Integrated Environmental Biotechnology 245The climate of the growing location, the irrigation needs of the particular
trees being grown, the available nutrients in the soil and the management regime
all play major deciding roles in the ultimate delivered biomass energy to land
area ratio. While the climate must be simply accepted, the last three production
variables can be optimised by judicious interventions.
The irrigation requirements of SRC have been the subject of much debate
and consternation over the years. In this respect, some confusion has crept in
between the needs of poplars and willows. While the former has a very deep
tap root and in close planting can lower the water table by up to 10 times its
grassland level, the latter has a much shallower root system, making no greater a
demand than a normal crop like winter wheat or sugar beet (MacPherson 1995).
Even so, at the equivalent of conventional arable requirements there still remains
a large irrigation need and it is obvious that for locations with soils of poor
water-holding capacity, this could form a major constraint, however well suited
they might otherwise be for biomass production.
Integrating biowaste products
The potential for nutrient and humus recycling from biowaste back into the soil,
via composted, digested or otherwise biologically treated material was mentioned
in Chapter 8. Without digressing into detailed examination of the general options
open for the utilisation of such soil amendments, they do have water-holding
applications and form another example of the natural potential for environmental
biotechnologies to self-integrate.
Much of the evidence for this has come from the field, with research conducted
throughout the UK highlighting the major water-holding benefits to be gained by
large-scale use of biowaste compost. It has been shown that at an application rate
of around 250 tonnes of composted material per hectare, the land is able to hold
between 1000 and 2500 tonnes of rainwater (Butterworth 1999). Perhaps the most
significant evidence in this respect comes from the trials of large-scale compost
treatment in the loose, sandy soils of East Anglia, which seem to suggest that
this would allow SRC crops to be grown without any further watering in all but
the most exceptional of years (Butterworth 1999). According to the same study,
even under such circumstances, the additional irrigation required would be very
greatly reduced. The same work established that relatively immature composts
are particularly effective in this respect, as they can absorb and retain between
two and 10 times their own weight of water. The situation appears similar for
dewatered AD digestate, when applied to soil and permitted to maturein situ.
Digestate sludges are often aerobically stabilised in a process sometimes rather
inaccurately termed ‘secondary composting’; this approach simply extends the
same idea. The end result of this process is a high humus material, with good
microbiological activity and excellent water-retaining properties, which appears
to match the performance of ‘true’ composts at similar application levels. More-
over, it would also seem that biologically derived soil amendment materials like