Limited information is available on the response of local cactus pear cultivars to environmental factors that influence fruit yield. Eleven cultivars were evaluated in three diverse agro-climatic areas over seven production seasons in the Limpopo Province to assess their environmental adaptability. Significant differences between cultivars, areas and production years for five fruit yield components were evident. A strong genotype by environment interaction was observed, although some cultivar characteristics were genetically controlled. The most suitable production area is the cool mid-altitude area of Limpopo Province. Cultivars that can be recommended for fresh fruit production are: âAlgerianâ, âAmerican Giantâ, âGymno Carpoâ, âMaltaâ, âMoradoâ, âNudosaâ and âZastronâ. Fruit yield was significantly influenced by minimum temperature and plant macro nutrients. Soil phosphorus levels above 20 mg kg-1 and applied nitrogen higher than 100 kg ha-1 year-1 had a positive effect on fruit yield. Soil pH did not influence the fruit yield of the cultivars tested. None of the cultivars tested had a winter chilling requirement to become fertile. Vegetative growth was stimulated by increased solar radiation. Cactus pear plants can be considered to be fully mature from the fifth year onwards. Environmental adaptability is related to species differences rather than plant morphological differences. Plant growth habit changed markedly in different environments. To obtain high fruit yields, it is important to match a cultivar with prevailing environmental conditions of the area. Fruit yield in cactus pear is a function of the number of fertile cladodes, the number of fruit set, the number of fruit left after thinning and individual fruit mass. Research into orchard practices, in particular pruning, and evaluation of the existing cactus pear germplasm should receive attention. As a ânewâ cultivated fruit crop it offers real solutions towards mitigation of the effects of drought in arid and semi-arid parts of Limpopo Province.

Downy Mildew (Plasmopara viticola) is known as one of the most important vineyard diseases in the Western Cape, because it has the capability to develop and spread very fast, and so cause large crop losses in certain years. In 1992 an Austrian researcher developed the Metos automatic weather station and associated software, to predict the occurrence of primary and secondary infection of downy mildew. This Metos weather stationâs software was adapted for South African climatic conditions during 1995 and is known as the Metos-2 model. The Metos-2 model however had some shortcomings that needed to be improved. The most important of this was that the model was not sensitive enough to accurately calculate infections, and furthermore it gives only a âYes/Noâ warning of possible primary and/or secondary infections. The Metos-2 model makes use of measured leaf wetness values from a leaf wetness sensor that is probably considered as one of the most inaccurate meteorological sensors. During 1995 - 2005 the Metos-2 model has been thoroughly tested and used by the disease management division of ARC Infruitec-Nietvoorbij, to warn the industry of possible downy mildew outbreaks. Results over these years have shown that more sprays were needed within the preventative spraying programs, as opposed to recommendations of the Metos-2 model, for the same or even improved control of downy mildew. On the other hand the results of the Metos-2 model compared to the Metos model, gave similar warnings for both primary and secondary infections. It is however very difficult to get clear similarities/differences between what the Metos-2 model has calculated and what had really occurred in the vineyards. This can be attributed mainly to the accumulation effect of downy mildew infections. With the development of the Downy Mildew Early Warning Model (DSVWmodel), two important changes were made, namely the leaf wetness was replaced with a mathematical, non-linear regression and the Metos-2 modelâs âYes/Noâ warnings for downy mildew infections were replaced with four classes of possible risks. The calculated leaf wetness of the DSVW-model, that uses measured relative humidity and air temperature as input values, had a significant coefficient of determination of 0.70, compared with measured leaf wetness. The DSVW-modelâs four risk classes of possible infections (primary and secondary) are as follows: zero infection (0 %), low infection (1 - 34 %), medium (35 - 74 %) and a high risk class (75 - 100 %). To test the DSVWmodelâs accuracy and reliability, historical weather data (1998 - 2003) and measured disease outbreak data in the Stellenbosch, Robertson and Paarl areas were used to run both the Metos-2 and the DSVW-models. Primary as well as secondary infections were predicted by the models. When the DSVW-model and the Metos-2 modelâs infection warnings were correlated with disease outbreaks, of the two, the DSVW-model showed consistently similar or better correlations with the measured disease outbreak data. The DSVW-model also calculated on a regular basis more primary and secondary infections, than the Metos-2 model, which at times did not warn of any downy mildew infections, although outbreaks of downy mildew did occur soon after. Producers can use the new DSVW-model with confidence, together with one or other prevention spray program, for the control of downy mildew.

