Soil Redox and Acidity
Soil redox and acidity are master variables that govern nutrient availability, metal mobility, and the chemistry of waterlogged and acidified soils.
Definition
The study of acid-base and oxidation-reduction conditions in soils and their control over chemical speciation and mobility.
Scope
This topic covers the sources and buffering of soil acidity, the role of exchangeable aluminum, the redox transitions that occur when soils become waterlogged, and how pH and redox jointly control element speciation and mobility.
Core questions
- What produces and buffers soil acidity?
- Why is exchangeable aluminum central to acid-soil chemistry?
- How do redox changes follow soil saturation?
- How do pH and redox together control metal and nutrient availability?
Key theories
- Aluminum control of acid-soil chemistry
- Below moderate pH, hydrolysis of exchangeable aluminum buffers soil pH and releases toxic aluminum species, making aluminum chemistry central to the behavior and management of acid soils.
Mechanisms
Soil acidity arises from carbonic and organic acids, nitrification, and acid deposition, and is buffered in stages by carbonates, exchangeable bases, and ultimately aluminum hydrolysis. When soils flood, oxygen is depleted and microbial respiration drives reduction of nitrate, manganese, iron, and sulfate, altering pH and mobilizing or sequestering elements.
Clinical relevance
Soil pH and redox determine nutrient availability, aluminum and heavy-metal toxicity, and the behavior of contaminants, guiding liming, drainage, and remediation decisions.
History
The recognition of aluminum, rather than free protons alone, as the key agent of soil acidity reshaped acid-soil science in the mid-20th century.
Related topics
Seminal works
- sparks2003
- sposito2008
Frequently asked questions
- Why does liming reduce aluminum toxicity?
- Raising soil pH causes dissolved and exchangeable aluminum to precipitate as hydroxides, lowering the concentration of toxic aluminum species.