Field trials were conducted on the West Campus facility of the Department of Soil, Crop and Climate Sciences of the University of the Free State in Bloemfontein during 2009 and 2010. The first trial during 2009 investigated the response of onion (Allium cepa L.) cultivars to sowing date. Cultivars namely; Charlize, Jaquar, Python and South Wester were used in 2009. Onions were sown on 31 April, 7 May and 21 May during 2009. The second trial was conducted during 2010, where cultivar Ceres Gold was used to replace South Wester as the latter was no-longer available in the market and sowing was done on 11 May, 25 May and 8 June. In both seasons, experiments were laid out as a randomized complete block design with each treatment combination replicated three times. During 2009, plant population of 41 plants m-2 was used, while in 2010 plant population of 61 plants m-2 was used. Plots of 1.8 m2 were used with each plot having five rows. Each row had fifteen plants during 2009 and twenty two plants during 2010. Before planting, soil sampling and analysis were made, thereafter, fertilizers were applied as per soil analysis results. A third field trial was conducted in 2010 to evaluate the three sowing dates (11 May, 25 May and 8 June) with a combination of five plant populations (95, 83, 74, 67 and 61 plants m-2) using one onion cultivar (âJaquarâ). The experiment was laid out as a randomized complete bock design, with three replications having 1.8 m2 plots. In each plot there were five rows. A bulb storage trial was also conducted under room (±25°C) and cold room temperatures (±5°C). This was done for all field trial in both seasons. In a trial investigating response of cultivars to sowing date, better plant height, number of leaves, bulb fresh mass, and yield were observed when sowing was done from the end of April to the end of May. Sowing date significantly influenced bulb and neck diameters only during 2009. Bulbs were becoming more firm as sowing date was delayed, and the opposite was observed for bolting. Cultivar South Wester bolted more, followed by cultivar Jaquar while other cultivars did not bolt. The shape of bulbs was not significantly influenced by sowing date but it showed to be cultivar authentic. No split bulbs were observed. In a trial of sowing date and plant population, significantly taller plants were obtained with early sowing date than the two later sowing dates. Leaf production was not significantly influenced by sowing date. Sowing date and plant population affected bulb fresh mass, yield, bulb and neck diameters as well as firmness. Sowing date did not influence bulb shape while plant population did. None of the bulbs bolted from this trial. Mid-intermediate day cultivars (âSouth Westerâ and âCeres Goldâ) recorded the shortest duration (105 days and 63 days respectively), while on average other cultivars were stored for 126 days in 2009 and 105 days in 2010. Storage disease (black mould), sprouting and loss of moisture from the bulbs were the contributing factors for reduction in storage duration. These factors were promoted by both field and storage conditions. Onion producers should have adequate information on the cultivars and the production

The soaring prices of inorganic fertilisers among other reasons has persuaded companies to commence producing biological enhanced substances herein refers as industrially manufactured biological amendments (IMBAs) with claims that they could increase crop growth and yield, and also revitalize the soil. Such claims are often without substantial empirical agronomic data to proof the efficacy of these IMBAs. A glasshouse pot trial was conducted during 2008/09 season to assess the effects of graded rates of nine IMBAs (Biozone, Gliogrow, Gromor, Promis, Growmax, Crop care, K-humate, Lanbac and Montys) on maize seedlings establishment and growth over six-weeks. These were assessed at 50, 75 and 100% of the recommended rates together with optimum inorganic NPK fertiliser and a control as check. The IMBAs exerted in many instances a deleterious effect on percent maize seedling emergences when applied at 100% rate. Application rates of 50 and 75% appeared sufficient amongst most IMBAs for encouraging better growth and phenological development of maize, although the most appropriate rate is dependent on the IMBA type. Rainfed trials were conducted for three seasons (2006/07-2008/09) at four localities (Bethlehem, Bothaville, Ottosdal and Potchefstroom) to assess the effects of the same nine IMBAs used above on maize performance and on soil health in a randomised completely block design. The IMBAs were applied based on product manufactures and/or supplier recommendations along with optimum inorganic NPK rate and the unamended control as check. All trial sites were planted to one maize cultivar PAN 6479. Every season, observations on phenological growth traits, grain yield and yield components, nitrogen and phosphorus content, uptake, and agronomic use efficiency, soil chemical and microbial properties and on grain quality traits were measured. The manure-based IMBAs like Gromor, Promis and Growmax generally raised pH (H2O) to between 6.0 and 7.0 which was not always the case with the other IMBAs that