Large areas of cultivated land in Ethiopia frequently suffer from drought, causing low crop yields and food insecurity. It was hypothesized that it may be possible to alleviate this problem by employing infield rain water harvesting (IRWH). Three representative semi-arid ecotopes in the Rift Valley were selected to test this hypothesis. They were the Melkassa Hypo Calcic Regosol, The Dera Calcic Fluvic Regosol and the Mieso Hypo Calcic Vertisol. The climate, topography and soils of the ecotopes were characterized in detail. Rainfall runoff studies were carried out over two rain seasons on replicated plots on these ecotopes comparing two soil surface treatments. They were conventional tillage (CT), simulating the initially fairly rough surface which results after normal tillage; and no tillage (NT), simulating the flat crusted surface expected on the runoff strip of the IRWH system. Rainfall amounts, rainfall intensity at one minute intervals, and runoff, were measured for each storm during the two rain seasons on each ecotope. The results were used to calibrate and validate the Morin & Cluff runoff model in order to enhance the extrapolation capability of the study results to other similar ecotopes. The study yielded the following useful results. · The rainfall pattern on all the ecotopes was characterized by occasional storms with fairly high amounts and high intensities (Pi) which greatly exceeded the final infiltration rates of the soil, causing a high proportion of the rain (P) to runoff (R), i.e. producing a high R/P ratio. For the NT treatment final overall R/P values for the two seasons on the Melkassa, Dera and Mieso ecotopes were 0.45, 0.52 and 0.32, respectively. These high values indicate that IRWH should produce a significant increase in yield due to its ability to reduce R to zero while concentrating the runoff in the basin area and increasing the water available for transpiration and therefore increasing yield. · Because of the textural and mineralogical properties of the topsoils, particularly the two Regosols soils; they disperse and form crusts easily when impacted by high intensity rain. The result was that after cultivation at the start of the rain season the surface of the CT treatment soon became very similar to that of the NT treatment. Accordingly no significant difference was found between the runoff from the NT and CT plots on the Melkassa Regosol and Dera Regosol. There was, however, a significant difference in this respect on the Mieso Vertisol with a more stable surface. · Runoff prediction in all the ecotopes were well done by the M & C model. · Two separate strategies were developed to estimate the maize yield increase that could be expected on the Melkassa Regosol by employing IRWH. From the nearby Melkassa Research Station it was possible to obtain maize yields for 15 seasons (1989-2003). These were used together with climate data, the CROPWAT model, and the runoff measurements, to estimate the benefit of IRWH. The two strategies produced yield increase estimates of 33% and 40% compared to CT.

No abstract available.

