Chemistry Reference
In-Depth Information
easier incorporation of nutrients; (vi) relief from soil compaction; and (vii)
control of some soil-borne diseases and insects.
111
However, by creating a bare
soil surface, such tillage also has the undesirable consequences of substantially
increasing rates of water and wind erosion and of the oxidation of soil carbon.
In many regions of the world there has been a move away from such
inversion tillage towards more minimal soil disturbance - a move that has been
assisted by the development of herbicides to control weeds. Conservation
tillage is a wide-ranging term used to describe the many practices that have
evolved, initially to retain at least 30% surface coverage by crop residues, but
to varying extents to also conserve time, fuel, earthworms, soil water, soil
structure and nutrients.
112
Several studies have shown that minimum tillage
coupled with residue retention increased infiltration and profile moisture
storage (e.g. in Kenya),
113
reduced soil erosion by water by 10- to 100-fold
because of greater aggregate stability and reduced runoff,
114,115
d
n
1
r
2
n
g
|
1
and increased
soil organic matter content.
116
Conservation tillage has been widely adopted in the USA, Brazil, Argentina,
Canada and Australia and now covers over 100 Mha of land worldwide. This
tillage and other practices form the basis of ''conservation agriculture'' which
combines three principles of:
117
N
d
n
6
h
3
reduced tillage - ideally zero tillage, but may involve controlled tillage
during seeding to disturb no more than 20-25 % of the soil surface;
N
retention of crop residues and surface cover - variable rates of retention to
reduce erosion and runoff and enhance soil properties associated with long-
term production; and
N
use of crop rotations - moderation of weed, disease and pest problems
coupled
with
utilisation
of
beneficial
effects
of
some
crops
on
soil
conditions.
These principles are applicable to a wide range of production systems from
low-yielding, rain-fed conditions to high-yielding, irrigated conditions. In
practice, though, the techniques used will vary with the specific situation,
depending
on
farmer
circumstances
and
biophysical
and
management
conditions.
4.3 Improving Root Systems
There is much interest in exploiting genotypic differences in the uptake of
nutrients (especially of N and P) and, particularly, in improving the efficiency
with which resources are used.
118
The root factors contributing to P-uptake
efficiency have been summarised as:
119
(i)
root geometry - differences in root length and its distribution in soils,
root-hair length and density, root diameter, etc.;
(ii)
mycorrhizal effects - differences in the extent or rate of infection, or
species of mycorrhizal fungus; and
