The Soil Environment

References...................................................................................................... 21

A central problem in any discussion about soil as a microbiological habitat

is peoples’ conceptual visualization of soil. The most commonly used unit of

reference is 1 g. At present, this appears to be an optimum mass from which to

gain biodiversity indices, themselves indicative of habitat richness. This gram

of soil will comprise inorganic and organic fractions. Soil inorganic particles

are classified into three major groups according to their size: sand, silt and

clay. The proportions of these in any one soil determine the soil texture. The

number of soil particles per gram can range from 90 (pure coarse sand) to 90

billion (pure clay), depending on the proportions of sand, silt and clay in the

sample. Assuming spherical shapes, the surface area of soil particles can range

from 11 cm

g

to 8 million cm

g

. Thus, although an innate truism, it

is necessary to emphasize the high degree of physical heterogeneity present in

any given sample. Although the concept of an average soil is not entirely

meaningful, a hypothetical soil aggregate is presented (Figure 1.1) as the basic

unit of the soil habitat. Many biogeochemical processes occur at scales

relevant to this unit, particularly processes such as gas diffusion and water

movement, which create a mosaic ofmicrosites and gradients [1]. Most, if not

all, aggregates are potential niches or habitats for one or more species of soil

microorganisms. In the hypothetical sample presented here, the gram of soil,

the arrangement of the soil particles (soil structure, see Table 1.1) will be vital

to the microbial habitat. Approximately half the volume of an aggregate will

be void spaces (pores) connected by tortuous pathways presenting a range of

pore neck sizes. It is the relationship between the interconnecting pathways,

channels and pores in soil that provides the microhabitat space (niche, used

here as the term to describe habitable space in soil) for the soil microbiota.

The physical niches themselves will mainly be pore walls, although water in

channels may transport significant numbers of freely motile bacteria

(Figure 1.2).