5813 Sulfur

Sulfur in Soil Management

Introduction to Sulfur in Soil

• Sulfur (S) is an essential nutrient for plant growth and is a key component of proteins and enzymes.

Sources of Sulfur

Inputs

• Atmospheric Deposition: Sulfur is deposited from the atmosphere.

• Crop Residues: Decomposing plant material contributes sulfur.

• Fertilizers: Both mineral and organic sulfur fertilizers are used.

• Naturally Occurring Sulfur: Derived from geological processes.

Losses

• Leaching: Movement of sulfur in water away from the root zone.

• Volatilization: Release of sulfur compounds into the atmosphere.

• Crop Removal: Sulfur is removed through harvested crops.

Fluxes

• Mineralization/Immobilization: Conversion of sulfur between organic and inorganic forms.

• Reduction/Oxidation: Changes between different oxidation states of sulfur.

• Adsorption: Binding of sulfur to soil particles.

Atmospheric Deposition

• Historical Data (2000-2002 & 2019-2021): Shows the total sulfur deposited (kg-S/ha) in affected regions.

• Chemical Reaction: SO2 + 1/2 O2 + 1 H2O → SO4-2 + 2H+

Naturally Occurring Sulfur

• Natural Presence: Sulfur is about 0.05% in Earth’s crust.

• Forms: Exists as metal sulfides (like pyrite), sulfates (like gypsum), and elemental sulfur.

• Weathering and Processes: Becomes available through weathering, volcanic activity, mining, and fossil fuel processing.

Sulfur Extraction Processes

Frasch Process

• Description: Involves injecting high-pressure air and steam into sulfur wells within salt domes.

• Mechanism: Steam melts the sulfur, and air forces it to the surface.

Claus Process

• Chemical Reaction: Utilizes catalysts to convert hydrogen sulfide (H2S) and sulfur dioxide (SO2) into elemental sulfur.

• Reaction: 2H2S + SO2 → 3S + 2H2O

Pyrite Sulfide Smelting

• Reaction 1: 4FeS2 + 11O2 → 2Fe2O3 + 8SO2

• Reaction 2: 2CuFeS2 + 4O2 → 2FeO + 2SO2 + 2CuS

• Relevance: Highlights the conversion of sulfide minerals to sulfates.

Plant Uptake of Sulfur

• Requirement for Uptake: Elemental sulfur must be oxidized to sulfate form (SO4-2) before it can be taken up by plants.

• Acidification Risk: Addition of free hydrogen ions (H+) can lead to soil acidification.

• Solubility: Elemental sulfur is insoluble, making its release in soil a slow process.

Fertilizers

Sources of Sulfate Fertilizers

• Types:

  • Ammonium Sulfate (24% S)

  • Potassium Sulfate (18% S)

  • Calcium Sulfate (21% S)

  • Magnesium Sulfate (14-22% S)

• Thiosulfates: S2O3-2 (17-26% S)

Manure Sulfur Content

• Poultry Manure: 0.5% to 1.5% S (dry weight basis)

• Swine Manure: 0.2% to 0.5% S

• Sheep Manure: 0.2% to 0.4% S

• Cattle Manure: 0.1% to 0.3% S

• Horse Manure: 0.1% to 0.3% S

The Sulfur Cycle

Fluxes

• Processes: Mixed sulfur products through atmospheric deposition, harvest, mineralization, and various processes lead to losses through leaching and volatilization.

Organic Sulfur Forms in Soils

• Non-Calcareous Soils: >90% S in organic matter.

  • Organic S Types: HI-reducible (40-60%), carbon-bonded (10-20%), residual (30-40%).

• Calcareous Soils: Less sulfur is tied up; presence of gypsum can lead to high sulfur concentrations.

Sulfur Turnover and Nutrient Management

Sulfur Turnover Rates

• C:S Ratios:

  • <200:1: Mineralization dominant

  • 200:1 to 400:1: Mineralization = Immobilization

  • 400:1: Immobilization dominant

Inorganic Sulfur Uptake

• Primary Form: Sulfate (SO4-2) is mainly absorbed by plants; thiosulfate can also be utilized.

• Uptake Mechanism: Mass flow to roots is the primary uptake mechanism.

• Concentration Needs: Soil concentrations of 3-5 ppm of sulfate are generally sufficient for crops.

Adsorbed Sulfate in Soil

• Concentrations in Humid Regions: Highly weathered soils may have up to 100 ppm of adsorbed SO4-2 in subsoil.

• Influencing Factors: Iron and aluminum oxides, clay minerals, and soil organic matter play critical roles.

Redox Reactions in Sulfur Cycling

• Anaerobic Environments: SO4-2 reduction is a vital process.

• Reaction: SO4-2 + 8H+ + 8e− → H2S + 4H2O

• Conditions: Marshlands can accumulate sulfide compounds; draining can lead to low pH levels.

Factors Affecting SO2 Oxidation

• Microbial Populations: Thiobacillus are active sulfur oxidizers.

• Soil Conditions: Temperature, moisture, aeration, and pH impact the oxidation processes.

Key Points to Remember

• Elements of the Sulfur Cycle: Understanding natural and fertilizer sources of sulfur, redox reactions, and plant roles.

• Symptoms of Deficiency: Recognizing and addressing sulfur deficiency in plants.

Nutrient Removal in Crops

Crop Yield and Nutrient Requirements

Questions?

• Engage with the material and clarify concepts surrounding sulfuranagement in soils.