Soil air

The soil air (sometimes referred to as "gas phase in soil", "soil gas" or "basic air") is the gaseous portion of the soil . The spaces between the solid soil particles are filled with air if they do not contain water. This gas phase is either in contact with the earth's atmosphere or it is enclosed by solid particles and water. For both cases there are typical compositions and pressure conditions in the soil air.

The gas phase in the soil is a complex system that is influenced by many factors. These factors can be climatic, geological and biological in nature, among others. Last but not least, anthropogenic interventions should be mentioned, which can have negative effects, especially in the area of soil compaction, due to increasingly larger agricultural machines. A healthy gas phase is a necessary condition for life in the soil; In particular, the diffusion movements of the oxygen necessary for many living things and the metabolic products produced ensure that the biological diversity in the soil can be maintained over a long period of time.

Distribution of the gas phase

Under field conditions all solid particles of the mineral soil are covered with water films that are in equilibrium with the water vapor pressure of the soil air. As long as this state is maintained, there is no direct contact between the soil particles and the gas phase in the soil. These water films make a very strong contribution to the thermal conductivity of the soil, because the air in the soil has an insulating effect, and the heat conduction through the soil particles hardly occurs due to the few contact surfaces. Heat conduction is only made possible by the presence of even the smallest amounts of water.

The gas phase in the soil is usually limited to the coarse pores under our field conditions. The middle pores form the basis of the usable field capacity (nFK), and the fine pores are occupied by the dead water .

In terms of distribution, it can generally be assumed that the proportion of gas decreases with increasing depth and thus closer to the groundwater surface (GWO), since the water content increases here. Deviations from this can be found as soon as large amounts of water are applied to the soil surface, which then seep into the soil , such as. B. after heavy rain or sudden flooding. The gas phase is divided into air channels, which are in contact with the atmosphere, and air pockets, so-called inclusions. The latter form when water flows down from a higher layer and temporarily clogs the air ducts because the air cannot escape from them quickly enough. This happens, among other things, in the above-mentioned heavy rain or overflows. Inclusions can also form under anaerobic conditions if there is enough easily decomposable organic material. Here microorganisms then produce, among other things, H ₂ and CH ₄ . This is especially visible when the water levels fall in the ground, so the water tensions increase and decrease the gas absorption capacity of the water. The hydrogen and methane formed are then in gaseous form and thus form the inclusions. The increasing water content towards the depth influences the distribution of the gas phase insofar as the decreasing water tension causes the air to migrate into the pores, in which the radii of curvature of the air bubbles allow the least air overpressure compared to the surrounding water. This means that the gas phase is limited to a few coarse pores with increasing depth.

Composition of soil air

The composition of the gas phase in the soil sometimes differs considerably from the distribution of the gases in the atmospheric air. Much higher CO ₂ contents occur in the soil , which can be up to five times higher at a depth of around 1.5 m than in the atmosphere. On the other hand, the O ₂ content decreases by about the same factor with increasing soil depth. This is mainly due to the biological processes in the soil. The root respiration of higher plants, respiration of the soil animals ( microbial respiration , see also soil respiration ) and the metabolic processes of the aerobic and anaerobic microflora consume considerable amounts of oxygen and in return form carbon dioxide. If there is a lack of O ₂ , ie under reducing conditions, small proportions of CH ₄ , H ₂ S, N ₂ O, NH ₃ , H ₂ and gases from the hydrocarbon group, which produce the smells typical of the soil, are also formed. The high water vapor content of the soil air is remarkable. The relative humidity in the soil is usually always close to 100%. The H ₂ O vapor pressure only begins to decrease when it becomes very dry ; this happens at water tension values beyond the PWP ( wilting point ), i.e. pF> 4.2. This fact contributes to the fact that most soil organisms and plants are not adapted to survive at lower water vapor contents in the soil. The high H ₂ O vapor pressure is due to the finely branched pore system and the very large interfaces between water and air in relation to the volume. Furthermore, the composition of the gas phase depends on the temperature, as this influences the solubility of the individual gases in water differently. Thus, O is ₂ more water soluble than, for example, N at low temperatures ₂ , with the result that the percentage of oxygen at low soil temperatures is lower than at higher.

