🔆Environmental Chemistry I Unit 7 – Soil Chemistry: Nutrients and Pollutants

Key Concepts and Terminology

• Soil chemistry studies the chemical properties and reactions occurring in soil
• Soil composition refers to the proportions of solid, liquid, and gaseous components in soil
• Soil structure describes the arrangement of soil particles and pores
  • Influences water retention, aeration, and root growth
• Soil nutrients are essential elements required for plant growth and development
• Macronutrients are needed in larger quantities (nitrogen, phosphorus, potassium)
• Micronutrients are needed in smaller amounts but are still crucial (iron, zinc, manganese)
• Nutrient cycles describe the movement and transformation of nutrients in soil
• Soil pH measures the acidity or alkalinity of soil on a scale from 0 to 14
• Soil pollutants are substances that contaminate soil and harm the environment

Soil Composition and Structure

• Soil is composed of mineral particles, organic matter, water, and air
• Mineral particles are derived from weathered rocks and make up the bulk of soil
  • Classified by size into sand, silt, and clay
• Organic matter consists of decomposed plant and animal residues
  • Provides nutrients, improves soil structure, and enhances water retention
• Soil structure refers to the aggregation of soil particles into larger units called peds
• Soil structure types include granular, blocky, prismatic, and platy
• Soil texture describes the relative proportions of sand, silt, and clay particles
• Soil porosity is the volume of soil occupied by air and water
• Soil bulk density is the mass of dry soil per unit volume

Essential Soil Nutrients

• Nitrogen (N) is essential for plant growth and chlorophyll production
  • Deficiency leads to stunted growth and yellowing of leaves (chlorosis)
• Phosphorus (P) is crucial for root development, energy transfer, and fruit formation
• Potassium (K) regulates water balance, enzyme activation, and disease resistance
• Calcium (Ca) is important for cell wall formation and root growth
• Magnesium (Mg) is a central component of chlorophyll and aids in photosynthesis
• Sulfur (S) is required for protein synthesis and chlorophyll formation
• Micronutrients such as iron (Fe), zinc (Zn), and manganese (Mn) are needed in trace amounts
  • Deficiencies can cause specific symptoms (interveinal chlorosis, stunted growth)

Nutrient Cycles in Soil

• Nitrogen cycle involves the transformation of nitrogen between various forms
  • Nitrogen fixation converts atmospheric N2 to ammonia (NH3) by bacteria or lightning
  • Nitrification converts ammonia to nitrite (NO2-) and then to nitrate (NO3-) by bacteria
  • Denitrification reduces nitrate to nitrogen gas (N2) under anaerobic conditions
• Phosphorus cycle is the movement of phosphorus through soil, plants, and animals
  • Weathering of rocks releases phosphorus into soil
  • Plants absorb phosphorus as phosphate ions (H2PO4-, HPO42-)
  • Decomposition of plant and animal residues returns phosphorus to soil
• Carbon cycle involves the exchange of carbon between the atmosphere, soil, and living organisms
  • Photosynthesis fixes atmospheric CO2 into organic compounds
  • Respiration and decomposition release CO2 back into the atmosphere

Soil pH and Its Effects

• Soil pH influences nutrient availability, microbial activity, and plant growth
• Acidic soils (pH < 7) can result from rainfall, organic matter decomposition, or fertilizer use
  • May lead to aluminum toxicity and reduced availability of certain nutrients (phosphorus, molybdenum)
• Alkaline soils (pH > 7) can occur in arid regions or due to excessive liming
  • May cause iron, manganese, and zinc deficiencies
• Optimal soil pH range for most plants is between 6.0 and 7.0
• Soil pH can be adjusted through liming (to raise pH) or applying sulfur (to lower pH)
• Buffer capacity is the soil's ability to resist changes in pH

Pollutants in Soil

• Heavy metals such as lead (Pb), cadmium (Cd), and mercury (Hg) can accumulate in soil
  • Sources include industrial activities, mining, and improper waste disposal
  • Can be toxic to plants, animals, and humans
• Persistent organic pollutants (POPs) are stable compounds that resist degradation
  • Examples include pesticides (DDT), dioxins, and polychlorinated biphenyls (PCBs)
  • Can bioaccumulate in the food chain and pose health risks
• Petroleum hydrocarbons can contaminate soil due to oil spills or leaking storage tanks
  • Can hinder plant growth and soil microbial activity
• Excess nutrients from fertilizers or manure can lead to soil and water pollution
  • Nitrate leaching can contaminate groundwater
  • Phosphorus runoff can cause eutrophication of surface waters

Soil Testing and Analysis

• Soil testing involves collecting soil samples and analyzing their properties
  • Determines nutrient levels, pH, organic matter content, and other parameters
• Soil sampling should be representative of the area and follow proper procedures
  • Collect samples from multiple locations and depths
  • Avoid sampling near field edges, fertilizer bands, or unusual areas
• Laboratory analysis provides accurate measurements of soil properties
  • Extractants are used to determine plant-available nutrient levels
  • Spectrophotometry, atomic absorption, and other techniques are employed
• Interpretation of soil test results helps guide fertilizer recommendations and management decisions
• Regular soil testing (every 2-3 years) is important for monitoring soil health and fertility

Environmental Impact and Management

• Soil degradation refers to the decline in soil quality due to various factors
  • Erosion, compaction, salinization, and loss of organic matter
  • Can lead to reduced agricultural productivity and ecosystem services
• Soil conservation practices aim to prevent or mitigate soil degradation
  • Contour farming, terracing, and cover cropping reduce erosion
  • Reduced tillage and crop rotation improve soil structure and organic matter
• Sustainable soil management focuses on maintaining soil health and productivity
  • Balanced fertilization based on soil tests and crop requirements
  • Integrated pest management to minimize pesticide use
  • Incorporation of organic amendments (compost, green manure) to enhance soil properties
• Phytoremediation uses plants to remove or stabilize soil pollutants
  • Hyperaccumulator plants can absorb and concentrate heavy metals in their tissues
  • Rhizodegradation involves the breakdown of organic pollutants by root-associated microbes