Aeration is the process of air passage though pore spaces in soil to provide an adequate supply of oxygen. Because roots require oxygen for growth and the uptake of nutrients and water, insufficient oxygen in the root zone (aeration deficit, sometimes called waterlogging) results in root asphyxiation and is a serious, often life-threatening problem for plants. Insufficient soil oxygen, excess soil moisture, and root decay pathogens often act in combination to damage or kill plants.
A short-term (acute) aeration deficit (for hours to days) commonly causes wilting and premature leaf drop. Sometimes the entire plant dies.
Chronic aeration deficit (for weeks to months) kills roots, stunts growth, causes abnormally short shoots and small leaves, and may lead to the gradual decline and death of plants. Leaves can suddenly collapse and turn brown and branches may die back from the tips, although only a few branches or one limb may die.
In some plants, chronic aeration deficit causes stem or bark cankers, resinous or gummy exudate on bark, adventitious (atypical, not normally present) roots near the basal stem, or unusually abundant and large lenticels (corky pores, or rows of narrow openings) in bark.
To diagnose aeration deficit, examine soil and roots and suspect aeration deficit if
- Soil or roots smell like rotten eggs.
- Soil is bluish gray or black and roots appear discolored, rotted, or water-soaked.
- Water ponds (accumulates) on the soil surface or drains slowly (e.g., topsoil remains soggy).
Aeration deficit is especially common in irrigated landscapes. Excess soil moisture (waterlogging) prevents adequate aeration by impeding oxygen movement through soil and leaving few water-free pore spaces for oxygen to occupy. Irrigating too frequently or applying excess amounts of water, especially when drainage is poor (slow), is the most common cause of aeration deficit.
Compaction, flooding, hardpan (an impermeable soil layer), insufficient soil volume for root growth, poor soil structure and texture, a shallow water table, and surface barriers such as pavement are other common causes of insufficient oxygen for roots. Planting in too deep of a hole, especially in poorly drained soil when the hole is backfilled with amended soil, can cause aeration deficit.
To determine the cause of aeration deficit, assess drainage, check soil moisture, determine whether the grade has been changed, and especially evaluate irrigation practices.
Send soil samples to a diagnostic laboratory to determine soil texture and whether excess sodium is present; movement of air and water are restricted in sodic soils. Consider measuring soil bulk density, a soil's weight per volume, by core sampling or carefully excavating specific volumes of soil, then thoroughly oven-drying and weighing them. Bulk density higher than critical levels for that soil texture as determined from published tables indicates that soil is compacted and lacks sufficient porosity for good plant growth. A percolation test measures soil drainage (infiltration rate), which is one indicator of aeration.
Prevent aeration deficit through good site preparation, appropriate planting, and proper cultural practices. If periodic soil saturation is unavoidable, consider replacing flood-sensitive plants with species more tolerant of wet soils; certain plant species tolerate very low soil aeration or tolerate it during dormancy. For more information, consult publications such as Abiotic Disorders of Landscape Plants and "Soil and Fertilizer Management" in the California Master Gardener Handbook.
Shoot dieback from aeration deficit
Premature leaf drop due to waterlogging