The Royal Society Report - Push-Pull

Figure 2.1. Global soil degradation. Source: UNEP (2009).
attributes, both beneficial and adverse. The latter relates to
the population densities and identities of resident pests
and diseases as well as the beneficial soil flora and fauna
that sustain soil ecosystem functions (eg nitrification,
aeration, nutrient cycling) and counter adverse impacts (eg
denitrification or regulation of pest populations).
2.5.1 Microbiological properties of soil
The microbial diversity in a fertile soil has been compared
to the biodiversity of a tropical rain forest (Beneddeti et al.
2005). Soil fungi and bacteria are critical for the recycling
of carbon and major nutrients, particularly nitrogen, from
organic inputs derived from plants and animals. Inputs of
organic material in the form of crop residues and animal
manures encourage the maintenance of an active microbial
population, although the impact of soil use (eg for
different crops) on microbial diversity is not well studied.
Much soil microbial diversity is maintained in a dormant
condition (spores and other resting structures) and the
majority of microbial activity is associated with the zone
surrounding plant roots (rhizosphere) where other impacts
such as enhanced nutrient uptake (mycorrhizae) and
amelioration of root diseases (biocontrol) can occur. Soil
microbes also contribute to the maintenance of a friable
soil structure.
2.5.2 Physical properties of soil
The physical properties of soil are determined by the
underlying geology, the way it has been managed in the
past and the way it is currently managed. A soil that is
resistant to wind and water erosion is usually also a soil
that readily allows water infiltration (ie is well drained) and
has a high water-holding capacity. These characteristics are
strongly correlated with adequate organic matter content
resulting from animal manures and return of crop residues.
Organic matter also encourages microbial activity and
nutrient recycling.
A well-drained, well-aerated, friable soil that is not
compacted promotes high crop productivity when water
and nutrients are not limited. Good seed beds conducive to
the germination, emergence and establishment of annual
crops raised from seed are easier to prepare from well
structured soils. In addition, the energy required for
cultivation is significantly less in well structured soils. For
example, it has been demonstrated that the energy savings
from incorporating wheat straw into arable soils to improve
soil conditioning are greater than the use of that straw as
an off-take feedstock for the production of biofuels or
electricity via combustion (Powlson et al. 2008).
In regions where soil of appropriate quality is in short
supply, artificial growing media can be used. These may be
solution culture, rockwool or coir in glasshouse production.
Increasingly waste products may be digested to produce
an inert growing substrate to which microbes and nutrients
can be added. This approach can not only contribute to the
production of artificial ‘soils’ but also result in the
generation of CO 2 and energy that can be used in the
production process.
2.5.3 Salinity
Of the land farmed in dry-land agriculture, about 2% is
affected by secondary salinity. Of the irrigated land, 20% is
14 I October 2009 I Reaping the Benefits The Royal Society