| dc.description.abstract | A reconnaissance soil survey was carried out of
sample strip about 2km wide and running 14km west from
the Nguu Volcano. The main objectives of the study
were to find out the effect of the Nguu pyroclastics on
the soils and to classify the soils.
The thickness of the pyroclastic surface soils and
their dominant particle sizes decrease away from the
Nguu Volcano (source of the pyroclastics). On the
volcano, there are bombs and blocks (>2.00mm diameter)
which constitute a coarse particle zone. Here the entire
soil profiles are developed from the pyroclastics per se.
Between 2-10km west of the Nguu Volcano, is a sand
(2.0-0.2mm) sized particle zone where the thickness of
surface soils decreases gradually westwards from 65cm to
46cm. In this sand zone the soil profiles show surface
soils, with additions of the Nguu pyroclastics and the
paleosols being developed from the gneisses or basanites.
West of the sand zone is a silt one «O.2mm) which
extends from lOkm to the end of the project area (14km).
In the silt zone, the depth of surface soils increases
from 46cm to 60cm corresponding to rising altitude and
overlie the basanite paleosols.
Using the FAO-UNESCO 'Legend' eight soil classification
units were established which are found on four
physiographic forms. On the Nguu Hill are found the
Vitric Andosols unit (Tv~6D/H/V), and Ochric Andosols
(To-SC/U/V). These Andosols are characterised by tex~
tural breaks which mark the different layers of
volcanic pyroclastics from which the soils were developed.
Further, the soils are dark co loured (dark
brown) reflecting the dark vitreous pyroclastic parent
materials. They also contain the pyroclastic index
min~rals (volcanic glass, olivine, augite and allophane)
which shows that they are developed on the pyroclastics
per see Compared with other soils in the area, the
.andosols (Tv- and To-) have abundant reserve of plant
nutrients as reflected by; weatherable minerals (>20%),
total bases (13-46 me/lOOg soil) and CEC (20-45 me/lOOg
soil). But their agricultural use would be hindered
by the steep slopes, stony-bouldery surface and pockets
Of very shallow soils.
From the foot of the Nguu Hill to the end of
the project area are the Orthic Ferralsols with units
(Fo-3BC/U/U) on uplands and (Fo-SA/P/B) on plains.
These Ferralsols are characterised by a layer of
imrface soils having additions of fine «2.0mm) pyroclastics
overlying the gneissic paleosols (Fo-3BC) and
basanitic paleosols (Fo~SA). The boundary between the
surface soils and paleosols is marked by 'textural
breaks' as indicated by; the sandy loam to sandy clay
loam textures of the surface soils and clay loams of
the paleosols. The distinction is also shown by darker
(dark brown) surface soils compared with the respective
paleosols that are strong brown. Further, the surface
soils of Ferralsols (Fo-3BC and Fo-SA) contain volcanic
glass, olivine, augite and allophane - the index
minerals of pyroclastics. On the other hand, the paleosols
of unit (Fo-3BC) have muscovite, microcline and
hornblende minerals indicative of the metamorphic
gneisses. But, the paleosols of (Fo-SA) contain augite
and olivine which are constituent minerals of pyroclastics
as well as basanites. However, unlike the pyroclastic
surface soils the basanite paleosols of unit
(Fo-SA) lack volcanic glass or allophane (index minerals
of the pyroclastics). The nutrient reserve in the surface
soils (weatherable minerals >10%, total bases 7.0
me/100g soil and CEC 9.0 me/100g soil) is slightly
higher than in the paleosols (weatherable minerals <10%,
total bases 4.0 me/100g soil and CEC 7 me/100g soil).
These Ferralsols which are on gentle slopes, are very
deep and have surface fertility, and are therefore suitable
for cultivation of adaptable crops.
A valley association of soils occurs along the
intermittent Minor Valleys in the area. The Chromic
Luvisols (Lc-3BC/U/F) and Orthic Luvisols (Lo-3B/U/F)
are on the Valley sides while Pellic Vertisols
(Vp-1AB/V/X) and Dystric Planosols (Wd-6AB/V/F) occur
in the Valley bottoms. The valley sides are affected
by severe soil erosion therefore conservation measures
are required. The valley bottom soils (Vertisols and
Planoso:s) are imperfectly drained and have compact
B-horizons. If the valley bottoms were to be cultivated,
artificial drainage and deep tillage would be
nec:essary.
Thus the Andosols (Tv~ and To-) though with high
basic fertility, their cultivation would be hindered
by steep slopes of the volcanic cone~oarse pyroby
clastics on the surface and in places shallowness. On
the other hand, the Ferralsols (Fo-3BC and Fo-SA) would
occur
be suitable for cultivation since they on gentle
slopes, have moderate fertility in the surface soils
and are--very-deep | en |
