Soil pH – Agriculture Dictionary

Soil pH

Definition: Soil pH refers to the measurement of acidity or alkalinity in soil, indicating the concentration of hydrogen ions (H⁺) in the soil solution. It is a crucial soil property that influences nutrient availability, microbial activity, plant growth, and soil chemistry, affecting crop yields, soil fertility, and ecosystem health.

Description: Soil pH is expressed on a scale ranging from 0 to 14, with pH values below 7 indicating acidic soils, pH values above 7 indicating alkaline or basic soils, and pH 7 representing neutral soils. Soil pH is determined by the balance between acidic and alkaline components in soil, primarily hydrogen ions (H⁺) released by soil water dissociating into hydronium ions (H₃O⁺) and hydroxide ions (OH⁻) derived from mineral weathering, organic matter decomposition, and microbial activity.

Factors Affecting Soil pH: Soil pH is influenced by various factors, including:

• Parent Material: The geological origin, mineral composition, and weathering processes of parent materials contribute to soil pH, with acidic parent materials (e.g., granite, sandstone) producing acidic soils and alkaline parent materials (e.g., limestone, dolomite) producing alkaline soils.
• Climate: Climatic factors such as rainfall, temperature, humidity, and evapotranspiration rates influence soil pH by affecting soil moisture, leaching, evaporation, and organic matter decomposition rates, leading to pH variations across different climatic regions.
• Vegetation: Plant species, root exudates, litter decomposition, and rhizosphere interactions can impact soil pH by altering soil microbial communities, nutrient cycling processes, and organic matter inputs, affecting soil acidity or alkalinity over time.
• Soil Texture: Soil texture, including particle size distribution (e.g., sand, silt, clay), affects soil pH by influencing soil water retention, drainage, aeration, and cation exchange capacity (CEC), with clay soils typically having higher pH than sandy soils due to greater buffering capacity.
• Organic Matter: Soil organic matter content, quality, and decomposition rates influence soil pH by releasing organic acids, humic substances, and nitrogen compounds during microbial decomposition, contributing to soil acidity, buffering capacity, and nutrient cycling dynamics.
• Drainage: Soil drainage conditions, including natural drainage patterns, topographic features, and anthropogenic drainage practices, can affect soil pH by influencing soil moisture levels, oxygen availability, redox potential, and microbial activity in waterlogged or poorly drained soils.
• Land Use: Agricultural practices, land management activities, and land use changes such as tillage, irrigation, fertilization, liming, and cropping rotations can impact soil pH by altering nutrient inputs, soil erosion rates, organic matter levels, and soil structure over time.

Effects of Soil pH: Soil pH has significant effects on soil properties, plant growth, nutrient availability, and microbial activity, including:

• Nutrient Availability: Soil pH influences the solubility, mobility, and availability of essential plant nutrients, with acidic soils generally having higher levels of available aluminum (Al), manganese (Mn), and iron (Fe) and lower levels of available calcium (Ca), magnesium (Mg), and phosphorus (P) compared to alkaline soils.
• Plant Growth: Soil pH affects plant nutrient uptake, root growth, shoot development, flowering, fruiting, and overall crop productivity, with different plant species and cultivars exhibiting varying tolerance or sensitivity to acidic or alkaline soil conditions.
• Microbial Activity: Soil pH regulates microbial community structure, diversity, metabolic processes, and enzymatic activities, with acidic soils favoring acidophilic bacteria and fungi adapted to low pH environments and alkaline soils favoring alkaliphilic microorganisms.
• Soil Chemistry: Soil pH influences soil chemical reactions, mineral dissolution rates, cation exchange capacity (CEC), nutrient retention, anion sorption, and soil fertility levels, affecting soil buffering capacity, nutrient cycling, and soil solution composition.
• Soil Biology: Soil pH impacts soil biota, including earthworms, arthropods, nematodes, protozoa, bacteria, and fungi, by altering microbial symbioses, predator-prey relationships, decomposition rates, and nutrient transformations in soil ecosystems.

Soil pH Management: Managing soil pH is essential for optimizing soil fertility, crop productivity, and environmental sustainability, including:

• Soil Testing: Conducting soil pH tests using calibrated pH meters, soil test kits, or laboratory analyses to determine soil pH levels, identify pH imbalances, and develop appropriate liming or acidification strategies based on crop requirements, nutrient recommendations, and target pH ranges.
• Liming: Applying agricultural lime (calcium carbonate) or dolomitic lime (calcium magnesium carbonate) to acidic soils to raise soil pH, neutralize soil acidity, and supply essential calcium (Ca) and magnesium (Mg) nutrients, improving crop growth, nutrient availability, and soil structure.
• Acidification: Using elemental sulfur (S), sulfuric acid (H₂SO₄), or ammonium sulfate (NH₄) to lower soil pH in alkaline soils, acidify soil solution, and release sulfate (SO₄²⁻) or ammonium (NH₄⁺) ions, reducing soil pH and increasing availability of acidic soil nutrients.
• Crop Selection: Choosing crop species, varieties, or cultivars adapted to specific soil pH ranges, tolerance levels, and nutritional requirements, selecting acid-loving plants for acidic soils and alkaline-tolerant plants for alkaline soils to optimize crop performance and yield potential.
• Organic Amendments: Incorporating organic matter sources such as compost, manure, green manures, or cover crops into soils to improve soil structure, organic matter content, microbial activity, and nutrient cycling, buffering soil pH fluctuations and enhancing soil fertility and productivity.
• Irrigation Management: Using irrigation water with neutral pH levels and low salt concentrations to prevent soil salinization, alkalinity buildup, and pH drift in irrigated soils, avoiding excessive irrigation, waterlogging, or water quality issues that may affect soil pH and crop health.
• Nutrient Management: Balancing soil nutrient levels, ratios, and interactions to optimize plant nutrient uptake, crop performance, and soil pH stability, avoiding excessive fertilizer applications, nutrient imbalances, or micronutrient deficiencies that may impact soil pH dynamics and nutrient availability.

Conclusion: Soil pH is a critical soil parameter that influences soil fertility, plant nutrition, and agricultural productivity. By understanding soil pH dynamics, managing pH levels effectively, and adopting appropriate soil management practices, farmers can improve soil health, crop yields, and environmental sustainability in agricultural systems.

References:

• Brady, N. C., & Weil, R. R. (2008). The nature and properties of soils (14th ed.). Prentice Hall.
• Huang, P. M., et al. (2015). Handbook of soil sciences: Properties and processes (3rd ed.). CRC Press.
• Sparks, D. L. (Ed.). (2003). Advances in agronomy: Volume 78. Academic Press.

Originally posted 2007-07-21 10:45:18.