Very little is known about the response of Lachenalia to fertilisation when cultivated in soil. The objective of this study was therefore to quantify the effect of fertilisation on the growth, yield and quality of Lachenalia in both the nursery and pot plant phases. In order to achieve this two pot trials for the nursery phase and one pot trial for the pot plant phase were conducted in the glasshouse. For the first trial in the nursery phase the combined effect of nine nitrogen levels and three application times on the Lachenalia cultivars, Rupert and Ronina, were studied. The nitrogen was applied at levels equivalent to 0, 30, 70, 120, 180, 250, 330, 420 or 520 kg N ha -1 . Three different nitrogen application times were used namely: one third with planting and the rest 10 weeks after planting (T1); one third with planting and the other two thirds after planting (T 2) or one quarter with planting and the other three quarters after planting (T3). The leaf area of Ronina plants was larger than that of Rupert plants irrespective of nitrogen levels and application times. However, Ronina bulbs were larger and softer than Rupert bulbs. The nutrient (N, P, Ca and Mg) and carbohydrate (D-glucose, sucrose and starch) content of Rupert bulbs were higher than that of Ronina bulbs. Application of nitrogen had a positive influence on the leaf area, bulb fresh mass and circumference of both cultivars. Bulb firmness was negatively influenced by nitrogen application. The best results for most parameters were obtained when nitrogen was applied in four equal applications. In the second trial for the nursery phase the response of Rupert and Ronina to five nitrogen (0, 70, 180, 330 or 520 kg N ha -1 ) and five phosphorus (0, 10, 30, 50 or 80 kg N ha -1 ) or five potassium (0, 70, 180, 330 or 520 kg N ha -1 ) levels were studied. Neither the interaction between nitrogen and phosphorus levels nor the interaction between nitrogen and potassium levels had a large influenced on the growth and development of Lachenalia. Results obta ined in this trial with respect to the effect of nitrogen levels on the different parameters mainly confirm with the results obtained with the first trial. In the trial for the pot plant phase the effect of seven nitrogen levels (0, 30, 70, 120, 180, 250, 330, 420 or 520 kg N ha -1 ) on Lachenalia grown from 7-8 cm bulbs, whereof the fertilisation history in the nursery phase differed, was investigated. The fertilisation history of the bulbs in the nursery phase consisted of three nitrogen levels (0, 70, 250 or 520 kg N ha -1 ) combined with two nitrogen application times (T1, T2 or T3 as described earlier). The leaf area of Ronina plants was larger than that of Rupert plants. Nitrogen applied in the nursery phase promoted the leaf area of both Rupert and Ronina. Application of nitrogen in the nursery phase and in the pot plant phase increased the number of inflorescence per plant and the number of florets per inflorescence. The peduncle length increased with higher nitrogen levels in the nursery phase wherea s the peduncle diameter increased with higher nitrogen levels in the pot plant phase.

Seasonal rainfall is an important source of water for rainfed farming in the semi-arid regions of the world, where rainfall is marginal and variable. However, as rains are unpredictable in terms of onset, amount and distribution, there is a need to understand the variability and other basic rainfall features in order to use the information in agricultural decision making. More specifically, combining the seasonal rainfall prediction with crop water requirement and soil water information is the core component to successful agriculture. The ultimate objective of this study was to characterize and obtain a better understanding of the most important rainfall features that form the basis for classifying the areas into homogenous rainfall zones and then to develop a seasonal rainfall prediction model for the Central Rift Valley (CRV) of Ethiopia. The source data for the analyses was primarily obtained from the National Meteorological Services Agency (NMSA) and partly from Melkassa Agricultural Research Centre (MARC) and the web site of the International Research Institute for Climate and Society (IRI). Rainfall variability and time series analyses were done using INSTAT 2.51 and coded time method, respectively. Rainfall onset and March-April- May (MAM) rainfall totals are the two most variable features both at Miesso and Abomssa. For both stations, rainfall end date displays the least variability. Rainfall onset date at Miesso ranges from the lower quartile (25 percentile) of DOY 61 to the upper quartile (75 percentile) of DOY 179 with a 42% coefficient of variation (cv). At Miesso, the main rainy season terminates during the last days of September (DOY 272 - 274) once in four years and terminates before DOY 293 in three out of four years. At Abomssa, the c.v for the lower quartile (DOY 61) to the upper quartile (DOY 134) was found to be 40.5%. At both locations, planting earlier than 15 March (DOY 75) only proves successful once in every four years. Further, at Miesso this upper quartile statistic can extend up to the DOY 179, whereas at Abomssa planting earlier than 15 April (DOY 134) is possible in three out of four years (75 percentile). At Abomssa, rainfall terminates by DOY 286 and the end of October (DOY 305) for the 25 and 75 percentile points respectively. From the time series analyses, there was no conclusive evidence for the existence of a trend for both Miesso and Abomssa, information which is useful for long-term research and development planning, as well as seasonal rainfall prediction for the study area. The classification study for the spatial rainfall pattern resulted in four homogenous rainfall zones that form distinct development and research units, using the FORTRAN- 90 based NAVORS2 program. The south facing Alem Tena-Langano zone has a better rainfall pattern than drier zones and thus formed zone 1. The southern, southwestern and southeastern area has formed the wet zone (zone 2), the northwestern to northeastern facing part (Debre-Zeit-Nazerth-Dera) that receives a higher rainfall amount than zone 1 has formed zone 3 and finally, the drier northeastern part constituted zone 4. Twenty seven seasonal rainfall prediction models with varied performance skills that can be used for the operational farming were developed for the March-September monthly rainfall using the Climate Predictability Tool (CPT v.4.01) from IRI. It was understood that with increased observing networks and data availability, useful operational climate prediction could be achieved for a smaller spatial unit and with a short lead-time. The tempo-spatial water requirement satisfaction pattern analyses were conducted using AGROMETSHELL v.1.0 of the FAO. Fourteen concurrent sorghum-growing seasons that give a general picture of crop water requirement satisfaction were mapped. The southern, southwestern and southeastern parts (zone 2) of the CRV constitute the most favourable location for growing a range of sorghum maturity groups. The northwestern and central (zone 3) parts constitute the next most suitable zone. The wide northeastern drylands (zone 4) of the study area, except the pocket area of Miesso-Assebot plain, does not warrant economic farming of sorghum under rainfed conditions. From the growth stage-based Water Requirement Satisfaction Index (WRSI) analyses, mid-season / flowering stage of the sorghum cultivars was found to be three times more sensitive to changes in sorghum yields for both cultivars and experimental sites as compared to the WRSI from the rest of growth stages. The results from the water production function analyses (WPF) also indicated the potential of WRSI for prediction of the long-term sorghum yields. The cumulative density function (CDF) and stochastic dominance analyses for the 120-day grain sorghum cultivar grown at Miesso show the June planting to be the most efficient set by first degree stochastic dominance (FSD), while May was found efficient for Melkassa. The CDF for Arsi Negele shows April planting date to be the best set. Therefore, these planting dates are to be preferred by farmers seeking âmoreâ yield at the respective locations, regardless of their attitude towards risk. The sensitivity analyses conducted using different levels of the seasonal rainfall related input variable combinations (sorghum planting date, maturity date, number of rainy days and WRSI) for Miesso, Melkassa and Arsi Negele provide useful information. By keeping input variables other than WRSI at the most preferred level (i.e. early planting date, extended maturity date, and greater number of rainy days) and only changing WRSI from 100% to 75% resulted in a 49.7% yield reduction in case of Miesso, 40.8% in case of Melkassa and 24.3% in case of Arsi Negele. Further, when WRSI was reduced down to 50%, there was a total crop failure in the case of Miesso and Melkassa, while the reduction was 48.6% for the Arsi Negele case. Similar results were found when WRSI was varied across other input level combinations. Visual Basic v.6.0 was used to write the algorithm for the decision support tool (DST) relating sorghum planting dates in CRV, to which the name ABBABOKA 1.0 was given. By using the rainfall prediction information from three different sources (the new prediction model developed in chapter 3, NMSA and ICPAC), ABBABOKA suggests the best possible planting alternatives for a given homogenous rainfall zone and planting season. When decision making under this predictive information alone is not sufficient, soil water parameters need to be consulted for more reliable decision making. This simple and briefly constructed ABBABOKA is expected to provide a suite of guidelines to the users. Certainly, this constitutes a significant departure from the fixed âbest betâ recommendations I learned from research systems in the past. It is recommended that the time-space classification of agricultural areas into homogeneous zones needs to be extended to the rest of the country together with the tailored rainfall prediction information. Research needs to be geared towards crop water requirements, climate risks and simulation modelling aspects. A network of weather stations and soil database needs to be developed in order to promote the soilcrop- climate research in Ethiopian agriculture. More importantly, the use of decision support tools and the well-established models (like APSIM) need to be included in agricultural research and development efforts.

