The geosphere is the solid part of the Earth, including the core, mantle, and crust. For purposes of discussion, we are treating the geosphere as separate from the other spheres, but in actuality components of the other spheres, especially the biosphere and hydrosphere, are closely linked to the geosphere. Examples of links between the geosphere and other spheres are soil water, aquifers, sediment pore water, plant roots, and microorganisms in the subsurface. Many important processes in the geosphere are controlled by the biota and/or water.

The core consists primarily of iron. The amount of nitrogen (N) is unknown but probably insignificant, and would be inaccessible for cycling at the Earth's surface. The mantle is chiefly composed of magnesium and iron silicates. N is a minor element in the mantle, but due to the large volume of the mantle, there is a considerable amount of N in the mantle. The core and the mantle have been estimated to contain a total of over 1.6 x 10¹⁷ metric tons of N (see Nitrogen Properties). However, this N is not readily available to be cycled in the near-surface Earth environment. Some periodically enters the atmosphere and hydrosphere through volcanic eruptions, primarily as ammonia (NH₃) and nitrogen (N₂) gas.

The crust can be subdivided into rocks, unconsolidated sediments, and soil. N has an average concentration of only 20 parts per million (ppm) in the crust. It is not a primary mineral forming element, and is concentrated in crustal rocks only in unusual environments.

The most common N-containing minerals are nitrates formed in evaporative environments. Nitrate minerals are very soluble, and can persist only in very arid places. Soda niter (NaNO₃) is the most common nitrate mineral, with niter (KNO₃) less common. The only significant deposit of soda niter in the world is a narrow belt of deserts in northern Chile, where other unusual evaporite minerals are also found. At one time, these deposits were an important source of nitrate (NO₃^-) for fertilizers; with modern artificial fertilizer production processes, these deposits are no longer of value for their NO₃^-. There are about 30 ammonium (NH₄⁺) minerals known. Like nitrate minerals, they are highly soluble and thus rarely preserved. They occur most commonly as weathering products of organic-rich sediments. NH₄⁺ in minerals is more commonly found as an impurity in some common silicate minerals, such as feldspars, micas, and clay minerals, where it substitutes for potassium (K⁺).

Crustal rocks are formed in three environments, igneous, metamorphic, and sedimentary. The N in igneous and metamorphic rocks is present as NH₄⁺ substituting for K⁺ in silicate minerals; the concentration of NH₄⁺ is generally less than 20 ppm in igneous and metamorphic rocks. NH₄⁺ is absent from most volcanic rocks because it is volatilized during magmatic cooling. Some plutonic rocks such as granites, however, have NH₄⁺ concentrations greater than 100 ppm, presumably derived from originally organic-bearing sediments assimilated in the magmatic source. NH₄⁺ is readily transported by hydrothermal activity, and hydrothermal fluids and hydrothermally altered rocks can be significantly enriched in NH₄⁺.

N is more abundant in sedimentary rocks than igneous or metamorphic rocks, with concentrations between 200 and 4000 ppm. Most of the N present in sedimentary rocks is NH₄⁺ and organic-N derived from the decay of buried organic material. NH₄⁺ is found both fixed in the crystal structures of silicate minerals and held in an exchangeable form by adsorption onto mineral surfaces or in interstitial fluids. Faure (1991) estimated that there are approximately 7.14 x 10¹⁹ moles (1.01 x 10¹⁵ metric tons) of N stored in sediments and sedimentary rocks, 80% as organic-N, mostly in shale. This pool of N is the same order of magnitude as the N found in the atmosphere and dwarfs the N found in the biosphere and hydrosphere by several orders of magnitude; however, it is cycled much more slowly than in the other spheres. Faure (1991) estimated that the residence time of organic-N in sediments and sedimentary rocks is 400 million years. Under certain conditions, such as when sediments or rocks containing significant N are exposed to oxygen, the reduced N can be oxidized and converted to NO₃^-, forming surface and subsurface waters with naturally elevated concentrations of NO₃^- (Hendry et al., 1984; Holloway et al., 1998).

An important source of N in sedimentary rocks is fossil fuels, including coal, oil, and natural gas. The N content of coal is generally between 0.1 to 2% and for most oil less than 0.1%. N₂ gas can be a major component of natural gas, usually less than 5%, but in some deposits greater than 85%. This pool of N is actively being removed from the geosphere through mining and drilling. In 1998, Illinois produced approximately 40 million tons of coal, which represents a removal of over 5 x 10⁵ metric tons of N from the geosphere. Most N is not used in the production of energy, and is primarily a waste product of combustion processes. Most of this waste N is released to the atmosphere as gases, such as N₂, NO, and N₂O.

From the cycling perspective, soils are the most concentrated and active reservoir for N in the geosphere. Almost all of the soil N exists in organic compounds, which on average contain 5 percent N by weight. Soil N contents vary widely; Schreiner and Brown (1938) reported that natural N concentrations in soils of the north central U.S. ranged from 6,500 to 218,000 kg N/ha. The sources of N for soil organic compounds include fixation of N₂ from the atmosphere, atmospheric deposition, human and animal waste, artificial fertilizer, and plant residues. The N is converted into organic compounds, or humus, the source of nutrients for plants and microorganisms. N is typically the most limiting nutrient in soils, which is why replenishment of N in agricultural soils by fertilization is often necessary.

Organic compounds in the soil are available for transfer into the biosphere, hydrosphere, and atmosphere via various biological and physicochemical processes, including compound degradation, leaching, erosion, plant and microbial uptake, denitrification, volatilization, etc. It has been estimated that 1 to 3 percent of total organic-N in temperate zone soils is mineralized in a single year, resulting in a complete turnover of soil N every 30 to 70 years (Foth, 1990). The state soil of Illinois is the Drummer soil, which covers much of east-central Illinois. Drummer soil is a productive soil with ample organic matter in its plow layer (6 %) (Fehrenbacher, 1984). To calculate the soil N content in the top meter of Drummer soil, we assume that soil organic matter contains 5 percent N (Foth, 1990), although some authors use a higher value (Stevenson, 1994), and use the reported ranges of organic matter content and bulk densities (Mount, 1982). Using these values, the top meter of Drummer soil is calculated to contain 29,200 to 48,800 kg N/ha. At mineralization rates of 1 to 3 percent per year, production of inorganic N in the top meter of Drummer soil ranges from 290 to 1470 kg N/ha-yr. This mineralized N is available for plant uptake and other biological and physicochemical processes listed above, such as leaching, denitrification, erosion, etc.