Masters Degrees (Soil, Crop and Climate Sciences)
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Browsing Masters Degrees (Soil, Crop and Climate Sciences) by Advisor "Hensley, M."
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Item Open Access Comparison of field and laboratory measured hydraulic properties of selected diagnostic soil horizons(University of the Free State, 2009-11) Chimungu, Joseph Gregory; Van Rensburg, L. D.; Hensley, M.An adequate characterization of soil hydraulic properties is a necessary solution for agriculturally and environmentally oriented problems such as irrigation, drainage, runoff and pollutants movement. The three approaches to determine hydraulic properties of soils are field measurements, laboratory measurements and mathematical models. In situ measurements, though representative, have the inherent limitation of being costly and time consuming. Laboratory and mathematical techniques are more convenient but require extensive comparison to field results as bench mark for evaluation. The objective of this study was to characterize the hydraulic properties of Bainsvlei and Tukulu form soils utilizing the above mentioned three approaches and to compare the results. The laboratory methods selected were hanging water column and pressure plate apparatus. Undisturbed soil samples were used to determine θ-h relationships at 0-100 kPa suctions and disturbed soil samples up to 1500 kPa. The water retention characteristics for both soils were generally well defined with little variability between replicates. The main variations were due to texture differences between the horizons. The θ-h relationships were used to estimate textural and structural domains using empirical pore class limits and derivative curves. The suction value separating the structural domain from the textural domain varies from horizon to horizon. The boundary between soil pore categories cannot be taken as a fixed value for all soils and all types of soil use. The measured water retention data corresponded well with the fitted curve via the van Genuchten (1980) model, indicating that the model can be successfully used to describe θ-h relationships for Bainsvlei and Tukulu soils. Soil water sensors were calibrated using undisturbed soil samples in climate controlled room for five horizons of a Bainsvlei form soil and three horizons of a Tukulu form soil. Soil water sensors and circuitry show extremely low sensitivity to temperature fluctuations. Horizon specific calibration is essential to get accurate water content estimates from the sensors if used in different soil horizons. Our study demonstrate that horizon specific calibrations of the water sensors improves the accuracy of soil water content monitoring compared with the manufacturer‟s generic calibration equation for the soils tested in this study. Hydraulic conductivity was obtained by measuring the hydraulic head and water content of the Bainsvlei soil form in situ with tensiometers and horizon specific calibrated ECH2O EC-20 probes, respectively. The profile was characterized with several relations of hydraulic conductivity and varied with depth. The reason for this was attributed to heterogeneous nature of the profiles due to variation in particle size distribution. The van Genuchten (1980) model laboratory method was used to predict K-θ relationships utilizing laboratory determined θ-h relationships. The K-θ relationships predicted from the θ-h relationships of the soil cores corresponded well with those determined by the instantaneous profile field method for water contents which they have in common. Thus it appears that this laboratory method is applicable to the soils studied, but the accuracy of the predicted values is quite sensitive to the matching factors. Thus, accurate measurement of these parameters is necessary for its successful use. The instantaneous profile field method is regarded as a reference method to measure in situ unsaturated hydraulic conductivity for both homogenous and layered soils (Hillel et al., 1972). There are, however, several site or profile characteristics that may limit this method (Bouma, 1983). Our studies show that it is not applicable on duplex soils with slow permeable C-horizons i.e. the Tukulu form profile at Paradys, because of negative hydraulic gradients within the profile due to impaired internal drainage. There is a need to adapt this method to duplex soils. Overall results indicate that from a practical perspective, the prediction of K-θ relationship from laboratory determined water retention data can be a viable alternative for determining the hydraulic properties of diagnostic horizons. The prediction of DUL using θ-h relationship has been found to be satisfactory.Item Open Access Evaluating maize production potential of selected semi-arid ecotopes using a water balance model(University of the Free State, 2003-06) Bairai Zere, Teclemariam; Van Huyssteen, C. A.; Hensley, M.English: The quantitative evaluation of crop production potential is important for sustainable and wise land use as well as for food security where subsistence farmers are involved. It is of particular importance in arid and semi-arid areas where rainfall is marginal and variable. This study aims at making a quantitative evaluation of the maize production potential of the Glen/Hutton and Glen/Oakleaf ecotopes which are located at the Glen Agricultural Research Station in the semi-arid Free State Province of South Africa. The objective was to characterize the ecotopes, and to make long-term yield predictions with a yield prediction model using long-term climate data. A detailed profile description, soil analyses and an in situ drainage curve were made for the Glen/Oakleaf ecotope. Similar data for the Glen/Hutton ecotope was obtained from previous research work (Hensley et al., 1993; Hattingh, 1993; Hensley, personal communication, 2002). A neutron water meter (NWM) was calibrated for each horizon of the Oakleaf soil on the Glen/Oakleaf ecootpe. The plant available water (PAW), defined as the differences between the drained upper limit (DUL) and the lower limit (LL), for maize grown on the Glen/Hutton and Glen/Oakleaf ecotopes was 133 mm and 120 mm respectively. Considering a mature maize crop growing in summer on these two ecotopes, PAW can be defined as the difference between the crop modified upper limit (CMUL) and LL. Results for this parameter were 183 mm and 192 mm for the Glen/Hutton and Glen/Oakleaf ecotopes respectively. The reason for the relatively high value of the latter is its slower drainage rate, which enables the crop to extract more water while drainage proceeds between field saturation and DUL than in the rapidly draining Hutton soil. Yields measured on experiments on the two ecotopes for 12 seasons on the Glen/Hutton and 10 seasons on the Glen/Oakleaf ecotope indicate that these two ecotopes have similar production potentials. For