## Ped structure

The overall architecture of the ped determines the amount of space occupied by air and water spaces, and that filled by organic detritus and mine ral particles. The spaces that can be occupied by air and water are called the soil pore space. Soil texture and structure affect the amount of pore spaces in the soil and the size of pores. Tortuosity of soil pores refers to the semi-continuous network, or reticulum, of these spaces. Soil organisms live inside the pore space reticulum and on the particles. The pedosphere refers to this habitat in the peds. However, not all pore spaces are large enough to hold living organisms. At any one time, a substantial fraction of the spaces are not accessible to aeration, nutrient flow or entry by living organisms. The habitable pore space refers to the fraction of pore spaces that is accessible to soil organisms (Fig. 2.4). It is possible to estimate the volume of some of these spaces.

If soil only consisted of solid material, it would have the density of its solid constituents. However, because it consists of solids and pore spaces, the weight of a volume of soil is much less than the density of the solid matrix alone. The bulk density is an estimate of how tightly packed the soil structure is. It is the dried weight of a known volume of soil expressed in g/cm3, or megagrams/m3 (Mg/m3). Bulk soil density (Sp) is obtained as Sp = WD/Fwet, where WDry is the weight of soil dried to constant weight at 105°C and Vwet is the bulk volume of the sample before drying. The lower the bulk density, the more loosely packed the soil is, and the more pore spaces there are. The bulk volume is an estimate of total volume of soil solid and pore spaces. This gives no indication of the mean size distribution of pore spaces, just an indication of how compact the soil is. Porosity is the ratio of pore space to bulk volume. In general, soil bulk density ranges between 0.9 and 1.7 g/cm3 with corresponding air porosity of 35-5%. The particle density (Pp) is the mass of a volume of soil excluding the pore spaces, i.e. the mass of only the solid matrix. The difference between the bulk density and particle density is an estimate of the percentage pore space (A) in the soil: A = (Pp - Sp)/Pp X 100 . An average value of particle density is 2.65 g/cm3, with a usual range of 2.5-2.8 g/cm3.

Soil pore spaces are occupied by air and water. The more moisture in the soil, the more space is occupied by water (Fig. 2.5). The air space volume can be obtained from the change in volume of a water column. When a known volume of soil is added to a known volume of water, the change in volume is the amount of water displaced by the soil solids. This difference is the sum of the soil solid particles (mineral and organic) as well as its water content. The more wet a soil, the less air volume in the pore space. Thus, dry soils which have more air spaces cause less volume change. By obtaining a series of measurements at different moisture contents, one can estimate the fraction of pore space occupied by air and water. When all pore spaces are filled with water, no air spaces remain and the soil is water saturated. Therefore, water saturation interferes with gas exchange between soil and the atmosphere. Air porosity depends on soil water content as well as soil structure. The presence of macroinvertebrates (such as earthworms) and burrowing small mammals can greatly affect soil porosity to air and the quality of soil air, by creating large gas exchange spaces.

Fig. 2.5. Soil pore spaces. Water moisture content of soil during drainage. (A) Soil pore spaces flooded with gravimetric water after heavy rain; air bubbles remain. (B) Most gravimetric water has drained, with air pockets reforming; capillary water remains. (C) Drying soil, with some capillary water remaining and mostly air spaces; relative humidity will decrease and soil temperature will be closer to air temperature.

Fig. 2.5. Soil pore spaces. Water moisture content of soil during drainage. (A) Soil pore spaces flooded with gravimetric water after heavy rain; air bubbles remain. (B) Most gravimetric water has drained, with air pockets reforming; capillary water remains. (C) Drying soil, with some capillary water remaining and mostly air spaces; relative humidity will decrease and soil temperature will be closer to air temperature.