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The West Usambara Mountains in Tanzania are severely affected by soil degradation
caused by water erosion that includes rill, interill, gully and landslides. To a large
extent the area is also affected by soil degradation that is caused by declining soil
fertility and harvesting of tuber, bulb and root crops. The problem of soil degradation
in the area has triggered adverse effect on crop productivity and is a serious threat to
livelihood. Many scientific „Soil and Water Conservation‟ (SWC) measures such as
bench terraces, Fanya Juu terraces, cut off drains, contour strips and agroforestry
have been promoted in the area to combat the escalating problem of soil degradation.
However, these technologies were rejected or minimally adopted because most of
them were laborious and expensive. In the West Usambara Mountains, farmers have
their own local SWC measures such as miraba (rectangular grass bound strips that do
not necessarily follow contour lines), micro ridges and stone bunds, technologies
which unfortunately have received very little considerations. Miraba is the most
preferred and widely practised indigenous SWC measure in the West Usambara
Mountains because it is cheaper in implementation and provides fodder for livestock.
Despite all the efforts in combating soil degradation, little success has been achieved
as the process has been active even in places where SWC measures are practised.
The general objective of the current study was to enhance knowledge on indigenous
SWC measures under smallholder farming conditions for preventing soil degradation
and improving crop yields in the West Usambara Mountains. Specifically the study
aimed to i) evaluate potentials and constraints of indigenous SWC technologies for
minimizing soil degradation and enhancing crop yields in various landscape types in
farmers‟ fields ii) determine the effectiveness and performance of selectediii
indigenous SWC measures for improved crop yields and iii) investigate the mass of
soil and nutrient losses due to crop harvesting under different indigenous SWC
measures. The study was conducted in Majulai and Migambo villages in Lushoto
District, Tanzania. The studied villages belong to two major contrasting agro-
ecological zones of the West Usambara Mountains. The former village belongs to
dry warm and the latter to the humid cold agro-ecological zone.
Participatory Rural Appraisal (PRA) coupled with soil fertility and crop yield
surveys under various SWC technologies in farmers‟ fields were conducted. Low soil
fertility and spatial variability of soil fertility were revealed as major constraints to
high crop yields under miraba. Thus, miraba were integrated with mulching and
spacing of grass strips adjusted to rectify the observed constraints such that i) the
spacing of grass strips that form miraba across the slope was reduced from
traditionally very wide (10 m - 30 m apart depending on the size of the farm plots) to
5 m apart to mimic the recommended maximum width of hand made bench terraces;
and ii) mulching applications using leaves of readily available plants namely
Tithonia (Tithonia diversifolia) and Tughutu (Vernonia myriantha). These plants are
also reported to have appreciable contents of N, P and K.
The effectiveness and performance of miraba that were adjusted to 5 m and with
above-mentioned mulching materials were tested in runoff experiments that were set
in Majulai and Migambo villages, in which climatic data were also collected using
standard rain gauges and tipping bucket rain gauges. Furthermore, root properties of
Guatemala grass (Tripsacum andersonii), Napier grass (Pennisetum purpureum) and
Tithonia shrub (Tithonia diversifolia) were investigated in farmers‟ fields and theiriv
erosion-reducing effects predicted. Since these plants are used for establishing
miraba and stabilizing the edge of bench terraces, it was deemed rational to
investigate the erosion-reducing potential of their roots since during drought or fire
outbreak, the above biomass disappears but roots remain and these could
significantly contribute to reduction of soil runoff. A survey was also conducted in
farmers‟ fields to investigate the magnitude of soil and nutrient losses resulting from
harvesting of root, tuber and bulb crops under miraba. The aim was to determine the
magnitude and effect of this process on soil degradation and extent to which it may
contribute to frustrate soil conservation efforts in the area.
The results of the current study showed that: at 5 m spacing of miraba grass strips,
there was formation of progressive bench terraces which significantly demonstrated
their effectiveness in controlling soil erosion in the West Usambara Mountains.
Formation of progressive bench terraces as a result of miraba implementation is by
far cheaper than mechanical construction of bench terraces which is not favoured by
farmers due to the labour costs that are involved. The roots of Guatemala grass had
higher (p < 0.05) potential to reduce soil erosion rates by concentrated flow than
Napier grass and Tithonia shrub in the 0-40 cm soil depth. These findings have
implications on the selection and use of appropriate plants for soil erosion control.
