🍂Environmental Chemistry II Unit 7 – Soil Chemistry: Processes & Nutrient Cycling

Key Concepts and Definitions

• Soil is a complex mixture of minerals, organic matter, water, air, and living organisms that supports plant growth
• Soil texture refers to the relative proportions of sand, silt, and clay particles in a soil
• Soil structure describes the arrangement of soil particles into aggregates or clumps
• Cation exchange capacity (CEC) is a measure of a soil's ability to hold and exchange positively charged ions (cations)
• Nutrient cycling involves the transfer of nutrients between the soil, living organisms, and the atmosphere
• Soil pH is a measure of the acidity or alkalinity of a soil, which influences nutrient availability and plant growth
• Soil organic matter (SOM) consists of decomposed plant and animal residues that improve soil structure, water retention, and nutrient holding capacity
• Soil degradation occurs when soil quality declines due to factors such as erosion, compaction, salinization, or contamination

Soil Composition and Structure

• Soil is composed of mineral particles, organic matter, water, and air
• The mineral component of soil is derived from weathered rock fragments and consists of sand (0.05-2 mm), silt (0.002-0.05 mm), and clay (<0.002 mm) particles
• Soil structure is influenced by the aggregation of soil particles into peds or clumps, which can be granular, blocky, prismatic, or platy in shape
• Well-structured soils have good porosity, allowing for adequate water infiltration, drainage, and aeration
• Soil texture and structure influence soil properties such as water holding capacity, nutrient retention, and root penetration
  • Sandy soils have high infiltration rates but low water and nutrient holding capacities
  • Clay soils have high water and nutrient holding capacities but may have poor drainage and aeration
• Soil organisms, such as bacteria, fungi, and earthworms, play a crucial role in the formation and maintenance of soil structure

Chemical Processes in Soil

• Chemical weathering of rocks and minerals releases nutrients into the soil solution, making them available for plant uptake
• Ion exchange involves the adsorption and release of ions (nutrients) between the soil solution and the surfaces of clay particles and organic matter
• Cation exchange capacity (CEC) is a measure of a soil's ability to hold and exchange cations, such as calcium (Ca2+), magnesium (Mg2+), and potassium (K+)
  • Soils with high CEC (e.g., clay soils) can retain more nutrients and are less susceptible to nutrient leaching
  • Soils with low CEC (e.g., sandy soils) have a lower nutrient holding capacity and are more prone to nutrient leaching
• Soil pH affects the solubility and availability of nutrients, as well as the activity of soil microorganisms
• Redox reactions in soil involve the transfer of electrons between chemical species, influencing the mobility and availability of nutrients such as iron (Fe) and manganese (Mn)
• Adsorption and desorption processes control the retention and release of nutrients and contaminants in soil

Nutrient Cycling Fundamentals

• Nutrient cycling is the continuous transfer of nutrients between the soil, living organisms, and the atmosphere
• Primary nutrients essential for plant growth include nitrogen (N), phosphorus (P), and potassium (K)
• Secondary nutrients, such as calcium (Ca), magnesium (Mg), and sulfur (S), are also important for plant growth but are required in smaller quantities
• Micronutrients, such as iron (Fe), manganese (Mn), and zinc (Zn), are needed in trace amounts but are crucial for plant growth and development
• Nutrient availability is influenced by factors such as soil pH, organic matter content, and soil moisture
• Plants absorb nutrients from the soil solution through their roots and incorporate them into biomass
• Nutrients are returned to the soil through the decomposition of plant litter, animal wastes, and soil organic matter
• Soil microorganisms play a vital role in nutrient cycling by breaking down organic matter and releasing nutrients into forms available for plant uptake

