It’s a nightmare to end up with anaerobic compost or soil with little or no oxygen—they are sticky, stinky, slow to decompose, and are not suitable for growing plants.
It often happens when compost piles get too wet and saturated with water. It is also found in areas in your garden that are waterlogged, compacted, or over-cultivated.
You can fix anaerobic soil by simply turning it, adding carbon-rich materials like sawdust and cardboard to it, adding dry bulking soil amendments, draining away excess water, and breaking up any hydrophobic soil or clay layer.
In this article, we will look at the problem of anaerobic soil and 7 effective ways to fix it.
At first glance, anaerobic soils may look like ordinary soils, but certain signs can tell you when soil is anaerobic. They include color, texture, smell, and the existence of soil organisms.
Anaerobic soils that have been waterlogged for an extended period of time have a uniform dull gray color rather than brown, red, or black. This coloration is due to a process called “gleying” which is a reduction or consumption of iron and manganese by anaerobic bacteria.
For soil that has been anaerobic or waterlogged for long, the soil has a mottled pattern with patches of gray mixed in with the original soil color. (University of Wisconsin, 1998).
Regular, healthy soil is light, crumbly, and moist. But anaerobic soils are wet, sticky, and heavy most times, even when it hasn’t rained in a while.
This is because the soil is waterlogged, either because the water table is high or there’s a layer of clay preventing further infiltration of rainwater, making the soil heavier due to the water and constantly wet.
Anaerobic soils typically have a foul odor that smells rotten, sulfurous, and like ammonia, urine or sweat.
The anaerobic odors come from a wide range of substances, mostly the reduced sulfur compounds, such as hydrogen sulfide, dimethyl disulfide, dimethyl sulfide, and methanethiol. Ammonia is the most common odor.
1.4 Absence of soil organisms
Normal and healthy soil teems with insects, earthworms and other small animals.
But for anaerobic soil, all these creatures are absent (or are significantly fewer) because of the lack of oxygen in the soil, which is needed to support biological life.
Healthy soil smells fresh and earthy, but anaerobic soil stinks.
There are several reasons why your soil stinks, such as the lack of oxygen, too much water, the existence of anaerobic bacteria, and too much nitrogen.
2.1 Lack of oxygen
When there’s no aeration or airflow through the soil, the soil becomes an environment where aerobic bacteria die and only anaerobic bacteria can survive. The anaerobic bacteria reduce elemental sulfur and other sulfates in the soil to hydrogen sulfide and other foul-smelling gasses like ammonia. (Richards, n.d.-a)
2.2 Too much water
Similar to the lack of oxygen, too much water in the soil can make it stink as the water saturates the soil and fills up the air spaces. This will create favorable conditions for anaerobic bacteria that go on to produce stinking hydrogen sulfides and other foul-smelling gasses.
This can be due to overwatering, poor drainage in the soil (e.g. too much clay in the ground), flooding, high water table in the ground.
2.3 Too much nitrogen
For compost or soil with an excessive amount of nitrogen, anaerobic bacteria convert nitrogen into ammonia gas which will be released into the atmosphere. The foul, stinky smell of ammonia is a sign that nitrogen is in excess.
Substances that are high in nitrogen, also referred to as the “greens”, include fruit and vegetable scraps, grass clippings, flowers, green leaves, manure, urine, blood meal, etc. These can be balanced out by substances that are rich in carbon, or “browns”, such as dried leaves, sawdust, twigs, cardboard paper, straw, etc.
A significantly higher amount of “greens” than “browns” in the compost or soil will produce an ammonia-like odor, as the excess nitrogen is lost to the atmosphere. That is why landfills and composting facilities frequently have odors of ammonia or urine.
2.4 Production of biogas
Biogas, officially known as methane, is produced from the anaerobic decomposition of organic matter in the soil. While methane itself is an odorless and colorless gas, such uncontrolled anaerobic decomposition also produces foul-smelling gasses like ammonia and hydrogen sulfides (Penn State University, 2015).
First, insoluble materials in the soil are converted into soluble substances, that are in turn converted into volatile acids that produce odors. Finally, methane-forming bacteria convert the acids to biogas, which is a mixture of primarily methane, carbon dioxide, and smaller amounts of hydrogen sulfide and ammonia.
3.1 Turn the compost
If it is a small area or a pile of compost of manageable size, turning the compost pile regularly with a pitchfork or shovel will improve aeration.
This allows air and oxygen to get into the pile. This will also kill anaerobic bacteria, as oxygen is toxic to anaerobes.
