Effectiveness and performance of indigenous soil and water conservation measures in the west Usambara mountains, Tanzania

Abstract

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.