$), and fine ($0.10\,\text{to}\,0.25\,\text{mm}$).

  • Silt: Intermediate particles ($0.002\,\text{to}\,0.05\,\text{mm}$).

  • Clay: Smallest particles (less than $0.002\,\text{mm}$).

• Porosity: The network of empty spaces (pores) between solids. Large particles (sand) create larger pores, facilitating air and water movement (aeration and infiltration).

• Surface-to-Volume Ratio: Smaller particles (clay) have a much higher surface-to-volume ratio, allowing them to coat themselves in nutrients and hold water tightly.

• Comparison of Textural Properties:

  • Sand: High infiltration and aeration; poor water and nutrient holding capacity.

  • Clay: Poor infiltration and aeration; high water and nutrient holding capacity.

  • Loam: A relatively uniform distribution of sand, silt, and clay. It is medium/average in all properties, making it the most suitable texture for agriculture.

The Soil Textural Triangle

• Purpose: A tool used to categorize soils based on the relative percentages of their mineral fractions (sand, silt, and clay).

• Laboratory Preparation: Organics must be burned off before testing. A stack of sieves with specific mesh sizes is used to capture different diameter fractions.

• How to Use the Triangle:

  • Sand Axis: Located at the bottom; read from right to left using diagonal lines.

  • Silt Axis: Located on the right-hand side; read from top to bottom using diagonal lines.

  • Clay Axis: Located on the left-hand side; read from bottom to top using horizontal lines.

• Example Calculation: A soil with $50\,\%$ sand, $10\,\%$ silt, and $40\,\%$ clay is classified as "sandy clay."

Stokes' Law and Particle Settling

• Concept: Used to determine the diameter of fine particles that cannot be measured by sieves. It relies on the principle that larger particles settle faster in a fluid due to the pull of gravity.

• The Stokes' Law Equation:

$v = \frac{g \times D^2 \times (\rho_p - \rho_f)}{18 \times \mu}$

• Constants and Variables:

  • $v$: Settling velocity (cm/s).

  • $g$: Gravitational acceleration constant ($900\,\text{cm/s}^2$).

  • $D$: Particle diameter (cm).

  • $\rho_p$: Particle density (typically $2.7\,\text{g/cm}^3$).

  • $\rho_f$: Fluid density (water is $1\,\text{g/cm}^3$).

  • $\mu$: Viscosity of the fluid ($0.01\,\text{g/cm}\cdot\text{s}$ or dynes/s/cm²).

• Sample Problem Solution:

  • Given: $1\,\text{meter}$ ($100\,\text{cm}$) settling tube. Fraction settles in $8\,\text{minutes}$ ($480\,\text{seconds}$).

  • Velocity ($v$) = $\frac{100\,\text{cm}}{480\,\text{s}} \approx 0.21\,\text{cm/s}$.

  • Rearranging for $D^2$: $D^2 \approx 2.27 \times 10^{-5}\,\text{cm}$.

  • Root square: $D = 4.76 \times 10^{-3}\,\text{cm}$.

  • Conversion to mm: $4.76 \times 10^{-2}\,\text{mm}$.

Global Soil Degradation and Management

• Status: Soil degradation is a worldwide problem affecting North America, South America, Africa, Asia, Australia, Europe, and the Arctic.

• Primary Causes: Over-cultivation, misuse of fertilizers, deforestation, and over-grazing.

• Over-grazing Feedback Loop: Excess livestock remove native grasses and their root systems. This removes the "glue" holding soil in place, exposing bare topsoil to wind and water erosion. Animal weight collapses soil pore spaces (compaction), reducing water infiltration and aeration, which prevents grass regeneration.

• Management Solutions:

  • Crop Rotation: Changing crops annually based on differing nutritional requirements to prevent soil depletion.

  • Terracing: Creating horizontal levels on steep mountain slopes to impede downward water movement and erosion.

  • Contour Farming: Planting rows that follow the topography of the land to manage water flow.

  • No-till Farming: Leaving soil and plant cover intact to reduce erosion and allow organic matter to replenish nutrients.

  • Agroforestry: Keeping or planting trees to provide shade, stability through root systems, and secondary income sources (e.g., fruit).

Groundwater Withdrawal and Land Subsidence

• Cone of Depression: When water is pumped from an aquifer faster than the recharge rate, the water table drops around the well, forming a V-shaped depression. This can cause shallower neighboring wells to run dry.

• Land Subsidence: The compression and lowering of the land surface relative to sea level. This occurs when groundwater is overdrawn, removing the buoyant support that water provides to soil pore spaces.

• Case Study: San Joaquin Valley, California: Significant subsidence has occurred due to agricultural pressure on aquifers. Between 1965 and 2016, one area dropped by $8.6\,\text{feet}$ ($2.6\,\text{meters}$). Another area dropped $6.2\,\text{feet}$ between 1988 and 2016. Such displacement damages infrastructure like bridges and wells.

Salinization and Desertification

• Salinization: The accumulation of salt in soil. When freshwater evaporates, it leaves a white crust of salt behind. High salt content destroys soil structure, causing cracks and crevices.

• Gravity Flow (Flood) Irrigation: This method, involving flooding ditches with water, is a primary cause of salinization in dry climates because much of the standing water evaporates rather than reaching plant roots.

• Desertification: The deterioration of land in arid, semi-arid, and dry sub-humid areas. It affects approximately $1\,\text{billion}$ people and $25\,\%$ of the total global land area. Key areas include the Sahel (sub-Saharan Africa), the Midwest USA, and parts of China and Australia.

• Irrigation Solutions:

  • Drip Irrigation: Pipes provide water drip-by-drip directly to roots, matching the rate of plant absorption and minimizing evaporation.

  • Center Pivot Irrigation: Distributes water as a gentle mist to mimic natural rain, improving effectiveness and reducing water waste.

Biogeochemical Cycles and Nutrient Pollution

• Linear vs. Circular Cycles: Natural systems (carbon, nitrogen, phosphorus, water) are circular and balanced over long timescales. Human activity has made these cycles linear: we extract minerals/nutrients, use them for crops, and discharge them as waste into waterways or landfills.

• The Haber-Bosch Process (1913): Created just before World War I as part of chemical weapons research, it was later adapted to fix atmospheric nitrogen for synthetic fertilizers. This has led to a dramatic increase in global nitrogen flow.

• Nitrogen Statistics: By the year 2000, human activity put as much nitrogen into the environment via fertilizers as does the natural nitrogen cycle ($100\,\text{teragrams}$), exceeding the natural processing capacity of ecosystems.

Eutrophication, Hypoxia, and Lake Erie

• Eutrophication Process: Excess nutrients (nitrogen and phosphorus) from farms and urban runoff enter water bodies. This triggers uncontrollable algal blooms. When algaes die and decay, bacteria consume dissolved oxygen to break them down.

• Oligotrophic vs. Eutrophic: Oligotrophic lakes are low in nutrients and clear; eutrophic lakes are nutrient-rich and murky with decaying organic matter.

• Hypoxia and Anoxia:

  • Hypoxia: Dissolved oxygen levels below $2\,\text{mg/L}$.

  • Anoxia: A complete absence of oxygen.

• Lake Erie Case Study:

  • Lake Erie is the shallowest of the Great Lakes and undergoes thermal stratification in summer. Warm surface water does not mix with cold, dense bottom water (the hypolimnion).

  • Non-point source phosphorus enters the Western Basin (e.g., from the Maumee River and Canadian farms), triggering massive blooms visible from space.

  • Oxygen depletion in the Central Basin causes major fish kills and impacts drinking water safety due to toxic algaal species.

• Solutions to Eutrophication:

  • Short-term: Dredging nutrient-rich sediments (expensive and aesthetic only).

  • Long-term: Improving wastewater treatment, controlling construction runoff, protecting coastal wetlands to act as natural nutrient filters, and creating closed-loop farm systems to capture and reuse manure leachate.

Questions & Discussion

• Final Exam Scope: The final exam exclusively covers information from the lectures. While e-learning modules (like Water Balance) may help clarify concepts, students are only tested on lecture content and specific questions from Assignments 1 and 2.

• Email Correspondence: When contacting the instructor, students must include their course code and student ID number to ensure a response.