Desertification is one of the most serious environmental and socio-economic problems of our time. Desertification describes circumstances of land degradation in arid, semi-arid and dry sub-humid regions resulting from the climate variation and human activities. The fundamental goal of this thesis was to monitor the extend and severity of the land degradation and examine climate variability and change in the Butana area of northeastern Sudan. To explore the climate variability and climate change in terms of rainfall, temperature and the aridity index for the period from 1941 to 2004, the monthly and annual time series for four weather stations (El Gadaref, Halfa, Wad Medani and Shambat) across the Butana area were analysed. The trend of the rainfall at Wad Medani and Shambat shows significant decline, while that of Halfa and El Gadaref does not show a significant decrease or increase. The Cumulative Rainfall Departure (CRD) was used to detect the periods of abrupt changes in the rainfall series. A significant decrease in the annual rainfall was observed at Shambat (p = 0.00135) and Wad Medani (p = 0.0005) from 1968 to 1987, there after the rainfall amount is close to the long-term mean. In El Gadaref there was a decline in the annual rainfall from 1971 to 1974 (p = 0.35) but it was not significant, with a recovery from 1975 to 1982 to a value higher than the long-term mean, followed by another downward turn from 1983 to 1994. In Halfa there was a significant decrease (p = 0.0304) from 1982 to 1993. The trends of maximum and minimum temperature were examined for the summer (March-May), autumn (June-October) and winter (November-February) seasons for the four weather stations. At Halfa and Shambat the trend of maximum and minimum summer and winter temperature was increasing but not significant, while in Wad Medani there was a significant increase for summer and winter minimum temperatures. El Gadaref station showed a significant increase in maximum and minimum temperature (p = 0.00005, p = 0.00016) respectively. The minimum autumn temperature for Halfa increased significantly, while this was the case for both the minimum and maximum autumn temperature at Shambat and Wad Medani. This significant increase in temperature, associated with autumn, is partly due to dry conditions observed during the late 1960s. The relationship between 8 km2 AVHRR/NDVI and rainfall data (1981-2003) was tested in the Butana area. The relationship was strong between the peak NDVI (end of August through the beginning of September) and cumulative July/August rainfall, but weak relationships resulted when annual rainfall and cumulative NDVI were used. The Departure Average Vegetation method showed that the area had a high percentage of departure, reaching about 40% of the long-term average during the drought years and the NDVI recovered during the following year if the rainfall was above average. There were increased trends in NDVI in the study area during the period from 1992 to 2003, despite some years during this period having higher departure although that departure was less than for the period 1981-1991. To monitor the impact of human activities on land degradation it is essential to remove the effects of rainfall on vegetation cover. Using the Residual Trend Method the differences between the observed peak NDVI and the peak NDVI predicted by the rainfall was calculated for each pixel. This method identified degraded areas that exhibit negative trends in NDVI. The human impact is more clear in the northern part. Satellite imagery provides an opportunity to undertake routine natural resource monitoring for mapping land degradation over a large area such as Butana over a long time period. This facilitates efficient decision making for resource management. Five classes of land use were achieved using unsupervised classification, whereafter an image difference technique was applied for 1987-1996 and 1987-2000. This analysis showed that the bare soil and eroded land increased by 3-7% while the vegetated area decreased by 3-6%. Also when comparing the aerial photographs (1960s and 1980s) for Shareif Baraket, Kamlin and El Maseid with Landsat images (2000) severe degradation of the vegetation cover was visible at all the three sites. The Moving Standard Deviation Index (MSDI) is calculated by performing a 3Ã3 moving standard deviation window across the band 3 Landsat images (1987, 2000). MSDI proved to be a powerful indicator of landscape condition for the study area. The MSDI increased considerably from 1987 to 2000, especially for Sufeiya, Sobagh and Banat areas, which are referred to as severely degraded sites in the literature. The Bare Soil Index (BSI) supports the finding from the MSDI. The BSI for the degraded sites Sufeiya, Sobagh and Banat increased from 0-8 in 1987 to 32-40 in 2000. The image difference of the BSI indicated that the index increased by about 14-43 over the 13 years. A Microsoft Excel macro was used to write the algorithms for a decision support tool relating the factors that trigger and propagate desertification in arid and semi-arid areas. This was named âTashurâ. Rainfall, aridity index and NDVI were used to evaluate the condition of the landscape. If these three parameters alone were not sufficient to make a decision, then soil and human activity parameters need to be consulted for more reliable decision making. This simple and concise decision support tool is expected to provide guidelines to planners and decision makers. Different ecosystems in the Butana area are subjected to various forms of site degradation. The desertification has led to sand encroachment and to accelerated development of dunes and also increased the water erosion in the northern part of the area. The area has also been subjected to a vegetation cover transformation. Pastures have deteriorated seriously in quality and quantity, but in many parts the degradation is still reversible if land use and water point sites are organized.