coincided with inorganic NPK fertiliser. Generally, Gromor and Gliogrow recorded most cases of significant pH increases compared to the NPK treatment. The frequency of significant increases in organic C, mineral N and extractable P were only four instances and less of all 12 potential cases in relation to the NPK check. Gromor resulted in no cases of significantly higher mineral N and extractable P than the NPK check. The IMBAs promoted higher microbial biomass-C immobilisation at 4-weeks after planting while biomass-C mineralisation was predominant at flowering and crop harvest, although it tended to decline at crop harvest. The different IMBAs exerted in many instances no significant effect on biomass-C and -P compared to the NPK check. The IMBAs had no positive effect on maize growth and phenological traits compared with the NPK treatment. Application of Gliogrow resulted in constant reduction in plant phenological growth in the 9th leaf and silking growth stages due to poor emergence, particularly from soils with higher clay content. Gromor and Promis exerted no significant positive effect on grain yield and yield components compared to the NPK check. Despite the consistent poor stand count, Gliogrow resulted in significant increases for all the yield parameters measured than any other IMBA. Compared to the NPK check, the IMBAs resulted also in few cases of significant increases on harvest index while no positive significant effect was observed on cob length. Treatments with Biozone, Gliogrow and Promis at 9th leaf, Gliogrow and K-humate at silking, and Biozone and K-humate at harvesting significantly increased plant N content and uptake at the respective growth stages. None of the IMBAs exerted a significant effect on the agronomic use of the applied N compared to the applied N from the NPK check, except in one case with Promis. The P content and uptake recorded at 9th leaf, silking, and harvesting increased significantly in three to four instances due to the application of Promis, Growmax and Montys. The efficiency of applied P from the IMBAs was not in one case significantly better than the applied P from the NPK check. Application of Gliogrow, Crop care and Lanbac significantly increased thousand kernel mass in two to three cases, and milling index in two to seven cases in comparison with the NPK check. Gliogrow gave solely significantly higher percentage of >11 mm, and 10-11 mm kernels than the NPK check. Equally, Gromor gave significantly higher percentage of 8-9 mm kernels, and Growmax of 7-8 mm kernels.

Soil sustainability is a long-term goal. Although physical and chemical properties of soil have been utilized extensively to evaluate soil quality, the application of biological indicators is becoming more important. In order to assess soil quality, soil enzymes and other biological parameters need to be considered. In semi-arid Bethlehem, South Africa, samples were taken at a wheat (Triticum aestivum L.) monoculture trial which was established in 1979 by the Agricultural Research Council-Small Grain Institute. The treatments were: no-tillage (NT), stubble-mulch (SM), and conventional tillage (CT); all paired with chemical weed control, the absence of burning residues, and 40 kg nitrogen ha-1 as limestone ammonium nitrate with single superphosphate as the fertilizer sources. The study period lasted from October 2010 to October 2011 with eight sampling times conducted over this year and two depths sampled (0-5 cm, 5-10 cm). Oat (Avena sativa L.) was growing in the plots from the start of the study until December 2010 when it was harvested. A fallow period then lasted until the planting of wheat in August 2011 which was harvested after the end of the study period. Potential enzyme activities were assayed for β-glucosidase, urease, acid- and alkalinephosphatase, and dehydrogenase at all eight sampling times, along with soil texture, total carbon, total nitrogen, Olsen-extractable phosphorus, and pH. Whole microbial community profiling using BIOLOG EcoPlatesTM was employed at the first sampling time and phospholipid fatty acid (PLFA) analysis for the first, third, and fifth sampling times. It was found that NT and SM had higher values than CT across all enzymes except alkaline phosphatase, which ranked NT higher than both SM and CT. BIOLOG EcoPlatesTM and PLFA showed similar results across tillage treatments. Microbial biomass, estimated from both potential dehydrogenase activities and PLFA values, was higher in NT and SM than in CT. Over the study period the values for all parameters varied but the average ranking of tillage treatments stayed consistent. In comparing the two soil depths, soil quality was easily shown to be higher in NT and SM in the 0-5 cm depth, but often in the 5-10 cm depth the differences faded. Potential acid phosphatase activity was the only measured parameter which was consistently higher in the 5-10 cm depth. If the parameters can be used as an index of soil quality, then it can be accepted that NT has higher quality than CT and often SM has higher quality than CT, but is not at the same level as NT; it can then be recommended that in semi-arid South Africa, NT will enhance soil quality under a monoculture cropping practice.

