ESS Exam Review: Soil Science, Food Production, and Conservation

Soil is a complex system composed of minerals, organic matter, gases, and water. The organic component, humus (spelled H‑U‑M‑U‑S), originates from decaying leaves and plants and should not be confused with hummus, the food. Soil acts as a vital carbon sink, storing up to 10 % of global CO₂ emissions, and it also stores heat, influencing worldwide weather patterns and rainfall. As a system, soil receives inputs such as organic matter, rainfall, oxygen, and solar energy, while outputs include plant mineral uptake, erosion, gas diffusion, and evaporation.

“Soil is effectively a vital, vital part of our ecosystem.”

Soil Properties and Classification

Particle size determines many soil characteristics. Clay particles are smaller than 0.002 mm, silt ranges from 0.002–0.05 mm, and sand can be up to 2 mm. The classic soil triangle illustrates the relative proportions of these three fractions.

• Loam—the ideal agricultural soil—contains roughly equal parts sand, silt, and clay, offering a balance of drainage and nutrient retention.
• Clay has high porosity due to numerous micro‑pores, making it nutrient‑rich, but its low permeability restricts root penetration and can lead to acidity‑related fertility issues.
• Sandy soils possess macro‑pores that provide excellent drainage, yet they dry out quickly and typically lack sufficient organic matter.

Food Production Systems

Agricultural systems are classified by scale and practice: commercial versus subsistence, arable versus pastoral, and intensive versus extensive. Terrestrial farming is generally more energy‑efficient than aquatic farming because it lies closer to the sun’s energy source.

Large‑scale agri‑business often relies on fossil fuels, monocultures, and antibiotics, rendering it unsustainable. In contrast, subsistence farms frequently employ polycultures, which enhance pollinator success and biodiversity.

“Humus is spelled like this H‑U‑M‑U‑S… please do not spell it like this because this is… hummus the food.”

Food Security and Future

Food security means providing enough affordable, nutritious food for the population. Malnutrition appears in three forms:

• Undernourishment – insufficient calorie intake.
• Overnourishment – obesity and excess calorie consumption.
• Unbalanced diets – deficiencies or excesses of specific nutrients.

Globally, 150 million children under five suffer from stunting, 45 million from wasting, and 39 million are overweight. Because arable land is finite, future yield gains must come from technology and irrigation, not from expanding cultivated area. The Green Revolution demonstrated how biochemical and mechanical advances can boost yields, and ongoing strategies include genetically modified organisms (GMOs), plant‑based diets, and waste reduction.

Soil Degradation and Conservation

Key drivers of soil degradation are:

• Overgrazing – creates bare patches, accelerates wind and water erosion, and damages root systems.
• Overcropping – depletes mineral nutrients and dries the soil.
• Deforestation – removes vegetation that moderates rainfall impact, leading to severe water erosion and nutrient loss.

Sustainable management techniques mitigate these impacts:

• Terracing and contour farming reduce slope‑driven erosion.
• Liming and adding organic matter improve pH and fertility.
• Crop rotation, especially with legumes, fixes atmospheric nitrogen and restores soil health.
• Proper irrigation prevents salinization, which occurs when evaporating water leaves a hard, salty crust that hinders crop growth.

“24 billion tons of soil are being eroded annually. It is a disaster that is not being treated as a non‑renewable resource.”

Mechanisms Behind Soil Processes

Decomposition slows in cold or water‑logged conditions because bacterial enzyme activity declines and oxygen becomes limited. Detritivores such as earthworms fragment dead organic material, after which saprotrophic bacteria and fungi complete decay, releasing nutrients back into the soil. These nutrient cycles sustain plant growth and maintain soil health.