The sustainability of maize production in western Ethiopia is in question despite of favorable environmental conditions. A major reason for this phenomenon is severe soil degradation in maize fields. This soil degradation manifested often in low soil N fertility which inhibited maize yields. The situation is worsened by the financial inability of most farmers to purchase N fertilizer for supplementation. In these conditions two basic approaches can be followed to improve maize productivity in a sustainable way. Firstly, integrated cropping practices can be developed for maize to make better use of N from organic and inorganic sources. Secondly, maize genotypes can be selected that are superior in the utilization of available N, either due to enhanced uptake efficiency or because of more efficient use of the absorbed N. In this context, experiments were conducted to determine the integrated effects of tillage system, residue management and N fertilization on the productivity of maize, and to evaluate different maize genotypes for N uptake and use efficiency. The experiments on integrated cropping practices were done from 2000 to 2004 at five sites viz. Bako, Shoboka, Tibe, Ijaji and Gudar in western Ethiopia. They were laid out in a randomized complete block design with three replications. Three tillage systems (MTRR = minimum tillage with residue retention, MTRV = minimum tillage with residue removal and CT = conventional tillage) and three N levels (the recommended rate and 25% less and 25% more than this rate) were combined in factorial arrangement. Every year yield response, usage of applied N and changes of some soil properties were measured. In 2004 the same experiments were used to monitor the fate of applied N in the soil-crop system. Labeled urea was applied at the recommended rate to micro plots within the MTRR and CT plots for this purpose. During the initial two years of the experiments, there was no significant difference in grain yield between MTRR and MTRV and both were significantly superior to CT. However, during the final two years of the experiments, there was no significant difference between MTRV and CT and both were significantly inferior to MTRR. On average, the grain yield of MTRR was 400 and 705 kg ha-1 higher than that of MTRV and CT, resulting in consequent increases of 6.6 and 12.2%, respectively. The application of N increased the grain yield regardless of tillage system. An application of 92 kg N ha-1 was significantly superior to 69 kg N ha-1, but on par with the 115 kg N ha-1 application. Hence, the recommended fertilization rate of 92 kg N ha-1 for conventional tilled maize was also found adequate for minimum tilled maize in western Ethiopia. This rate remained economically optimum with a 20% decrease in the maize price and a 20% increase in fertilizer cost. The grain differences resulted from the tillage systems and concomitant residue management were attributed to significant changes in some soil fertility parameters, especially in the 0-7.5 cm layer. After five years both indices of organic matter, viz. the organic C and total N contents were significantly higher in the MTRR soils when the CT soils serve as reference. Similarly, the extractable P and exchangeable K contents of the MTRR soils were also higher than that of the CT soils. The only negative aspect of MTRR in comparison with CT was a decline in soil pH. A significantly higher grain N content was recorded with MTRR than with MTRV and CT. The stover N content was not significantly affected by the three tillage systems. However, grain, stover and total N uptake were consistently superior with MTRR compared to MTRV and CT. The NAE, NRE and NPE of maize for the same tillage system were consistently higher at the lower N level range of 69-92 kg ha-1 than at the higher N level range of 92-115 kg ha-1. At the lower N level range NAE and NRE were larger with CT than with the other two tillage systems. Both indices were higher with MTRR than with the other two tillage systems at the higher N level range. The NPE was not significantly affected by the tillage systems. However, the trend at both N level ranges was higher with MTRR than with MTRV and CT. The labeled urea study