SOIL222 - Lecture 6 - Sulfur & Cations

Sulfur in Plants

  • Essential for plant functions, required similarly to phosphorus (P).

  • Deficiency leads to reduced photosynthesis and chlorosis.

  • Critical S concentration: 0.15-0.30% in whole tops or index leaves.

Deficiency Symptoms

  • S deficiency appears as chlorosis throughout the plant, unlike nitrogen (N) deficiency (older leaves only).

  • Similar symptoms in wheat and sunflowers.

Sulfur in Soil

  • In well-drained soils, >90% S is organic; exceptions include saline and acid sulfate soils.

  • Four types of organic S: reducible by HI acid (e.g., ester sulphates) and non-reducible (C-S bonded organics).

  • Plants uptake S as sulfate ions (SO₄²⁻); mineralization is key for availability.

Sulfur Cycle

  • Analogous to the nitrogen cycle; involves biological and biochemical mineralization.

  • Leaching is a significant loss mechanism.

Agricultural Cycles for Sulfur

  • Crop removal rates (kg/ha): Corn (9-11), Wheat (10-13), Cotton (13-17), Alfalfa hay (22-27).

  • Replacement of S requires fertilizers or organic matter mineralization.

Sources of Sulfur

  • S input from rainfall decreases with distance from the coast.

  • Common S fertilizers and their sulfur contents:

    • Ammonium sulfate - 24%

    • Potassium sulfate - 16%

    • Calcium sulfate - 18%

  • Typical fertilization rates: Pastures (5-30 kg/ha), Wheat (10-15 kg/ha).

Cations in Soil: Ca, K, Mg

  • Typical optimal nutrient concentrations (%) for crops:

    • K: 1.5-3.0, Ca: 0.2-0.5, Mg: 0.15-0.5.

  • Plant deficiencies are rare due to high soil reserves.

Importance of Potassium (K)

  • Essential for enzyme function, photosynthesis, disease resistance, and turgor.

  • K deficiency manifests in older leaves, leading to necrosis.

Importance of Calcium (Ca)

  • Involved in many plant functions: ion absorption, growth, and protection against toxic ions.

  • Deficiency affects younger leaves and fruit development.

Importance of Magnesium (Mg)

  • Vital for chlorophyll synthesis and enzyme activation.

  • Deficiency symptoms include interveinal chlorosis, primarily in older leaves.

Soil Content and Distribution

  • Soil K, Ca, Mg concentrations: 1-3% or higher, variable due to weathering and leaching.

  • Forms: Readily available (inorganic), non-exchangeable (primary/secondary minerals), and exchangeable (soil solution).

Availability to Plants

  • Uptake as K⁺, Ca²⁺, and Mg²⁺; affected by soil concentration and cation saturation.

Soil Testing

  • Assessed through exchangeable concentrations or % cation saturation; comparisons can help yield response.