Soil loss was significantly (p < 0.05) higher in cropland with no SWC measure than
under miraba with mulching (e.g. 184 Mg ha -1 yr -1 vs. 8 Mg ha -1 yr -1 ). The annual
nutrient losses (kg ha -1 yr -1 ) were higher (p < 0.001) in croplands with no SWC
measures (e.g. 307, 0.8, 14 NPK) than under miraba with mulching (37, 0.1, 4.0
NPK respectively). Soil fertility was higher (p < 0.05) under miraba with Tughutu
than under miraba with Tithonia and miraba sole. Similarly, maize and bean yieldsv
(Mg ha -1 ) followed the same trend e.g. 3.8 vs. 1.6 for maize and 1.0 vs. 0.6 for beans
under miraba with Tughutu mulching vs. cropland with no SWC measures
respectively. The crop yields did not vary between segments under miraba or miraba
with mulching, whereas, under cropland with no SWC measures, maize yields
differed significantly (p < 0.05) with lower position segments having higher yields
than the upper position segments. Climatic conditions had an influence on the
effectiveness and performance of miraba such that miraba were found more effective
in Migambo village which is humid than in Majulai village which has drier climate.
During dry spells, Napier grasses forming miraba were found to die out and
rejuvenate during the rainy seasons, hence the formed Napier grass strips become
weaker and less effective. On the other hand, soil loss due to crop harvesting (SLCH)
under miraba was significantly (p < 0.05) higher for carrot 7.1 than 3.8 for onion and
0.7 Mg ha -1 harvest -1 in the case of potato harvesting. Soil nutrient losses in kg ha -1
harvest -1 were higher (p < 0.05) for carrot than for onion and potato harvesting. Soil
water content at harvest time played a significant role at 5 % level in inducing SLCH
for onion crop. Bulk density and soil texture played only a minor role to SLCH of the
studied crops. These observations imply that soil degradation due to crop harvesting
under miraba is substantial and poses a challenge that calls for immediate attention
on the harvesting practices.
Based on the findings, it is concluded that i) low soil fertility and spatial variability
of soil fertility and crop performance under traditional miraba and micro ridges are
the major constraints to high crop yields in smallholder farmlands of the West
Usambara Mountains. ii) soils of the West Usambara Mountains are susceptible to
erosion as indicated by their very low values of K factors and very high rates of soilvi
degradation by water erosion. iii) roots of Guatemala grass are more effective in
reducing concentrated flow erosion rates in the 0 - 40 cm soil depth than the roots of
Napier grass while the roots of Tithonia shrub are the least effective. Thus selection
of plants with effective rooting characteristics for controlling concentrated flow
erosion is important. iv) improved miraba are effective in reducing runoff, soil and
nutrient losses, but, improved miraba with either Tithonia or Tughutu mulching were
more effective. v) Tughutu mulches had higher potential in soil fertility restoration
than Tithonia mulches; and thus improved miraba with Tughutu mulching was the
best SWC measure for improving crop yields. vi) although miraba and miraba with
mulching were effective in reducing soil and nutrient losses, significant rates of soil
and nutrient losses under miraba that were revealed due to harvesting of root, tuber
and bulb crops could frustrate the success of soil conservation efforts that have been
achieved.
The following recommendations are made: i) due to the vulnerability of the West
Usambara Mountains to soil degradation, it is recommended not to cultivate in these
areas without the use of appropriate SWC measures. ii) in dry areas such as Majulai
village drought resistant grasses such as Guatemala should be used for establishing
miraba because Napier grasses mostly preferred are sensitive to drought, thus
leading to reduced effectiveness of miraba. iii) the spacing of miraba grass strips
across the slope is recommended at 5 m apart for effectively controlling spatial
variability of soil fertility and crop yields and for allowing miraba to form
progressive bench terraces that are effective in controlling soil erosion in the West
Usambara Mountains. iv) the use of Tughutu shrub should be strongly promoted for
use as mulching materials under miraba as the shrub has demonstrated itsvii
effectiveness in controlling soil erosion and at the same time improving soil fertility
and crop yields. v) furthermore, Tughutu shrubs should be planted along the borders
of farm plots so that the plants can easily be available for use as mulching materials.
vi) farmers should remove as much as possible soil stuck on the harvested crops at
their farm plots to avoid losses of soil and nutrients from farm lands. vii) further
studies should be carried out on the scaling up of the application of improved miraba
in other areas not only in the West Usambara Mountains but also in other areas of the
country with similar socio-economic and environmental conditions for reduced soil
degradation and improved crop productivity. viii) the potentials of the studied
mulching materials should be tested for the productivity of vegetables such as
cabbage, tomatoes, onions and carrots which are widely cultivated in the West
Usambara Mountains. ix) further research should be carried out to investigate the
effectiveness of the studied soil conservation practices on watershed protection to
mitigate river stream sedimentation. x) more studies should be carried out to
investigate SLCH for other crops in different climatic conditions and soil types tovalidate further this process under low input farming. |
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