Pore Space

Between the solid (mineral and organic) components of soil is space forming pores that vary in size. On a volume basis, mineral soils are about 35-55% pore space, whereas organic soils are 80-90% pore space. But total soil pore space can

Environmental factors Disturbance

Fine-scale effects of roots, organic particles and soil structure

Plot-to fieldscale effects of burrowing animals, individual plants and plant communities

Large-scale gradients of texture, soil carbon, topography and vegetation systems

FIGURE 2.5 Determinants of spatial heterogeneity of soil organisms. Spatial heterogeneity in soil organism distributions occurs on nested scales and is shaped by a spatial hierarchy of environmental factors, intrinsic population processes, and disturbance. Disturbance operates at all spatial scales and can be a key driver of spatial heterogeneity, for example, through biomass reduction of dominant organisms or alteration of the physical structure of the soil substrate. Feedback between spatial patterns of soil biotic activity and heterogeneity of environmental factors adds further complexity (dotted arrows) (from Ettema and Wardle, 2002, with permission from Elsevier).

Environmental factors Disturbance

FIGURE 2.5 Determinants of spatial heterogeneity of soil organisms. Spatial heterogeneity in soil organism distributions occurs on nested scales and is shaped by a spatial hierarchy of environmental factors, intrinsic population processes, and disturbance. Disturbance operates at all spatial scales and can be a key driver of spatial heterogeneity, for example, through biomass reduction of dominant organisms or alteration of the physical structure of the soil substrate. Feedback between spatial patterns of soil biotic activity and heterogeneity of environmental factors adds further complexity (dotted arrows) (from Ettema and Wardle, 2002, with permission from Elsevier).

vary widely for a variety of reasons, including soil mineralogy, bulk density, organic matter content, and disturbance. Pore space can range from as low as 25% for compacted subsoils in the lower vadose zone to more than 60% in well-aggregated clay-textured surface soils. Sandy-textured soils, though having a higher mean pore size, tend to have less total pore space than do clay soils.

Soil pore space is defined as the percentage of the total soil volume occupied by soil pores:

% pore space = (pore volume/soil volume) X 100. (2.1)

Direct measurement of the soil pore volume is somewhat tedious to carry out; it is usually estimated from data on soil bulk density and soil particle density using the following formulas:

soil bulk density—Db (Mg/m3) = soil mass (Mg)/soil bulk volume (m3),

soil particle density—Dp (Mg/m3) = soil mass (Mg)/soil particle volume (m3)

(usually assumed to be 2.65 Mg/m3 for silicate minerals, but can be as high as 3.25 Mg/m3 for iron-rich tropical soils and as low as 1.3 Mg/m3 for volcanic soils and organic soils),

FIGURE 2.6 A 3-D CT X-ray scan of a soil core 6.4 cm in diameter X 10 cm in length, with a resolution of 10 pm, showing pore space distribution. (Courtesy of R. Heck, University of Guelph.)

Though total pore space is important, fundamentally it is the size, shape, and interconnection of soil pores that are the key in determining the habitability of the soil, for they directly control soil aeration and water relationships; this is largely a function of soil texture and structure (Fig. 2.6).

Total pore space is usually divided into two size classes, macropores and micropores, based on their ability to retain water after water drainage under the influence of gravity (Table 2.2). Macropores are those pores larger than ~10 pm in diameter that allow rapid diffusion of air and rapid water infiltration and drainage. They can occur as the spaces between individual sand and coarse silt grains in coarser-textured soils and in the interaggregate pore space of well-structured loam- and clay-textured soils. Macropores can also be created by roots, earthworms, and other soil organisms, forming an important type of pore termed "biopore." Biopores are typically lined with cutans rich in organic matter and clay and are ideal habitats for soil microorganisms. They provide continuous channels extending throughout the soil, often for lengths of a meter or more.

Soil pores less than 10 pm in diameter are referred to as micropores. Though important for retention of water available for plants and providing an aqueous habitat for microorganisms, the restricted size of micropores can limit interactions of soil organisms and their access to potential substrates.

While the larger micropores together with the smaller macropores can accommodate plant root hairs and microorganisms, pores smaller than ~5 pm in diameter are not habitable by most microorganisms and may even be too small to permit entrance of their exoenzymes.

TABLE 2.2 Pore-Size Characteristics across a Textural Range and Their Relation to Soil Water Potential (% of total soil porosity)

Textural classes

TABLE 2.2 Pore-Size Characteristics across a Textural Range and Their Relation to Soil Water Potential (% of total soil porosity)

Textural classes

General pore

Soil water

Equivalent

Sandy

Loam

Clay

categories

potential (kPa)

diameter (pm)

loam

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