For each grain production system, there is an optimum row spacing and plant density that optimises the use of available resources, allowing the expression of maximum attainable grain yield in that specific environment. Introduction of the ultra-fast maize hybrids raised the question whether existing guidelines for row spacing and plant density were still applicable. This necessitated the integration of optimum row spacing by plant density to maintain productivity and sustainability the yields with the intention to increase water use efficiency. Field experiments were conducted for two successive cropping seasons (2008/9 to 2009/10) at Kenilworth Experimental Station of the Department of Soil, Crop and Climate Sciences, University of the Free State to evaluate the growth, agronomic performance, phenological development and water use efficiency of an ultra-fast maize hybrid at varying row spacing and plant densities under irrigation. The treatments involved in this study were three row spacings (0.225, 0.45 and 0.90 m) and five plant densities (50 000, 75 000, 100 000, 125 000 and 150 000 plant ha-1). The treatments were arranged in a factorial combination and laid out in a randomized complete block design (RCBD) with four replications. The largest block was used for periodic destructive sampling for growth analysis where a completely randomized design was adopted and replications consisted of five (5) single plants randomly selected. Regarding soil water monitoring, twenty neutron probe access tubes were installed prior to planting in the center of each plot in one of the three blocks of the agronomic study. Soil water content was measured at 0.3 m intervals to a depth of 1.8 m using a calibrated neutron probe. Measurements were made at weekly intervals from planting to crop physiological maturity where the volumetric reading was converted into depth of water per 1.8 m. Seasonal ET (water use) was determined by solving the ET components of the water balance equation. From this water use efficiency was computed as the ratio of total biomass/grain yield to seasonal ET. In each season crop growth, agronomic, phenologic and water use efficiency parameters were measured and the collected data were combined over seasons after carrying the homogeneity test of variances. Growth parameters, agronomic traits, phenology and water use efficiency of maize reacted differently to row spacing and plant density and the combination thereof. In general a slow increase in growth parameters during establishment was followed by an exponential increase during the vegetative phase. At the reproductive phase growth ceased following the onset of flowering. Photosynthetic efficiency (NAR) and CGR, averaged over row spacing, were highest at a plant density of 100 000 plants ha-1 at all growth phases. Reducing row spacing from 0.45 to 0.225 m and a plant density below or above 100 000 plants ha-1 showed LAI outside the optimum with respect to NAR for optimum seed yield. Row spacing, plant density and its interaction affected yield and yield components of maize significantly. Narrowing rows from 0.45 to 0.225 m and plant densities above 100 000 plants ha-1 as main or interaction effects led to the formation of smaller ears, a shorter ear length and diameter, low seed mass, favored plant lodging and development of barren plants with an obvious negative impact on grain yield. On other hand, plant densities below 100 000 plants ha-1 were insufficient to utilise growth-influencing factors optimally. Thus, growth analysis provided an opportunity to monitor the main effects and interaction effects of row spacing and plant density on crop growth at different growth and development phases. Row spacing and plant density combinations affected the phenological development of maize. Increasing row spacing from 0.225 to 0.90 m relatively prolonged the number of days to anthesis and silking. Regarding anthesis-silking interval (ASI), the lowest plant density had the shortest ASI while the higher plant densities had relatively longer ASI. Wide row spacing coupled with low plant density increased the number of days to physiological maturity and vice versa. Row spacing and plant density and their interaction affected water use efficiency of maize. Highest water use was observed at a plant density of 125 000 plants ha-1. Biomass WUE was highest at a row spacing of 0.45 m with a plant density of 125 000 plants ha-1 while the highest grain yield WUE recorded was at a row spacing of 0.45 m with a plant density of 100 000 plants ha-1. The overall combined effect of row spacing and plant density revealed that a combination of 0.45 or 0.90 m with 100 000 plants ha-1 to be the optimum for the selected ultra-fast maize hybrid under irrigation.

