World’s Soil Resources

The quality of the soil’s water is determined by the impurities and pollutants present in the soil water, which
may, or may not be adsorbed to and/or exchanged in some part with the soil’s reactive matrix materials.
The flow of soil water is determined by the gradient in the matric potential, and the soil’s hydraulic
conductivity, K (cm day -1 ) (Figure 6.14), which describes the ease with which water flows through the soil
pore space. The hydraulic conductivity curve is highly non-linear and strongly dependent on the soil’s water
content, θ, and hence matric potential (Figure 6.14). Soil water flow can vary from very slow in soil with small
pores, to very fast in soil with large interconnected pores.
Figure 6.13 The soil-water characteristic curve linking matric potential, to the soil’s volumetric water content.
Source: Tuller and Or, 2003.
The soil-water characteristic (Figure 6.14) is an important factor affecting soil microbiology and rhizosphere
ecology. It controls the stability of the spatial and temporal geometry of the soil pore space, which in turn
defines the allocation of resources to soil biota, the transport of liquids, gases and solutes to and from roots,
and the diversity of microbial habitats (Hinsinger et al., 2009). The soil micro-organisms are largely aquatic in
nature and do not inhabit the air-filled pores. They live instead in the liquid phase of the pores, the thickness of
which is controlled by the matric potential, which also controls the size and distribution of water-filled pores
that provide the hydraulic connectivity through soils.
Status of the World’s Soil Resources | Main Report Global soil status, processes and trends
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