360 Ecological Restoration and Design
- Phosphorus (P), often identified as a key constraint to crop production, is
actually abundant in most soils, with much less than 10 per cent of the total
supply ‘available’ at any one time. There is much potential for P solubilization
and mobilization through biological processes. Turner and Haygarth (2001),
in discussing their evidence that microorganisms significantly increase the P
available in soil when it is alternately wetted and dried, suggest that the same
mechanisms probably apply for other nutrients, but these have not been stud-
ied. It remains to be determined to what extent these processes can provide P
and other nutrients at the levels and rates needed to meet crop demand and if
it can be done sustainably.
There is a legitimate concern about ‘where the nutrients will come from’ if exoge-
nous inputs are reduced. An ICRISAT study addressed that question and showed
that organic inputs could match inorganic input-dependent practices in terms of
yield, with a concomitant build-up of soil resources both chemically and biologically.
A concern whether farmers can access sufficient organic resources for such produc-
tion is valid but may be solvable. Relatively little scientific research and experimenta-
tion have gone into producing biomass rather than just yield. Plant breeding efforts
over recent decades, aiming to maximize the Harvest Index, have sought to reduce
the biomass, which can feed soil microorganisms as well as livestock. Biomass pro-
duction has been raised from 6–8 t ha–1 to 25 t ha–1 in Brazil by introducing a calcu-
lated variety and sequence of plants into the system.
Significant research and experimentation have been done on N-fixing trees
and on green manures and cover crops grown in situ. If research and extension
efforts comparable with those that went into the Green Revolution were focused
on the production of biomass within agricultural systems as well as on otherwise
non-arable land, this nutrient and biomass constraint could, it seems likely, be
alleviated creating more scope for biomass-based soil fertility management. Work
would need to be done on implements such as cutting tools, shredders and equip-
ment for transport that could raise labour productivity when handling biomass for
agricultural purposes. A combination of enhanced productivity and reduced costs
based on innovations that alleviate this constraint could make second-paradigm
practices more profitable and would present producers with a different incentive
structure in the future.
There will in most soil systems be some nutrient constraints, following von
Liebig’s ‘law of the minimum’, based on the concept that there will always be some
nutritionally limiting factor operative in the soil (van der Ploeg et al, 1999). This
is why the second paradigm is better characterized as ‘biological’ than as ‘organic’,
since it does not reject the use of inorganic nutrient inputs.
Justus von Liebig, the first major contributor to our knowledge of soil fertility,
considerably expanded his thinking by the end of his scientific career. Rather than
focus on particular chemical elements in a reductionist manner, he advocated a
more holistic view of soil systems and paid more attention to their living compo-
nents. In 1865, reflecting on his life’s work, von Liebig wrote: