Many reasons account for why a soil can smell bad. A soil may also have its normal scent, but it simply does not agree with you. A healthy soil has a distinct smell and, even if it is strong, it is not typically thought to be a bad smell. Usually the soil does not smell unless it has been disturbed. The main cause of soil smells is the amount of moisture in the soil. A dry soil may become dusty and can be inhaled easily. A wet soil can harbour bacteria and other microorganisms that produce bad smells.
When a soil becomes water logged, it provides the perfect conditions for anaerobic bacteria, which are a type of microorganism that can only survive in the absence of oxygen. Soil with anaerobic bacteria will have a very bad smell. Soils in low, poorly-drained areas with lots of clay or organic matter are the most likely to provide the necessary conditions for this type of noxious bacterial. The same conditions can develop in houseplants if the pots are not well drained or if they are over-watered.
Organic matter in the soil absorbs lots of water. If a soil has lots of organic matter, it can smell bad due to anaerobic conditions or because of decaying materials. As plant and animal materials decay, they release different gasses that can be smelly. A soil with lots of peat moss, dead leaves or compost will stink like the decaying matter. Spreading the smelly material around will expose it to more oxygen and help it break down faster. Mixing organic materials into the soil is also a good way to help the smell go away.
Soils with lots of clay can absorb huge amounts of water, which can lead to anaerobic conditions. Clay particles are extremely small and cannot even be seen with the naked eye. Because of their tiny size, clay particles can trap water between themselves that smelly bacteria can grow in. Clay soils do not usually stink until they have standing water on them or they are dug into. Digging releases the gasses that have been building up in the ground from microorganisms.
A healthy soil has a balance of organic matter, clay and other materials. It is well oxygenated, which allows smelly materials to decay faster. A health soil will have a rich earthy smell. The smell of a soil can tell a lot about its fertility and is related to its colour. A dark soil has a bit more of an acidic smell. The dark colour of the soil comes from decayed organic matter which is more acidic. A pale soil may have a sandy smell since it is mostly mineral materials.