3.2 Add carbon
Compost piles that smell of ammonia have excess nitrogen materials decomposing in a pile. The way to correct this is to increase the amount of carbon to match the nitrogen content. Doing so will provide more energy to microorganisms to decompose the greens in a pile.
Substances that are rich in carbon are brown and woody materials, such as autumn leaves, wood chips, sawdust, shredded paper.
3.3 Add soil amendments
Adding coarse, dry and bulking soil amendments such as straw, wood chips, leaves, twigs can help increase the porosity, allow oxygen to penetrate, and thus improve the aeration of dense, compacted anaerobic compost.
Spread a layer (around three inches thick) of your preferred amendment over the soil and use a pitchfork or shovel to work it into the ground. Ensure that the amendment is evenly distributed throughout the soil so that no areas remain untouched.
It’s best to add the amendments 4 to 6 weeks before planting to allow enough time for soil organisms to further work the amendments into the soil.
3.4 Reduce watering
Waterlogging is one of the chief causes of anaerobic soils. Even soils like sand with good drainage can become waterlogged if too much water is applied within a short period.
If your soil has good drainage but is still waterlogged, cut down on watering, only add more water to the ground when it’s almost dry.
3.5 Break up barrier layers
Sometimes the waterlogging is not due to too much watering but because the water can’t infiltrate into the soil as it is hydrophobic or there’s an impermeable clay layer preventing the downward flow of the water.
In such cases, you can use a subsoiler plow or a broad fork to break up the clay layer to allow the water to pass. A subsoiler is a tractor-mounted implement used for loosening and breaking up soil at depths up to 16 to 20 inches below the surface.
3.6 Lower the water table
Your soil can be waterlogged and anaerobic due to having a high water table, i.e. near a stream, lake or marsh.
One approach to tackle this problem is to raise the soil level, but this can be expensive as it involves importing soil from a different area.
The preferred way is to lower the water table itself, by installing underground drains that collect and discharge the water into nearby streams or rivers.
Another way to lower the water table is by planting trees like willows, birch, and poplar, which grow in wet areas and absorb much water. But this method does not produce immediate results as the trees need to grow to a certain age to absorb any significant amount of water. Additionally, some trees have invasive roots that can cause trip hazards, go into septic lines, disrupt sidewalks and foundations.
3.7 Redirect water to ponds and swales
You can also create swales and ponds to redirect the water and plant on soil mounds.
Anaerobic soils can be found in natural wetlands, peat soils, flood plains, swamps and paddy soil. You can also find them in areas with a high water table and places with heavy rainfall coupled with poorly drained soils.
Soils can also become anaerobic from human and animal activities like over-cultivation, over-grazing from large livestock animals like cows and horses.
Soils that have become anaerobic because of human and animal activities are relatively easy to repair. But, naturally occurring anaerobic soils like those found in wetlands and floodplains aren’t easily fixed. It involves rerouting existing water bodies and destroying local ecosystems.
It is possible and fairly easy to fix anaerobic compost. The process mainly involves regularly aerating the compost by turning it so that enough air and oxygen is flowing through the compost.
Also, ensure that the compost bin or pile has good drainage to prevent or fix waterlogging.
Once proper air circulation has been restored, you will find that the foul smells will dissipate within a day or two. And as oxygen levels are restored, you would notice that the compost decomposes faster, which is because anaerobic decomposition is significantly slower than aerobic decomposition. (Texas A&M University, 2009)
Anaerobic soils are formed by a lack of oxygen caused by waterlogging, lack of aeration, and the compaction of soil particles.
The lack of oxygen encourages the growth and multiplication of anaerobic bacteria that remove the color from the soil, produce foul smells, and affect plant productivity.
You can fix anaerobic compost by turning it regularly to aerate it and increase oxygen levels in the system.
Similarly, anaerobic soils can be fixed by adding organic soil amendments like sphagnum peat moss, compost, straw, and wood chips. The amendments will improve aeration and soil drainage over time and have the added benefit of improving soil fertility.
University of Wisconsin (1998, April 10). Soil Science 230 FAQ: Basic Soils & Land Use.
Richards, T. (n.d.-a). Odor Management – Cornell Composting. Cornell University.
Richards, T. (n.d.-b). Ammonia Odors. Cornell University.
Penn State University Extension. (2022, March 2). Anaerobic Digestion for Odor Control.
University of Washington. (n.d.). Green Technology Research : Soil Amendments.
Texas A&M University. (2009, February). The Decomposition Process, Chapter 1.