Not available

Amaranthus (Amaranthus spp) and pearl millet (Pennisetum glaucum [L.] R. Br.) are drought tolerant crops with much potential that has not been well exploited as they can be cultivated under semi-arid climatic conditions. This study was carried out to characterize their water use and model their growth and yield in response to water. Experiments were carried out under a field line source sprinkler irrigation system for both crops for two seasons, as well as in a greenhouse with a pot experiment for amaranthus and in the lysimeter facility for pearl millet studies, each for one growth cycle. One genotype of amaranthus (Amaranthus crentus ex Arusha) and two lines of pearl millet (GCI 17, improved line and Monyaloti, local variety) were used in the trials with these crops in a semi-arid area near Bloemfontein, South Africa. The influence of water application on growth of amaranthus was contrary to the expectation that fully irrigated plants will perform better than the plants receiving less water. Fully irrigated plants produced shorter plants with less leaves and branches. However, irrigation improved the plant height in both lines of the pearl millet. A large amount of irrigation resulted in taller plants for both lines while the shortest plants were found in the rainfed plots. Another millet crop parameter that was affected by irrigation was flower emergence. Flower emergence was earlier in irrigated plots of both lines of pearl millet and during the two seasons. In both lines of pearl millet, irrigation increased leaf area index and biomass accumulation during both seasons. The two crops were able to exhibit the ability to tolerate water stress with different coping mechanisms and this influenced their water uptake and invariably also water use. Amaranthus was able to manage water stress in rainfed plots through the closure of stomata in the field and during the pot trials. Stomatal closure reduces water loss as a response to water deficit in the soil-crop-atmosphere continuum. Daily water use of amaranthus ranged from 1.2 to 6.5 mm day-1 while the seasonal water use was 437 mm for the first season and 482 mm for the second season. Higher water use in the second season was attributed to higher atmospheric evaporative demand recorded during the second amaranthus growing season compared to the first. It was observed that while water application can increase the production of amaranthus, it should also not be too much or it could have a detrimental effect on biomass production of the crop. This conclusion is due to the fact that the lowest irrigated plots produced higher fresh and dry mass of amaranthus during both seasons while production in the fully irrigated plots was low for the two seasons. The response of pearl millet to water deficit stress was to lower the leaf water potential (more negative) and also gradually decrease the leaf stomatal conductance. Pearl millet demonstrated a response to the water stress condition by closing of the stomata as leaf water potential declined (towards more negative) so as to conserve water and prevent water loss. This minimized water loss through transpiration when the soil water available is limited. The crop adjusted to severe water stress conditions by maintaining a leaf water potential that keeps the leaf turgid in order to avoid wilting when the stomata closes so as to prevent excessive water loss. The daily evapotranspiration of the two lines of pearl millet for the two seasons were between 2 and 8 mm day-1 for the first season and 1 and 6 mm day-1 for the second season. The difference could also be attributed to a higher atmospheric evaporative demand in the first pearl millet growing season than the second season. Overall, the improved (GCI 17) and the local variety (Monyaloti) of pearl millet had water use of 309 and 414 mm in 2008/2009 season. The water use for the two lines was higher in the 2009/2010 season with GCI 17 having water use of 401 mm and Monyaloti 457 mm which was probably due to high availability of water. High soil water content coupled with a higher amount of rainfall in the second season than the first season could be the reason for difference of the water use of the two lines of pearl millet for the two seasons. However, the water use of the plants of the two lines of pearl millet from the rainfed plots and water stressed treatments showed that the crop was able to reduce water use under water stress conditions as a coping mechanism and hereby increase water use efficiency of the crop. With the aid of the data from the field experiment, greenhouse and lysimeter trials, calibration and validation of AquaCrop crop model was performed successfully for both crops. Simulation of biomass production and cumulative evapotranspiration of both crops were performed adequately. The good performance in simulating these crop parameters were illustrated with a high index of agreement that was higher than 0.9 except for 2 cases of CC excluding the soil water comparisons. However, it was observed that more effort is needed to accurately simulate early canopy cover in amaranthus and also the soil water content and depletion patterns for both crops. Following successful validation, the model