Energetic position of the gas phase

The energetic position of the gas phase will only be dealt with briefly here in connection with the pressures occurring in the gas phase. We can calculate the water pressure in the ground using the basic hydrostatic equation. In this equation, the air pressure is used as a constant quantity in order to be able to calculate the absolute water pressure with its help:

{\ displaystyle p _ {\ mathrm {W}} = p _ {\ mathrm {L}} + h \ cdot d _ {\ mathrm {W}} \ cdot g}

${\ displaystyle p _ {\ mathrm {W}}}$ is the water pressure to be calculated, is the atmospheric air pressure, is the height of the water column, is the density of the water and the acceleration due to gravity . This equation shows that the water pressure is the sum of atmospheric pressure and the pressure exerted by the water column above the measuring point. This method produces only minor errors, since air pressure differences quickly equalize in an air body due to the lower viscosity of the air compared to water. For this reason, the influence of the air to be displaced or flowing in can generally be neglected when investigating water absorption or water release. Since this influence cannot be ignored in principle, it is common practice to define a gas potential when discussing the partial potentials of the soil water . (This differs from the pressure potential or .) As already described, in the event of heavy rain or overflows, gas inclusions can form, in which pressure differences arise compared to atmospheric pressure. These pressure differences can be up to 20 hPa depending on the height of the overflowing water column. The trapped air transfers the pressure exerted by the water column almost immediately to the solid particles and the water surrounding the air bubble. In this way, the equilibrium of potential is not changed and, apart from the change in volume of the gas inclusion caused by the ambient pressure, there is no flow movement of air masses. The pressure within such an enclosed body of air corresponds to the sum of outside air pressure and water pressure. This means that the pressure within such an inclusion can also be calculated using the basic hydrostatic equation. ${\ displaystyle p _ {\ mathrm {L}}}$ ${\ displaystyle h}$ ${\ displaystyle d _ {\ mathrm {W}}}$ ${\ displaystyle g}$ ${\ displaystyle \ Psi _ {\ mathrm {g}}}$ ${\ displaystyle \ Psi _ {\ mathrm {h}}}$ ${\ displaystyle \ Psi _ {\ mathrm {p}}}$

{\ displaystyle p _ {\ mathrm {L1}} = p _ {\ mathrm {W1}} = p _ {\ mathrm {L}} + h_ {1} \ cdot d _ {\ mathrm {W}} \ cdot g}

Here is the outside air pressure, the height of the water column, the density of the water and the acceleration due to gravity. This formula applies to air inclusions below the free water surface, i.e. H. as the height of the water column rises, so does the pressure. However, if the air inclusion is above the free water surface, i.e. in the area of negative water pressures (positive water tensions), the height of the water column has the opposite effect. The equation then changes as follows: ${\ displaystyle p _ {\ mathrm {L}}}$ ${\ displaystyle h}$ ${\ displaystyle d _ {\ mathrm {W}}}$ ${\ displaystyle g}$

{\ displaystyle p _ {\ mathrm {L2}} = p _ {\ mathrm {W2}} = p _ {\ mathrm {L}} -h_ {2} \ cdot d _ {\ mathrm {W}} \ cdot g}

In this case, the pressure within such an inclusion is lower than the atmospheric pressure. The models mentioned so far assume that the air bodies in the ground are separated from the water by flat interfaces. This is by no means the case in the narrow pore system of the soil, because the interfaces between air and water always form curved menisci , the surface tension and radius of which compensate for the pressure differences between water and air. Seen from the air, these menisci are concave. This has the consequence that the pressures within the air space are higher than in the surrounding water. This pressure difference has the amount:

{\ displaystyle \ Delta p = {\ frac {2 \, y} {r}}}

${\ displaystyle y}$ is the interfacial tension water: air and the radius of curvature of the largest meniscus involved in the limitation. The equation shows that the excess pressure within the gas confinement is greater, the narrower the limiting menisci are. Therefore, as already described under point 1), the gas inclusions also tend to move into the largest possible pores, since the larger radii of curvature of the menisci represent a form with less energy. These movements of the air in the ground will be discussed later under the term redistribution. The air inclusions try to assume as spherical a shape as possible and therefore press the solid particles apart with the excess pressure described by . This process can lead to the so-called air explosion. ${\ displaystyle r}$ ${\ displaystyle \ Delta p}$

Transport operations

The constant fluctuations in the production of carbon dioxide and the consumption of oxygen over the course of the year and day, as well as the above-mentioned urge for air inclusions to get into the largest possible pores, lead to transport processes in the soil. Different movements are possible here:

diffusion
Mass flow
Redistribution

Diffusion movements can arise when different distributions of the soil air components and thus different partial pressures occur. Mass flows, on the other hand, require differences in the total pressure, so they only occur if the total gas mass is unevenly distributed.

diffusion

Diffusion is the most important transport process in the gas phase. Two diffusion currents are predominant in the soil: on the one hand, the transport of CO ₂ from the depths upwards, and on the other hand the opposing flow of O ₂ downwards into the solum. In order for a diffusion flow to occur, changes in the concentrations and thus in the partial pressures are necessary. The gas flows that occur can be expressed by Fick's 1st law :