showed that the grain, stover and total biomass N derived from fertilizer was consistently higher for CT than MTRR. Conversely, grain, stover and total biomass N derived from soil was consistently higher with MTRR than CT. Therefore, the fertilizer N recorded in the MTRR soils was higher with MTRR than CT and mainly confined to the upper 45 cm. The fate of fertilizer N was in MTRR: 47% recovered by maize, 17% remained in the soil and 36% unaccounted for and in CT: 54% recovered by maize, 12% remained in the soil and 34% unaccounted for. The experiments on genotype comparison for N uptake and use efficiency were also done at Bako, Shoboka, Tibe, Ijaji and Gudar. In 2004 the response of five open-pollinated and five hybrid genotypes were evaluated at six N levels from 0 to 230 kg ha-1 with 46 kg ha-1 intervals. Only two out of the ten genotypes evaluated qualify as N use efficient. They were the openpollinated Ecaval 1 and the hybrid CML373/CML202/CML384. These two CIMMYT genotypes showed consistently higher NAE, NRE and NPE at low and high N applications as required. This was not the case with the two local genotypes that were included, viz. the open-pollinated Kulani and the hybrid BH 540. Based on the results that evolved from this study it is clear that: 1. Farmers should be encouraged to practice MTRR instead of CT since this change in tillage system could improve the productivity of maize on Nitisols in western Ethiopia. 2. On these Nitisols the conversion from CT to MTRR need not coincide with an adaptation in the recommended fertilization rate of 92 kg N ha-1. 3. The planting of N use efficient maize genotypes on Nitisols must be advocated to farmers, especially those who can not afford proper fertilization. Aspects that need to be investigated in future are: 1. Quantification of N mineralization and immobilization in the Nitisols when subject to MTRR and CT for maize production. 2. Losses of fertilizer N through volatilization, leaching and denitrification from the Nitisols when subject to MTRR and CT for maize production. 3. Suitability of other soil types which are used for maize production in western Ethiopia for MTRR instead of CT. 4. Performance of the N use efficient genotypes on other soil types which are used for maize production in western Ethiopia. 5. Crop rotation with N fixing crops.

Various studies have been conducted into redox potential (Eh), redox indicators and the measured soil water contents in soil (Franzmeier et al., 1983; Schwertmann & Fanning, 1976; Veneman et al., 1976). Although a measure of success has come from these studies, there are still vast knowledge gaps within this field. The degree of water saturation where reduction in the soil is initiated cannot be determined from literature, although it was approximated that 70% of water saturation (S0.7) was sufficient to initiate reduction (Van Huyssteen et al., 2005). This value will vary for different soil temperatures, varying bulk densities as well as soils with different organic matter contents. This study aimed to determine if it was possible to identify a degree of water saturation at which reduction is initiated for a soil in a closed system. It also aimed to determine the effect of bulk density on reduction. Reduction was defined by a decrease in pe (Eh) of a soil and an increase in the soluble Fe2+ concentration. There were three key aims to the study: to establish the relationship between the degree of water saturation (s) and the onset of reduction; to establish the relationship between the degree of water saturation (s) and the duration of reduction and to establish the effect of bulk density on the above-mentioned processes. A yellow brown apedal B horizon from an Avalon soil form (profile 234) in the Weatherley catchment was used in this study. A soil core experiment was carried out to determine the effect of degree and duration of water saturation on Eh, pH, Fe2+, Mn2+, Ca2+, Mg2+, K+, and Na+. Soil cores were packed to a bulk density of 1.6 Mg m-3 and individually saturated to S0.6 (60% of the pores saturated with water), S0.7 (70% of the pores saturated with water), S0.8 (80% of the pores saturated with water), and S0.9 (90% of the pores saturated with water). Measurements