Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 142

plus various physical properties including soil texture, porosity, and water retention,
among others.
Biological factors are a third classification to be reckoned with; but the many
difficulties in measuring as well as interpreting the soil's biological components
have made this aspect of soil systems a less-considered category. A basic principle
of soil science is that all three aspects of soil systems warrant concurrent attention,
because they function as three dimensions of the same phenomenon. However, work
on the biological dimension has lagged behind the chemical and physical focuses
in soil science. Research findings reviewed in this chapter should encourage a nar-
rowing of this disparity as they show profound and intimate relationships between
the soil microbiota and crop production, the latter heretofore attributed primarily
to the soil's chemical and physical properties. The chapter throws new light on the
contributions made by soil organisms to crop performance, not just in the soil but
in plants as well.
6.2 BIOLOGICAL ACTORS WITHIN SOIL SYSTEMS
It is well understood that many of the physical and chemical properties of soil such
as its aggregation and available forms of nitrogen are reflective of “the life in the
soil,” influenced by the presence and activity of earthworms and many other fauna,
as well by its abundance and diversity of microflora. Soil structure and functioning
are directly affected by fungi, for example, which produce the compound glomalin
that “glues” soil particles together. Bacteria and algae likewise produce extracellular,
adhesive hygroscopic polymers within the soil that contribute to its aggregation and
water-retaining properties. Both the stocks and availability of plant nutrients in the
soil are affected by uncountable numbers and species of microbes, through well-
known processes such as biological nitrogen fixation (BNF); phosphorus (P) solubi-
lization; nitrogen (N) and sulfur (S) cycling; and the conservation and concentration
of nutrients in soil, referred to as immobilization. This latter process coexists with
the complementary processes of nutrient mobilization and mineralization, critical
for both micronutrients and macronutrients, which are mediated by microbial activ-
ity (Coleman et al. 2004).
At a larger scale, soil nutrients are cycled through immensely complex food
chains or food webs with myriad organisms operating at multiple trophic levels
(Thies and Grossman 2006; Wolfe 2002). Their biodiversity parallels and even sur-
passes that observed above-ground (Wardle 2002). Much is being learned about how
plants and microbes interact to mutual benefit (Barea et al. 2005; Berg 2009; Badri
and Vivanco 2009; Hartmann et al. 2009). Still, the roles that soil organisms play
have been mostly considered in terms of how they modify the chemical and physical
parameters of soil systems.
Less recognized—except where soil organisms have observable negative effects
on crop yield as pathogens or parasites—is how biological agents in the soil them-
selves contribute to crop production, directly affecting the measures of yield that are
used as indicators of soil fertility. What appear to be the effects of soil physics or
soil chemistry may, on closer examination, be biological effects. Put another way,
they may be the outcome of inextricably meshed effects of all three dimensions of