{\ displaystyle Jg = -D '\ cdot {\ frac {\ partial c} {\ partial s}}}

This formula is a simple equation that describes the transport of a substance along a concentration gradient. Jg is the flow of a gas component, i.e. H. the amount of gas that passes through a surface in a time s. is the change in concentration that is effective over the distance . D 'is the so-called diffusion coefficient, a proportionality factor. The negative sign on the right-hand side of the equation indicates that the flow is always from the higher to the lower concentration. According to the general gas law, the partial pressure p of a gas is determined by: ${\ displaystyle \ partial c}$ ${\ displaystyle \ partial s}$

{\ displaystyle p \ cdot V = m \ cdot R \ cdot T}

Therefore one can also write for the concentration c as the ratio between mass m and volume V:

{\ displaystyle c = {\ frac {m} {V}} = {\ frac {p} {RT}}}

This reveals the possibility of considering the partial pressures instead of the concentrations. For using the partial pressures we then get:

{\ displaystyle Jg = -D '\ cdot {\ frac {\ partial p_ {g}} {\ partial s}} \ cdot {\ frac {1} {RT}}}

and

{\ displaystyle D = D '\ cdot {\ frac {1} {RT}}}

These transformations make it possible to work with the partial pressures as well as with the concentrations. The diffusion coefficient D records the hindrance of diffusion due to the different shape and size of the diffusion paths in the pore system. It is therefore a soil characteristic.

Since a partial pressure gradient is necessary for the diffusion movements in the soil, gas particles can only be transported in this way if the partial pressure gradient is not interrupted by diffusion-inhibiting zones. These can be cultivation horizons ( plow bottom ) or compacted soil horizons. They form diffusion barriers, for which a large amount of the available concentration gradients is used up. Only slight diffusion movements are then possible behind such a barrier.

Mass flow

The term mass flow is understood to mean transport processes in which the total amount of gas changes. Changes in pressure are therefore necessary to induce a mass flow. Such changes in pressure in a gas-filled room are possible according to the general gas law:

{\ displaystyle p \ cdot V = m \ cdot R \ cdot T}

The following mean: m = mass of the enclosed gas, R = gas constant, p = pressure, V = volume and T = temperature. Changes in pressure in the gas phase can occur due to changes in atmospheric pressure and changes in temperature. However, the influence of the outside air pressure is relatively small, so a change of 30 hPa only causes a change in volume of about 1/30. This means that in a 1 m thick layer of soil, only 2-3 cm of air is transported out or in. The displacement of the soil air by water, e.g. B. by rapid changes in the groundwater levels due to impoundment, as occurs in floodplain soils. This can lead to an exchange of almost the entire gas volume. The influence of temperature changes in its effect on pressure changes in the gas phase is similarly low as that of the outside air pressure. A significant mass flow can occur when new gas molecules form in the soil, as is the case under anaerobic conditions in the presence of easily decomposable organic matter. The gas flow occurring here is occasionally far greater than the diffusion flow of oxygen directed into the soil, so that reducing conditions are established.

Redistributions

Redistributions play a role when considering the air inclusions described. The delimiting menisci of such air pockets all have the same radius. The pressure inside the inclusions is higher than in the surrounding water. The menisci in the coarsest pores involved in the inclusion are therefore shallower than those in smaller pores. The air inclusions are pressed into these coarsest pores, since they can form the largest hemispherical meniscus there and thus achieve the lowest pressure difference compared to the surrounding water. This process forces the entire amount of air to be shifted towards the coarsest pores.

Gas budget

Regarding the gas balance, the regular changes in the gas content should be mentioned, which are closely related to the course of the seasons. The change in the air content is caused by the increase or decrease in the water content. The water content is relatively high in spring, only to decrease in the course of summer and with advancing vegetation development. In return, the air content increases. The gas phase is not only limited to the space above the GWO, but gases can also be found below the GWO. These are mostly in water-dissolved form or are located in bubbles enclosed by the water. The diffusion movements of oxygen and carbon dioxide also play a role in the gas balance. In the case of oxygen, the concentration decreases with increasing depth; in the case of carbon dioxide, it increases analogously. If the air content in the soil drops to about 4–6% of the total volume, the gas volume begins to split up into individual inclusions. This gas content also forms the limit below which the O ₂ partial pressure drops to below 18%, i.e. the content of O ₂ in the atmosphere. Below this limit the characteristics of anaerobic processes appear. This limit of 4–6% air content also applies to the thriving of most cultivated plants.

literature

KH Hartge, R. Horn: Introduction to soil physics. 3. Edition. Enke-Verlag, 1999, ISBN 3-8274-1239-0 .