Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 143

soil system capabilities and performance—in which biological actors are pivotal and
animating.
Studies in recent years reviewed here shed some different light on the present
paradigm for crop production. They give evidence that certain soil organisms can
directly enhance crops' productivity by affecting crops' expression of their genetic
potential, rather than by the indirect route of just improving the chemical and physi-
cal characteristics of the soil systems that the plants inhabit. This microbiological
knowledge may modify our current understanding of what constitutes, and contrib-
utes to, soil fertility.
The current paradigm regards the soil itself as a vast, differentiated substrate
for the nurture of crop plants, which are viewed as beneficiaries receiving nutrients
from the soil under favorable chemical and physical conditions. Biological activity
enhances these parameters to varying extents. Soil fertility in this view is a quality
that resides in the soil itself. Such a stylization accepts the premise that soil and crop
sciences can be pursued as separable, albeit complementary, endeavors. The respec-
tive domains of soil and crops are linked by transactions of nutrients and water (and
root exudates) between soil systems and plants, with some amount of plant material
and residues subsequently returned to the soil. Much attention is focused on anthro-
pogenic soil nutrient amendments. This view of separate domains, however, misses
that connective role between the domains of soil organisms that directly influence
the vegetative growth and reproductive success of crop plants.
Both crop and soil scientists should be taking into account the contributions that
are made by mutually symbiotic microbial endophytes. These organisms live within
plant tissues and not just in their many and varied soil habitats. That such organisms
do more than facilitate crops' access to nutrients presents a more complicated con-
nection between the crop and soil sciences than presently considered. Soil organisms
inhabit not only the rhizosphere, being many times more populous in the thin zone
of soil adhering to and immediately surrounding the roots than in the bulk soil (for
a recent review and references therein, see Dazzo and Gantner 2009); they also live
within root interiors, as discussed below. Moreover, soil organisms can benefit host
plants while residing within their sheaths, leaves, and even seeds, as also discussed
here. When present in plant tissues, soil microbial symbionts, through processes not
yet well understood, affect the expression of tissue cells' genetic potentials, thereby
contributing to plants' vegetative growth and reproductive success, which are the
principal markers of soil fertility.
Advances in soil science are likely to come from greater appreciation that soil sys-
tems' support for crop production is more than a consequence of certain measurable,
inanimate qualities of the soil itself, considering the easily assessed mineral compo-
nents of soil and its organic matter content made inert for more precise and replicable
analysis. Soil fertility will be understood differently and better if it is regarded as
an emergent and contingent property that can only be known and validated outside
(or above) the soil system , measured in terms of desired kinds and extents of plant
growth and productivity. Seeing how crop performance is more intimately affected
by the unseen realm of soil microbes than previously appreciated will expand our
understanding of what constitutes and causes soil fertility. It should also contribute
to more prominence for the biological principles in soil science.