was also applied to predict the performance of both crops under a range of proposed planting dates and choice of varieties in pearl millet as possible adaptation strategies under two climate change scenarios. The model was able to predict the production of the two crops under predicted climate change for the period between the year 2046 and 2065 and the most appropriate adaptation strategy as a recommendation is to delay planting for two months until the first half of January for both crops under the two future climate change scenarios (A2 and B1). In conclusion, the two crops under investigation can adjust to water limited conditions but through different mechanisms. Amaranthus can avoid water stress through restricting growth, while the pearl millet crop escapes water stress through speedy completion of growth stages before the water stress condition sets in. It was also revealed that there are possibilities of cultivating these crops in central South Africa. However, more studies should be carried out on the effect of interaction of nutrient and irrigation on amaranthus production to reveal the reasons for the unexpected response of amaranthus to water application. Studies on root development of the two crops are hereby recommended to aid in accurate simulation of water balance of the two crops in the field situations. The calibration and validation of AquaCrop for these two crops can also be improved by using datasets of more varieties or genotypes of the crops and from other agro-ecological regions. In general, underutilised crops provide means of food security and source of income for farmers. Due to the fact that they are drought tolerant, they require minimum amount of input which is a desirable quality for low resource farmers and can be used as alternative crops in semi-arid areas.

Food production in semi-arid areas principally depends on the availability of water. Consequently, improving rainwater productivity and modifying the available energy for unproductive water losses is an important and necessary step towards promoting rainfed agriculture in dryland farming. It has been convincingly argued that water management strategies on rainfed semi-arid areas, including in-field rainwater harvesting (IRWH) deserve considerable attention. However, integrated studies of water and energy balance on the IRWH technique in particular in optimizing runoff to basin area ratio and mulching levels (ML) was not comprehensively appraised. Therefore, in this thesis, the two main research questions concern: (i) what is the optimal runoff to basin area ratio to sustain maize crop yield? and (ii) how do the microclimatic conditions change under wide and narrow runoff strip length (RSL)? Field experiments were conducted (2007/08 and 2008/9) on the Kenilworth Bainsvlei ecotope, associated with high evaporative demand of 2294 mm per annum and relatively low and erratic rainfall (528 155.6 mm). Topographically the area had a gentle slope (< 1%) with reddish brown in colour (Amalia family) a fine sandy loam texture soil, thus was classified as a Bainsvlei form. The soil is regarded as very suitable for dryland agriculture, because it is deep (2000 mm) and drains freely in the top and the upper sub-soil. So the study was performed by quantifying and evaluating the soil-crop-atmosphere parameters. In the first part of the thesis, the soil water balance components and different efficiency parameters were assessed. In the second part of the thesis, the micrometeorological variable profiles within and above the maize canopy for the heat and water vapour exchange processes were characterized. Furthermore, comparison of available energy for evapotranspiration (ET) was evaluated for both wide and narrow runoff strips through the quantification of energy balance components. A multiple regression model was developed to predict in-field runoff by combining the effects of rainfall event characteristics and surface treatments. From the results of runoff-rainfall (RR) ratio a lower efficiency was observed from full mulch covered wide runoff strip length (RSL-3) i.e. only about 4% of the rainfall, while the highest mean RR was about 27% from bare, narrow RSL- 1. From the estimation of rainfall canopy interception (RCI) it was revealed that the highest interception was in the range of 4.5% to 9.0% of the precipitation. The RCI capacity of a maize field under IRWH reached a plateau at about 0.5 â 0.6 mm for narrow RSL and 1.0 â 1.1 mm for wide that would be evaporated eventually from the canopy. Furthermore the cumulative Es (ΣEs) was evaluated as influenced by both mulch (âdry-mulchâ) and shading (âgreen-mulchâ) effects. Thus, the proportion of water loss by Es from seasonal rainfall is about 62%, 64% and 66% in the bare treatments and as low as 28%, 30% and 32% for full mulch cover treatments under full shade, (FC), partial canopy shaded (PC) and unshaded (UC) respectively. This implies that, reduction of runoff and evaporation losses through surface treatments can promote improved water use efficiency, of the stored available water in the root zone and thus, enhance yield. The final grain yield decreased slightly as an order of increasing the length of the runoff strip. The performance