Soil chemistry forms the invisible backbone of soil fertility and ecosystem function. While physical and biological properties shape the structure and life of soils, the chemical environment determines nutrient availability, pH balance, and the mobility of elements essential for life. Soil chemistry is complex, encompassing processes such as the exchange of ions on clay and organic matter surfaces, redox reactions that determine whether nutrients are available or locked away, and the balance of salts and toxins that can either sustain or stress plants.
From an ecological perspective, soil chemistry regulates nutrient cycling, plant growth, water quality, and carbon storage. From an agricultural perspective, it determines whether soils can provide crops with essential minerals without building up harmful toxins. For environmental managers, soil chemistry provides insight into contamination, acidification, and salinization processes that threaten ecosystem health.
This module explores the fundamental components of soil chemistry: pH and redox dynamics, cation exchange capacity (CEC), nutrient balance, toxicity and salinity, and other key chemical processes. Together, these elements reveal how soils act not only as physical foundations but also as chemical reactors that sustain ecosystems.
https://www.youtube.com/watch?v=oIqmZwW_t7w
Soil pH is a measure of hydrogen ion concentration, expressed on a scale from acidic (below 7) to alkaline (above 7). Soil pH influences nearly every aspect of soil chemistry, from nutrient availability to microbial activity.
Soil organisms are also sensitive to pH. Bacteria typically prefer neutral to slightly alkaline soils, whereas fungi thrive under more acidic conditions. Thus, pH directly shapes the biological communities and nutrient dynamics of soil ecosystems.
| Soil pH Range | Chemical Characteristics | Nutrient Availability | Typical Soil or Plant Issues | Common Management Strategies |
|---|---|---|---|---|
| < 4.5 (Very Strongly Acidic) | High H⁺ and Al³⁺ activity; extremely acidic | N, P, K, Ca, Mg, Mo very limited; Fe, Mn, Al become highly soluble (toxic) | Aluminum and manganese toxicity, stunted root growth, poor microbial activity | Apply lime (CaCO₃) or dolomite; add organic matter to buffer acidity |
| 4.6 – 5.5 (Strongly Acidic) | Acidic, common in high rainfall areas | P and Mo limited; Fe and Mn often high | Reduced bacterial activity, nutrient leaching, P fixation | Liming, crop rotation with acid-tolerant species (e.g., oats, blueberries) |
| 5.6 – 6.5 (Moderately to Slightly Acidic) | Ideal range for most crops | Most nutrients optimally available | Few issues, though slight P fixation possible at lower end | Maintain organic matter, minimal liming if needed |
| 6.6 – 7.3 (Neutral) | Balanced H⁺ and base cations | Optimal nutrient availability for majority of plants | Very few chemical constraints | Maintain organic content, avoid over-liming |
| 7.4 – 8.4 (Slightly to Moderately Alkaline) | Dominated by Ca²⁺, Mg²⁺ carbonates | Fe, Mn, Zn, Cu less available; P may precipitate with Ca | Iron chlorosis (yellowing leaves), micronutrient deficiencies | Apply acidifying fertilizers (e.g., ammonium sulfate), foliar micronutrient sprays |
| > 8.5 (Strongly Alkaline / Sodic) | High Na⁺ concentration, poor structure | Most micronutrients unavailable, especially Fe, Mn, Zn | Sodicity, soil dispersion, poor infiltration | Apply gypsum (CaSO₄), improve drainage, leach salts with good-quality water |
Redox potential reflects the tendency of a soil to gain or lose electrons, influenced by oxygen availability and water saturation. Well-aerated soils have high redox potential (oxidizing conditions), while waterlogged soils develop low redox potential (reducing conditions).
Redox dynamics highlight how closely soil chemistry is tied to hydrology and biology. A flooded rice paddy, for example, is chemically very different from a well-drained wheat field.
Cation Exchange Capacity (CEC) is the ability of soil to hold and exchange positively charged ions (cations), such as calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), and ammonium (NH₄⁺). These nutrients are adsorbed onto negatively charged surfaces of clay minerals and organic matter, preventing them from leaching while keeping them accessible to plant roots.
CEC is also influenced by soil pH: as soils become more acidic, hydrogen (H⁺) and aluminum (Al³⁺) occupy exchange sites, reducing the availability of essential base cations. This is why lime (calcium carbonate) is often applied to acidic soils as it displaces hydrogen and aluminum, which is restoring the nutrient balance. CEC thus represents a crucial buffering property of soils, mediating nutrient availability, fertility, and ecosystem resilience.
https://www.youtube.com/watch?v=HmEyymGXOfI
Healthy soils supply plants with essential nutrients in the right balance. Nutrients in soil are divided into: