SG (Slot)

latter two treatments were expected to provide extremes, thus
enhancing the range of the treatments.
Conventional inversion tillage consists of ploughing with
a single-furrow ox-drawn mouldboard plough. The mouldboard
plough along with maize monocropping was launched in the
1920's in a major extension drive by the colonial authorities
to replace shifting cultivation (Page & Page, 1990). Today it
is the most widely used cultivation technique in the Communal
Areas of Zimbabwe, estimated to be practiced on 73-90 % of
the cultivated area (Working Document, 1990). The inclusion
of this practice provides a baseline for assessing the merits
of the conservation tillage treatments.
Clean ripping involves inter-row-ripping into bare ground
using a ripper tine attached to the ox-drawn plough frame.
Planting is carried out into the ripper tine line. This nonturning
but only soil-loosening tillage technique has been
promoted by the Department of Agricultural, Technical and
Extension Services (Agritex) for its potential in improving
yields but high soil losses and runoff are anticipated from
the unmulched condition. Because of the advantages it offers
in low animal draft, timeliness of operations and potentially
higher yields, it is important to assess whether this
practice is sustainable.
Mulch ripping into crop residues is one of the two basic
conservation tillage treatments, (the other is tied ridging),
currently being promoted by the IAE. Soil loss and runoff are
expected to be far }Olver than from clean ripping, but there
are no local data available to prove either its
sustainability or acceptability. Although returning crop
residues to sandveld soils is an important aspect of
improving rock-bottom fertility, crop residues for mulching
are scarce in most Communal Areas, since it is normally
grazed on-field after harvest or kraal-fed during the dry
season. However, the method could offer advantages on smallscale
commercial farms and in Communal Areas where cattle
numbers are reducing (scarcity of grazing land) or absent
(tsetse fly areas).
No-till tied ridging combines the conservation and yield
advantages of the conventional tied ridge system (Prestt,
1986; Hulugalle, 1990) with savings in draft power offered by
no-till methods. During the first year, the land is ploughed
to the recommended depth of 230 mm, cross-slope crop ridges
of not less than 250 mm unconsolidated height are then
constructed with an ox-ridger plough or with a single-furrow
mouldboard plough (Elwell & Norton, 1988). During the second
and subsequent years the land is not ploughed, the ridges
merely being maintained. Planting is carried out into the
crest of the ridge once the ridge is moist throughout. Early
weeding is hoped to be achieved entirely through re-ridging.
Moisture control is achieved by constructing the ridges at
gradients (1 in 250 grade minimum and 1% maximum) with lower
crossties (1/2 to 2/3 of the height of the ridge) constructed
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