of the harvest index (HI) was slightly variable among the treatments due to more water for yield being collected from bare plots than mulch covered plots. The higher mulch conserves much water by suppressing the soil evaporation. In expressing grain yield per unit ET (WUEET) and transpiration, Ev (WPEv) the RSL-2 m and RSL-1.5 m at lower mulch cover showed significant higher values than RSL-1 and RSL-3 treatments. However, the transpiring water for yield and unproductive evaporation losses more under IRWH should be evaluated in terms of micrometeorological profile characterization and available energy. With regard to micrometeorological variables, the growth stage had a strong effect on the vertical profiles of climatic variables. In wide runoff strips lapse conditions extended from lowest measurement level (LP) to the upper middle section (MU) of the canopy and inversion was apparent at the top layer (UP) of the canopy. The reason for the extension of temperature inversion into the upper part of the wide RSL canopy was as a result of higher air movements compared to narrow strips. From this result it was confirmed that the effect of wind on water vapour removal decreased downward from wind flow within the canopy. This had an influence on the resistance of the boundary layer and canopy and soil surface resistance. This is a clear indication that wide strips supply more drying power to respond to evaporative demand of the atmosphere compared to narrow strips. From the measurement of profiles within and above the canopy, it was suggested that, the presence of local advection in the wide runoff strips of IRWH could be a common phenomenon causing variations in water vapour removal under the heterogeneous nature of IRWH tillage system. Thus, profile characteristics within and above a plant canopy are playing a great role in determining the vapour pressure deficit and consequently, can explain the ET rate. Therefore based on micrometeorological measurements, results indicated that the latent heat (LE) was dominant and higher in wide compared to narrow runoff strips (RSL) under both dry and wet conditions. However, sensible heat (Hs) showed lower values on wide runoff strips during wet conditions due to the advective effect of the runoff area. Thus, the wide runoff strip with a higher basin leaf area ratio (BLAR) of 2.43 had higher ET and used more energy in evaporating water than the narrow runoff with a lower BLAR of 1.42. Wide runoff strips converted the higher available energy more efficiently into a higher biomass production. During wet days, the wide RSL used more than 70% of the available energy for evapotranspiration, while the narrow RSL response to the available energy (63%) was stronger during dry compared to wet days. In general the wide and narrow RSL used the available water and energy differently during dry and wet conditions under IRWH system. From this experiment finding, important implications were described such as better yield obtained from narrow RSL-1, however RSL-1.5 and 2 m with minimum mulch cover gave higher water productivity compared to narrow RSL-1 and wide RSL-3. On the other hand when quantifying and evaluating the cause behind the effect of available energy, the wide RSL converted available energy more efficiently into higher biomass production than the narrow RSL. Therefore, this challenge should be addressed on the basis of an integrated approach to water and energy resources in order to develop comprehensive management strategies. Furthermore, for improved rainwater use management strategies, it is recommended to link an integrated approach of water and energy resources with crop growth simulation models. The application of the crop models could be important by incorporating a range of planting dates and densities along with the selection of surface treatment management strategies

An investigation was initiated to obtain some understanding on the behaviour of soil at higher moisture content and to explore the potential of preparing paddy fields with reduced amounts of water. This investigation comprised of three separate studies. Based on existing information that water could be reduced when soil clods were initially formed prior to flooding, the effects of clod size, clod initial moisture content and confining states on the rate of water uptake were explored. The moisture gradients within clods wetted and dried for different period of times were also studied. The results of the clod wetting experiments show that· the rate of water uptake by capillarity was greatest when clods were initially very dry and smaller clods tended to absorb water faster than bigger clods when under confined conditions. Confining had no effect on infiltration when the initial condition was very wet. On drying, the smallest clod dried the fastest, reduced greater volume and increased its dry bulk density significantly. Larger clods required,longer drying period to arrive at a uniform moisture profile within as compared to smaller clods. Results from the wetting experiments were tested against the infiltration model of .Jarvis and Leeds-Harrison (1987) and a model developed based on linear flow of heat into a solid (Carslaw and Jaeger, 1959). A second