As a result, grayish soils are found in places with high quantities of both moisture and iron. Sulfur may be present in gray soil if it has a blue or greenish hue. A mottled gray, as opposed to a uniform gray or blue-gray, indicates that the soil is wet at times and dry at others. That mixture is called terra sigillata - sealed earth.
Gray soil can be caused by a number of factors including the presence of iron sulfide (methane) in the soil. Sealing of the soil occurs when organic matter decays and releases carbon dioxide, which combines with the iron in the soil to form iron sulfide. The sulfur prevents other minerals from forming under these conditions, so only iron sulfide can precipitate out. The amount of sealing material present in the soil determines how quickly the soil will become gray. If more organic matter were added to the soil over time, the rate of sulfurization would decrease until there was no longer enough carbon dioxide in the atmosphere to react with the iron in the soil. At that point, if the heating process was stopped, solid iron oxide would precipitate out instead.
Soils on planets with atmospheres composed primarily of nitrogen and oxygen are unlikely to be gray due to sulfur contamination. However, on some planets with significant amounts of sulfur in their crusts, the soil might be gray even if it contains little sulfur itself.
Blue-grey and blue-green hues indicate that the soil is saturated for the majority of the year. The colors are caused by iron (which is generally red as an oxide) being present in a reduced form (the opposite of being oxidised) and maybe combining with sulfur as a sulphide. These soils are good sources of iron but can be difficult to work because they're so hard.
The coloration of these soils results from bacteria in the soil reducing iron oxides into its metallic form which then combines with sulfur coming from other parts of the plant to create a grayish-blue pigment. The bacteria responsible for this process are called sulfate reducing bacteria and there are several genera within the family Desulfovibrionaceae that are capable of doing this. One common name for these soils is "black cottonwood soil" because the vegetation around the soil becomes black when it rains. This is due to the fact that the moisture in these soils promotes the growth of algae which uses the sulfur in water molecules to produce their own food source. As these organisms die and decompose, the sulfides they contain release gas which causes the soil to become acidic.
These are very acidic soils that are poor at holding any organic material such as nutrients or carbon. If the acidity is high enough, it will actually dissolve some calcium carbonates such as limestone rocks which can cause them to white out during snowmelt seasons.
Gleyed/grey/green soils are linked with extremely poor drainage or waterlogging. Because of the absence of air in these soils, iron and manganese may combine to produce compounds that give these soils their color. The presence of magnesium sulfate (MgSO4) in gleyed soils results from the dissolution of gypsum deposits created by evaporating water from flooded fields. Magnesium ions replace some of the sodium ions in the clay particles, giving them a negative charge. When enough magnesium ions bind to one clay particle, the result is flocculation or clumping. This prevents the particles from sinking to the bottom of the floodwater when it evaporates, so they remain in the top few inches of soil.
If you look at photos of ashy soils, you will see that they are usually very dark in color. This is because there is not much organic matter present in these soils which can absorb sunlight and reduce the amount of toxic substances being released into the atmosphere.
Asphyxiated soils often test high in alkalinity due to the release of carbon dioxide into the atmosphere. These gases react with moisture to form salts that can damage plant roots. In addition, the low levels of oxygen in asphyxiated soils cause metals such as iron and copper to combine with other chemicals instead of oxidizing like they should.
Iron generates red oxides in well-drained soils or under dry circumstances, lending a red color to the soil. Iron occurs in a reduced condition in damp soil with a lack of oxygen, providing the soil grey/green/bluish-grey colors. The iron reduces organic molecules, causing them to appear colored.
Cobalt generates blue oxides in wet conditions, giving earth cobalt blue colors. It does this in much the same way as iron but for different reasons: instead of red, it gives off blue. Cobalt is found in small amounts in most rocks. When coal was formed over three billion years ago, the remains of plants and animals were buried deep beneath the earth's surface, forming huge reservoirs of oil and gas. As these deposits decayed, they left behind solid beds of coal that are still present today. Coal contains high levels of carbon and hydrogen but very little other material. The only significant element not found in coal is nitrogen. Most nitrogen is locked up in animal and plant bodies, but some is also found in the atmosphere in the forms of ammonia and nitric acid. Ammonia is used by bacteria in soil to make nutrients available for plants. Nitrogen is important for healthy plants because they use it to build their tissues. Plants take in nitrogen through their roots and then pass away unharmed. Any remaining nitrogen is then absorbed into more organic matter or returned to the soil for further use.