Biomedical Engineering Reference
In-Depth Information
these, applied appropriately to soils either as a surface mulch or ploughed-in, can
not only lower supplementary watering demands enormously but also largely
offset any tendency to drought stress in the growing biomass. In addition, the
leaching of nitrate from the soil is also lessened significantly.
As an aside, it is interesting to note that this ability to retain large amounts
of water, together with its naturally high organic content has led to the use of
compost in the construction of artificial wetlands. The USA has been particularly
active in this area, in part due to the fact that federal environmental regulations
encourage the creation of this type of habitat as a means of water treatment.
This approach, which has been discussed more fully in an earlier chapter, has
as its main goal the manufacture of a wetland which behaves like a natural
system in terms of both its hydrology and biology. To achieve this, a humus
rich, biologically active medium, which closely replicates the normal physical
and chemical properties of local soils is required. Biowaste-derived composts
have been found to contribute well as constituents of manufactured wetland soils,
often allowing vegetation to become established on such sites more quickly than
usual (Alexander 1999).
Nutrient requirements
To return to the issue of minerals, one of the chief potential bulk end uses of
biowaste-derived compost is as a horticultural amendment and fertiliser replace-
ment. There is no clear consensus between those working in the field as to how
much nutrient is removed from the system when SRC wood is harvested, esti-
mates for nitrogen loss ranging between 30 kg and 150 kg per hectare. A study
by the UK's Forestry Commission produced figures of 135 kg per hectare for
nitrogen and 16 kg of phosphate, which is around one-fifth the demands made
by a cereal crop. On this basis, it seems unlikely that nutrient removal would
be a limit on fertile sites and certainly not for the first few harvest cycles. In
the case of soils with naturally low fertility, or those which have been used
for coppice cropping for some years, supplementary mineral input may well be
required. Clearly, if biowaste-derived material is used for its water-holding prop-
erties, the concomitant humus and mineral donation would represent what might
be described as a gratuitous benefit. Process integration in this fashion brings
evident economic advantages to any commercial coppicing operation.
There is another way in which composts can help SRC. Direct competition from
other plants is one of the largest factors in poor coppice crop growth and may even
lead to outright failure in some cases. Uncontrolled grass or weed growth around
the trees in their first season can reduce the dry matter yield by a fifth and halve
their overall growth. Even after they have become properly established, weed
control remains an important part of optimising the energy crop's performance,
particularly where a soil's intrinsic water-holding and/or nutrient levels are less
than ideal. Heavy mulching has been used very successfully in many operations
and, as is obvious from the previous discussions, biowaste soil amendments are
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