ABSTRACT

The gas phase in soils is called soil air. It is located in the pore space, which is defined as “the space in soil that is occupied by the gas and liquid phases.” This pore space is composed of macro-and micropores. The macropores are usually the spaces between the soil structural units and as such, they are the main channels for air movement. On the other hand, the micropores are the spaces within the structural units, and are the main spaces for water. The macropores will be filled first with water during a rainfall or by irrigation. This water then moves into the micropores, where it will be held for some time in the soil. Some scientists suggest distinguishing the pores in terms of sizes, e.g., transmission pores with an effective diameter >50 :m, storage pores with sizes between 0.5 to 50 :m, and residual pores with effective diameters <0.5 :m (Wild, 1993). The residual pores then fall in the category of the micropores and will retain soil moisture when the soil is “dry.” Transmission pores have an effective diameter of the size of fine silt and can be called mesopores, whereas the transmission pores are then the macropores. Wider pores are usually not called soil pores but reveal themselves as cracks in the soil. The amount of total pore spaces, called soil porosity, differs from soil to soil and from soil horizon to soil horizon. The percentage of pore spaces may range from 25% (by volume) or lower to 50% or higher. The lower number of pore spaces is usually found in the subsoil, where the soil

is more compacted and contains more solid space (less air and organic matter but more mineral matter). A surface soil with a loam texture and in optimum condition for plant growth has a total pore space of 50%, which is filled half with water and half with air. Pore space is inversely related to bulk density in soils of similar textures (Brady and Weil, 2008; Brady, 1990). The major gaseous components and their relative amounts in atmospheric air are listed in Table 4.1. Soil air is composed of the same type of gases commonly found in the atmosphere and differs from atmospheric air only in composition. Generally, soil air is higher in carbon dioxide concentrations and water vapor, but often lower in oxygen content. The magnitude of these differences depends on a number of factors, e.g., respiration, aeration, soil porosity and soil depth. For example, the consumption of oxygen and production of carbon dioxide, called respiration, by plant roots and other soil organisms, is the cause for the decline of oxygen and rise of carbon dioxide contents in soil air. The differences are also affected by aeration or the rate of gas exchange between atmospheric air and soil air. Such an exchange is often related to movement of soil moisture into and out of the soil pores, which will be addressed below. Sandy soils usually exhibit lower total porosity than clayey soils, because of

the presence of large pores. These macropores promote more rapid oxygen and carbon dioxide movement than micropores, because of a more rapid diffusion of air through the larger pores. Oxygen contents are also lower at deeper depth in the soil due to slower rates of diffusion from the surface down into the soil. For proper plant growth, the pore spaces must contain air in sufficient amounts and in the proper composition. The amount of soil air in the pore spaces is usually controlled by soil water. When the amount of soil water increases, air is pushed out of the pore spaces. When water content in the pore spaces decreases because of uptake by plants and/or evaporation, air content increases by mass flow and diffusion. As indicated earlier, for optimum plant growth, half of the pore spaces should be filled with air and half with water. The quality

2 2of this air is usually measured by its composition in O and CO content. Nitrogen (in gas form) is unavailable to most plants and, therefore, is not taken into consideration in this respect, though it is an important constituent in biological nitrogen fixation. Most

2 2biological reactions in soils consume O and produce CO ; hence soil 2 2air is generally lower in O (< 20%), but higher in CO content (>

0.03%) than atmospheric air above the ground, as indicated earlier. 2The CO content shows a marked seasonal trend and usually increases

with soil depth. Oxygen content in soil air appears to be low in soils saturated with water. It reaches a maximum level at field capacity, after which it remains almost constant with decreasing amounts of

2soil water (Brady, 1984). In general, oxygen deficiency is noted if O content in soil air decreases to 15% or less (Taylor and Ashcroft, 1972). Several biochemical reactions, mediated by soil organisms, are

2 2responsible for the O !CO balance in soil air, e.g., respiration of plant roots and microorganisms and aerobic decomposition of soil organic matter. Both reactions consume oxygen and produce carbon dioxide (see Tan, 1993). Respiration increases with the presence of abundant

2plant life; hence, soils under crops generally contain less O and more 2CO than bare or fallow soils. The application of manure or plant

2residues may also increase the consumption of O and the production 2 2of CO . An additional source of CO is the burning of crop residues.