project involved the study of soil deformation at high moisture contents in an attempt to produce clods with minimum draught force using simple relieved tines at various rake angles and depths in a soil tank. The principal. objective of the - ii - study was to utilise soil implement mechanics knowledge to improve the efficiency of soil preparation for wetland crops. Aspects like the nature of soil disturbance, extent of disturbance and draught requirement were investigated. The soil was in a plastic consistency prepared to three specified density states of 940, 1000 and 1250 kg/m3• The soil disturbance pattern was monitored using implanted coloured beads and glass sided tank studies. In addition, the extent and height of heave and surface disturbance were noted. Predictive models based upon Mohr-Coulomb soil mechanics theory were developed to predict the interaction between the soil and simple implements at three rake angles. These were based on the lateral failure theory of Godwin and Spoor (1977) and the two dimensional soil failure model of Hettiaratchi and Reece (1974). Results from the single tine study were tested against the models. A sliding resistance component and crescent effect were incorporated to improve the predictions for the 45° and 90° rake angle tines. The magnitude of each mode of failure is dependent upon the critical aspect ratio which varies with tine rake angles and soil conditions. The mode of failure is considered to be lateral when the tine aspect ratio is larger than the critical aspect ratio and an upward failure when the tine aspect ratio is lower than the critical aspect ratio. The predicted results are in close agreement with the results of the experimental studies. For the backward raked tine, a model was developed based on the formation of an elliptical wedge and bearing capacity type of failure ahead and below the wedge. This failure theory was based on the bearing capacity failure for deep footings. The model - iii - helped identify an additional parameter that influenced the draught force for a backward raked tine. This parameter is the sliding resistance component on both sides and beneath the elliptical soil wedge •. Results from multitine studies showed that draught force increased with tine spacinq but the increase was not significant. In the wet condition the tines merely cut slots and little or no interaction was noted. In an effort to find the optimum water level for soil puddlinq, a laboratory study was conducted to determine the influence of water-soil ratio on the ease of puddling air dry aqqreqates. Soil puddlinq was carried out usinq a·rotary stirrer simulatinq the rotary motion of a rotary cultivator commonly used in wetland preparation •. The results obtained showed that· the fastest dispersion of particles resultinq in a minimum wet bulk density of 1.23 Mg/m3, was achieved at a water-soil ratio of 1.2. (A supersaturated condition equivalent to a moisture content of 120% dry basis). Increasing the water-soil ratio above this value did not change the wet bulk density value for all stirring times.

No description available.

The studies reported here, used sorghum to: (i) investigate the effect of changes in the matric suction (m) of filter-paper on germination and water uptake (ii) assess the effect of seed vigour on seedling response to the m of filter paper and to soil mechanical impedance, (iii) investigate the effect of root-shoot interaction on establishment of seedlings subjected to mechanical impedance (MI), and (iv) evaluate the effects of different amounts of planting rain on crop establishment. A technique for controlling matric suction on seed test paper is described and used to study seed germination at m of 0.1, 1 and 10 kPa. Time to 50% germination was increased, and final lengths of both radicles and cleoptiles reduced by an increase in m from either 0.1 to 1, or 1 to 10 kPa. A link between filter paper m, seed-liquid contact area, seed size, the rate of water uptake and germination is discussed. Unimpeded, pregerminated low vigour and fresh seeds produced a similar root and shoot length, fresh mass, and number of leaves and lateral roots. When grown in compacted soil, low vigour seed took longer to emerge but only when seeds were severely aged. (i) Impedance to the shoot delayed emergence, more so when the root was also impeded. (ii) Shoots emerging through a mechanically impeding layer, had greater extension rates after emergence than unimpeded ones. (iii) Mesocotyls became thicker only when the root systems were impeded. (iv) Impeding the shoot system, increased root extension rate. (v) The length of the first internode, the number of leaves and the spacing of lateral roots were not changed by any of the treatments. Germination was >95% in sand columns watered with 5, 10 or 15 mm equivalents of rain. However, seedling emergence was <15% in the 5 mm compared to 77-100% in the 10 and 15 mm treatments. At harvest (9 DAP), all seedlings in the 5 mm treatment were dead. Seedlings given 10 mm rain showed signs of water stress. The interaction between seed vigour, the amount of water available to germinating seed, and soil mechanical impedance and their effect on crop establishment is discussed.