were done in triplicate. The cores were sealed with a double layer of plastic wrap and stored in a laboratory at 23°C until needed. Analysis started three days after initial water saturation. A set of cores (four degrees of saturation with triplicates of each) was analysed every 3.5 days for the first three months after which a set was analysed once a week for the remaining month of analyses. The experiment was terminated after 121 days. The same soil and experimental setup was used for the bulk density experiment. The experiment consisted of a set of three cores packed to an initial bulk density of 1.4, 1.6 and 1.8 Mg m-3. The cores were all saturated to S0.8, each packed in triplicate. The bulk density experiment was terminated after 23 days. There was a good correlation between an increase in degree of water saturation and pe (R2 = 0.95); Mn2+ (R2 = 0.91) and Fe2+ (R2 = 0.92) concentrations. Eh, pH, Fe2+, Mn2+, Ca2+, Mg2+, K+, and Na+ were significantly affected by duration of water saturation and all except Ca2+ and K+ significantly affected by degree of water saturation. Fe2+ and Mn2+ accumulations and depletions (visible segregations or mottles) occurred within 12 months of water saturation in a separate experiment where cores were packed to a bulk density of 1.6 Mg m-3 in a core saturated to S0.9. It was therefore evident that this soil with 0.22% organic carbon and a bulk density of 1.6 Mg m-3 will produce morphological features due to reduction within a year of water saturation at S0.9. An experiment was set up with cores kept at a constant degree of water saturation (S0.8) with varying bulk densities, namely 1.4, 1.6 and 1.8 Mg m-3. All the factors measured (Eh, pH, Fe2+, Mn2+, Mg2+ and K+) except Ca2+ and Na+ were significantly affected by a variation in bulk density. In another part of the experiment two different water temperatures were used to saturated the cores, namely 23°C and 30°C respectively. It was determined that the temperature difference of 7°C caused the cores to react significantly different to each other.. The higher water temperature caused the Eh to decrease more rapidly and therefore a higher Fe2+ concentration occurred in these cores. It was concluded that for this soil at 23°C, Fe3+ and to a certain extent Mn4+ will start to become reduced at a pe of 6 at S0.78. These findings show that the first approximation of Van Huyssteen et al. (2005) where S0.7 was found to be sufficient for reduction is very similar for this soil.

A Climate Predictability Tool was used to evaluate the relationship between sea-surface temperatures and stream flow at different lead-times in the upper Olifants catchment in Mpumalanga, South Africa. Four stream flow stations were selected from each of the subcatchments of the upper Olifants, namely the Groot Olifants on the eastern side and the Wilger on the western side of the catchment. Canonical correlation analyses were used to make three month stream flow forecasts for October-November-December (OND) and January-February-March (JFM) seasons. Monthly global-scale SSTs were used to evaluate the effect of lead-times on correlations between global Sea-Surface Temperatures (SSTs) and stream flow. Then the lead-times with Pearsonâs correlation values greater than 0.50 were selected to be used for evaluating possible origins of stream flow forecasting skill in the Equatorial Atlantic, Southern Atlantic, Equatorial Indian and Pacific Oceans. Although local climatic and hydrological characteristics were not considered in this study good hit score skill from the Southern Atlantic Ocean was found at a short lead-time of two months for both OND and JFM seasons. The equatorial Atlantic Ocean gave a good hit skill score at longer lead-times of seven and eight months. The equatorial Indian Ocean gave a higher Heidke score at a short lead-time of two months during OND and JFM seasons in the Groot Olifants sub-catchment. The oceanic domains adjacent to the southern African subcontinent gave a good Heidke score at a shorter lead-time as compared to the equatorial Pacific Ocean. These forecasts could be used for planning water storage and releases in dams that are down stream of these stream flow monitoring points.