Boron Fertilizers and Soil Mobility

Boron Fertilizers: Use, Challenges, and Slow-Release Sources

• Boron (B) is an essential plant nutrient, but toxic in excess.
• Boron is highly mobile in most soils, making it vulnerable to leaching.
• Boron deficiency is common in high-rainfall areas, especially sandy soils.
• Soluble sodium borates (e.g., borax) are commonly used boron fertilizers.
• Slow-release boron sources reduce the risk of seedling toxicity and leaching.
• Examples of slow-release sources include colemanite.
• Boron release rate from slow-release sources depends on fertilizer characteristics and soil properties.
• More research is needed to predict release rates for varying soil and climate conditions.

Boron Chemistry and Mobility in Soil

• Boron typically exists as boric acid (H3BO3) in soil solution.
• Boron adsorption is relatively weak in most soils.
• Adsorption increases with increasing pH, peaking around pH 9.
• Boron adsorption depends on soil properties (e.g., clay content).
• Mobility of boron is high, leading to leaching in high-rainfall environments.

Boron in Plants: Deficiency and Toxicity

• Boron uptake by plants occurs via passive and active transport mechanisms.
• Boron is important for cell wall integrity and metabolic activities.
• Boron is not phloem mobile in most plant species.
• Deficiency symptoms typically appear in meristematic tissues (buds and young leaves).
• Toxicity symptoms differ based on plant mobility (e.g., some species show marginal leaf burn, others dieback).
• Boron tolerance varies between plant species.

Soil Testing

• Total boron content in soil is not a reliable indicator of plant availability.
• Mild extracts (e.g., hot water, dilute CaCl2, mannitol) are better indicators of plant-available boron.
• Hot water extraction is commonly used to assess boron availability.
• Boron adsorption is soil pH and clay content dependent.
• The Langmuir isotherm describes boron adsorption behavior.

Sources of Boron

• Boron occurs naturally in soils in various minerals (e.g., boroslicates, borates).
• Irrigation water can contribute to high boron levels in soils.
• Boron fertilizers are sourced from borate mineral deposits.
• Colemanite, kernite, tincal, and ulexite are common borate minerals.
• These minerals vary in their solubility, which affects their slow-release properties in soil.

Boron Fertilization

• Rates of boron fertilizer application depend on the crop and soil.
• Banding or broadcasting are common soil application methods.
• Foliar applications are common for correcting deficiencies in fruit trees.
• Slow-release fertilizers can potentially reduce the need for repeated applications and the risk of toxicity or leaching.

Advantages of Slow-Release Fertilizers

• Reduce risk of seedling toxicity and leaching losses of readily available boron sources.
• Extended availability of boron for consistent plant uptake.
• Allow for lower application rates and less frequent applications