Chemistry Reference
In-Depth Information
d
n
1
r
2
n
g
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Figure 3
Binary images of vertical slices of soil (pixel size 107 mm, solid in black):
initial seedbed, bottom slow wetting (BS), top fast wetting then immersion
(TFI),
top
slow
wetting
(TS)
and
rainfall
simulation
(R).
(Source:
Bresson).
2
cultivation.
65,66
It has been known for decades that the slaking and coalescence
of soil structure increases if bonding by organic compounds is diminished.
67
2.3.1 Extent of Seedbed Damage
Hardsetting of soils poses a major risk to crop production in the sub-humid to
semi-arid tropics and Mediterranean regions where it is prominent. It affects
more than 110 Mha of agricultural land. Affected soils, once cultivated,
disperse under rainfall and then dry into dense, structureless layers that are
difficult for water or plant roots to penetrate.
68
Many studies suggest that a
shift to reduced tillage systems can decrease hardsetting over time, but the
longer-term impacts are not known.
69
Surface crusting and sealing affects all regions of the globe, but is most
prominent in Africa (18 Mha) and Asia (10 Mha).
35
Declines in organic matter
caused by intensive cultivation and trampling by livestock are the primary
causes.
Seedbed slumping occurs to some extent in all soils following tillage.
70
It is
very poorly characterised and only appreciated when associated with water-
logging, which affects about 11 Mha worldwide.
35
The maintenance of the
physical structure of seedbeds, however, is vital to crop productivity and offers
