SG (Slot)

TILLAGE AND CROPPING SYSTEMS EFFECTS ON CROP YIELDS ON A
SOUTHERN GUINEA SAVANNA SOIL AT ILORIN, NIGERIA
R. Lal
Department of Agronomy, The Ohio State University
Columbus, Ohio 43210 USA
INTRODUCTION
The savanna region in West Africa, with rainfall of 1000
to 1200 mm per annum but concentrated in five to six months
followed by a long dry season of six to seven months, is
naturally suited for cultivation of grain crop annuals. The
forest zone in the south, with rainfall of 1500 to 3000 mm
received in a monomodal or bimodal pattern, is suited either
for tree or root crop production. Rainfed agriculture in the
semi-arid and Sahel regions of the north is prone to severe
drought. The middle savanna belt has a potential to intensify
agriculture and enhance food production. The research data,
however, on appropriate methods of developing land, tillage
and seedbed preparation, and appropriate cropping systems and
crop combinations are not available for major soils and ecoregions.
Despite the high potential to enhance production of food
crop annuals, soils of the savanna regions are extremely
vulnerable to degradation by several physical and chemical
processes. Important physical processes of soil degradation
are compaction, crusting, hard-setting, and accelerated runoff
and erosion. These soils have low organic matter content,
coarse texture, and clay fraction of low activity, e.g. low
CEC. consequently, crusting and soil compaction are severe
problems (Adeoye, 1982; 1985; Ike, 1988). Ley et al. (1989)
reported that these soils are also structurally weak, and
prone to hard setting. The hard setting behaviour is
aggravated by rapid decline in soil organic matter content,
raindrop impact on exposed soil, reduction in biotic activity
of soil fauna during the cultivation phase, and ultradesiccation
that occurs during the prolonged dry season.
Because of weak structural attributes, these soils have
relatively high erodibility (Van Elslande et al., 1987).
Above all, high erosivity of rains is an important factor to
be considered in land use planning. Localized rainfall
intensities exceeding 100 mm h- 1 sustained for 10 to 15
minutes are frequently observed (Kowal and kassam, 1976).
Consequently soil erosion is a severe problem in the region
especially at the onset of rains. The problem of soil erosion
is further accentuated by the shallow depth of topsoil. A
large proportion of soils developed on the mid-slope region
along the catena are underlain by hardened plinthite at 15 to
30 em depth.
In view
of several soil-related constraints to crop
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