Anoxic soil is completely depleted of dissolved oxygen. This condition is distinct from hypoxic soil, which has a low oxygen concentration but is not entirely devoid of it. Anoxia creates a challenging environment where the roots of most terrestrial plants cannot survive, as they require oxygen for basic metabolic functions. The absence of oxygen alters the soil’s chemistry and biology, impacting plant health and soil stability.
Formation of Anoxic Soil
The primary cause of anoxic soil is prolonged waterlogging or saturation. When water fills the soil’s pore spaces, it displaces the air that plant roots and aerobic microorganisms depend on. This saturation can result from heavy rainfall, poor drainage, over-irrigation, or a high water table. The duration and frequency of these events contribute to the severity of the anoxic conditions.
Soil compaction encourages anoxia by reducing the volume and connectivity of pore spaces. Heavy machinery, high foot traffic, or the natural settling of fine-textured clay soils can squeeze out air channels. This makes it difficult for oxygen to penetrate from the atmosphere and for water to drain. This limitation on gas exchange means even moderate water can lead to oxygen depletion in compacted zones.
High microbial activity can accelerate the consumption of remaining oxygen. When large amounts of organic matter are present in saturated soil, decomposing microorganisms rapidly use the available oxygen for respiration. This biological oxygen demand creates an environment where microbes out-compete plant roots for the limited oxygen supply, pushing the soil into an anoxic state.
Identifying Anoxic Soil
A distinct indicator of anoxic soil is its smell. When soil is deprived of oxygen, anaerobic bacteria take over and produce gases during respiration. One of these is hydrogen sulfide, which gives off a characteristic “rotten egg” odor that becomes noticeable when the soil is disturbed.
The soil’s color provides another visual cue. Prolonged saturation leads to a process known as gleying, where the soil turns a bluish-gray or greenish-gray color. These gleyed colors are a clear sign of long-term anaerobic conditions.
In areas where the water level fluctuates, the soil may exhibit mottling. These are spots of reddish-brown mixed with the dominant gray colors. The reddish spots indicate where oxygen has periodically returned, allowing iron to re-oxidize, while the gray areas show where anoxic conditions persisted. This pattern reveals a history of alternating wet and dry cycles.
Chemical and Biological Consequences
The absence of oxygen triggers a shift in the soil’s microbial community. Aerobic microorganisms, which rely on oxygen for respiration, die off or become dormant. They are replaced by anaerobic microbes that thrive without oxygen, using other compounds for their metabolic processes.
A consequence of this shift is denitrification, where anaerobic bacteria convert nitrate into nitrogen gas, which is lost to the atmosphere and reduces soil fertility. Simultaneously, other chemical reductions occur. Manganese and iron compounds are transformed into soluble forms that can become toxic to plants in high concentrations. The reduction of iron is responsible for the gray, gleyed colors seen in anoxic soils.
These changes have severe effects on plant roots. Without oxygen, roots cannot perform aerobic respiration to generate energy for nutrient and water uptake, which leads to root death. The plant’s stressed metabolism can also produce toxic compounds like ethanol. Additionally, the hydrogen sulfide produced by anaerobic bacteria is directly toxic to plant roots, inhibiting their function.
Remediation and Management
The most direct way to remediate anoxic soil is to reintroduce oxygen by improving drainage. For large areas, installing subsurface drainage systems like French drains can channel excess water away from the root zone. In a garden, creating raised beds or grading the landscape can prevent water from pooling and keep the soil from becoming saturated.
Mechanical aeration is an effective technique for addressing compaction. This process involves using tools to create holes, which breaks up compacted layers and creates new channels for air and water. This physical disruption improves gas exchange between the soil and the atmosphere, allowing oxygen to penetrate deeper.
Long-term management involves practices that prevent compaction and improve soil structure. This includes minimizing foot or vehicle traffic, especially when the ground is wet. Incorporating well-decomposed compost can improve the structure of heavy clay soils, but it must be done carefully. Adding fresh organic matter to wet soil can worsen oxygen depletion, so using mature compost is advised.