Major Nutrient Cycles

• The nitrogen cycle involves the transfer of nitrogen between the atmosphere, soil, and living organisms
  • Nitrogen fixation converts atmospheric N2 into forms usable by plants, such as ammonium (NH4+) and nitrate (NO3-)
  • Nitrification is the microbial conversion of ammonium to nitrate
  • Denitrification is the microbial reduction of nitrate to gaseous forms of nitrogen (N2, N2O) under anaerobic conditions
• The phosphorus cycle is the movement of phosphorus through the soil, living organisms, and water bodies
  • Weathering of phosphate-bearing minerals releases phosphorus into the soil solution
  • Plants absorb phosphorus as orthophosphate ions (H2PO4-, HPO42-)
  • Phosphorus is relatively immobile in soil and tends to accumulate in the topsoil
• The carbon cycle involves the exchange of carbon between the atmosphere, soil, and living organisms
  • Photosynthesis fixes atmospheric CO2 into plant biomass
  • Decomposition of plant litter and soil organic matter releases CO2 back into the atmosphere
  • Soil organic carbon plays a crucial role in maintaining soil structure, water retention, and nutrient cycling
• The sulfur cycle involves the transformation of sulfur between various organic and inorganic forms
  • Weathering of sulfur-bearing minerals releases sulfate (SO42-) into the soil solution
  • Plants absorb sulfate and incorporate it into organic compounds
  • Mineralization of organic sulfur compounds releases sulfate back into the soil solution

Soil pH and Its Effects

• Soil pH is a measure of the acidity or alkalinity of a soil, expressed on a scale from 0 to 14
  • A pH of 7 is neutral, while values below 7 are acidic and values above 7 are alkaline
  • Most plants grow best in slightly acidic to neutral soils (pH 6.0-7.5)
• Soil pH influences nutrient availability and plant growth
  • Acidic soils (pH <5.5) may have reduced availability of nutrients such as phosphorus, calcium, and magnesium
  • Alkaline soils (pH >7.5) may have reduced availability of micronutrients such as iron, manganese, and zinc
• Soil pH affects the activity and diversity of soil microorganisms
  • Most soil bacteria and actinomycetes thrive in neutral to slightly alkaline conditions
  • Fungi are more tolerant of acidic conditions and dominate in low pH soils
• Soil pH can be modified through the application of amendments such as lime (to raise pH) or sulfur (to lower pH)
• Regular monitoring of soil pH is important for maintaining optimal growing conditions and preventing nutrient imbalances

Organic Matter in Soil

• Soil organic matter (SOM) is the fraction of the soil composed of decomposed plant and animal residues, as well as living soil organisms
• SOM improves soil structure by promoting the formation of stable aggregates, which enhances water infiltration, drainage, and aeration
• SOM increases the water holding capacity of soil, reducing the risk of drought stress in plants
• SOM is a reservoir of nutrients, slowly releasing them through mineralization as plants require them
• SOM provides a food source and habitat for soil organisms, supporting a diverse and active soil food web
• The decomposition of SOM is influenced by factors such as temperature, moisture, and the quality (C:N ratio) of the organic material
• Management practices that promote SOM accumulation include reduced tillage, cover cropping, and the application of organic amendments (e.g., compost, manure)
• Maintaining adequate levels of SOM is crucial for long-term soil health, productivity, and resilience

Environmental Impacts and Management

• Soil erosion is the detachment and transport of soil particles by water or wind, leading to the loss of topsoil and reduced soil productivity
  • Soil conservation practices, such as contour farming, terracing, and cover cropping, can help reduce erosion
• Soil compaction occurs when soil particles are compressed, reducing pore space and limiting root growth and water infiltration
  • Minimizing tillage, avoiding traffic on wet soils, and using controlled traffic farming can help prevent soil compaction
• Soil salinization is the accumulation of soluble salts in the soil, which can inhibit plant growth and reduce soil productivity
  • Proper irrigation management, drainage, and the use of salt-tolerant crops can help mitigate soil salinization
• Soil contamination occurs when pollutants, such as heavy metals, pesticides, or hydrocarbons, accumulate in the soil, posing risks to human health and the environment
  • Remediation techniques, such as phytoremediation or bioremediation, can be used to clean up contaminated soils
• Nutrient management involves the judicious use of fertilizers to meet crop nutrient requirements while minimizing environmental impacts
  • Soil testing, precision agriculture, and the use of slow-release or stabilized fertilizers can help optimize nutrient use efficiency and reduce nutrient losses
• Sustainable soil management practices aim to maintain or improve soil health while supporting productive agriculture and minimizing environmental degradation
  • These practices include crop rotation, cover cropping, reduced tillage, integrated pest management, and the use of organic amendments