the development of a yield prediction model it was necessary to find a way to estimate daily crop evapotranspiration (ET). Based on the semi-arid climate, soil morphological observations and results of soil analyses, deep drainage from these two maize ecotopes was considered to be negligible. Equations for predicting runoff from rainfall (P) were developed based on long-term runoff measurements made at nearby sites (Du Plessis and Mostert, 1965; Hensley, personal communication, 2002). Because of fairly good r² values (0.84 and 0.82) the equations can be considered as reliable enough for the purpose of this study. A procedure for estimating soil water content at planting, from the rainfall pattern during preceding fallow period and grain yield in the preceding season, was also developed based on measurements from previous research work (De Jager and Hensley, 1988; Hattingh, 1993). Using all this information it was possible to make a fairly reliable estimation of daily ET. Climate data was used to calculate daily potential evaporation (Eo) values. This enabled the degree of crop water stress to be defined as ET/Eo , on a daily basis. The maize growing season was divided into three stages i.e. the vegetative, flowering and seed filling stages. A stress index (SI), defined as the average ET/Eo value for each period, was then calculated. To develop an integrated stress index (ISI) for the growing season eight different methods of integrating the three SI values were formulated. Measured maize yields from experimental plots on the two ecotopes were available for 22 seasons (De Wet and Engelbrecht, 1962; De Bruyn, 1974; De Jager and Hensley, 1988; Hattingh, 1993). Integrated stress index values were then calculated for these seasons and correlated with the biomass yields. This made it possible to choose the best method of calculating the ISI value from the individual SI's. The ISI with the best correlation (r² = 0.69) was the one with formula ISI = (2A + 3B + 2C)/7, where A, B and C are the SI values of the three growth periods respectively. The equation to predict total biomass (Yb) is Yb = 15238 ISI + 1067 kg ha¹. The biomass prediction equation was used to generate maize yields for 80 seasons (1922/23 - 2001/02). Yb was converted to grain yield using a harvest index regression equation based on 38 yields from Glen for which both total biomass and grain yield had been measured. Four production techniques were compared, i.e., November planting with conventional tillage (CTN), January planting with conventional tillage (CTJ), November planting with in-field water harvesting and basin tillage (WHBN), and January planting with water harvesting and basin tillage (WHBJ). Cumulative probability functions (CPF's) of yields were computed for the four different production techniques. The CPF's indicated that the long-term mean yields (at 50% probability) were 2653, 2 685, 3 108, and 3 355 kg ha¹ for CTN, CTJ, WHBN and WHBJ respectively. The CPF's were compared using the stochastic dominance and the Kolmogorov-Smimov (K-S) tests (Anderson et al., 1977; Steel et al., 1997). Stochastic dominance results indicated that the WHBJ and WHBN production techniques have well defined first degree stochastic dominance over the CTN and CTJ techniques. January planting showed only second degree stochastic dominance over November planting. The K-S test indicated that the CPF's of the water harvesting techniques were significantly different from those of the conventional production techniques. No statistical significant difference was observed with the K-S test between the November and January plantings.Item Open Access Organic matter restoration by conversion of cultivated land to perennial pasture on three agro-ecosystems in the Free State(University of the Free State, 2002-01) Birru, Tilahun Chibsa; Du Preez, C. C.; Hensley, M.English: Understanding the process of organic matter degradation and restoration is important with regard to sustainable agricultural production on any agro-ecosystem, and of particular importance where degradation is relatively rapid, such as in the coarse textured savannah soils of the South African highveld. Organic matter degradation studies on such soils in three agro-ecosystems, Harrismith, Tweespruit and Kroonstad, have been undertaken by Du Toit et al. (1994), and Lobe et al. (2001). This study is concerned with organic matter restoration on the same agro-ecosystems, and is therefore complementary to the two earlier studies. The objective was to investigate organic matter restoration at three depths, 0-50, 50-100 and 100- 200 mm, on perennial pastures of different ages that had been established on lands which had been cultivated continuously for more than 20 years. Representative C and N values for degraded lands and virgin grasslands for the three agro-ecosystems were obtained from the studies of Du Toit et al. (1994) and Lobe et al. (2001), and used as reference values. To reduce within-site error samples were collected at six places, separated from each other by a few meters, at each site. At each of these places six subsamples of each layer were taken to make up the final sample. There were therefore 18 soil samples per site. A total of 28 sites, ranging in ages from 4 to 25 years, were identified and sampled on the three agro-ecosystems, All the samples were analyzed for C and N, and selected samples were analyzed to characterize the soil fertility levels and particle size distribution at each site. Results showed a wide variation in the rate of organic matter restoration between sites in each of the agro-eco systems , due mainly to differences in natural resource factors and management techniques. Most important of the latter was the application of N fertilizer. Where this was inadequate or absent, very low organic matter restoration rates were generally measured. An approximate threshold value of available N below which organic matter restoration is severely impaired appears to be about 15 mg kg". On pastures up to the age of25 years most of the C and N storage has been in the 0-50 mm layer, a little in the 50-100 mm layer, and very little in the 100-200 mm layer. This observation accentuates the importance of the sampling depth in such studies. These results are in accordance with those of Potter ef al. (1999). The mean C gains over all the sites in the three agroecosystems, excluding those with a Nfertility level considered too low to initiate efficient C sequestration, is 0.56 Mg ha-I yr' as compared to 0.8 Mg ha" yr" suggested by Bruce el al. (1999) for the United States of America and Canada. The relatively coarse texture of the Free State soils, and the lower aridity indices, may account for the difference. An attempt was made by pooling the data for the three agro-ecosystems, and adopting a normalization procedure, to identify common C and Nrestoration curves with time. Although a definite upward trend is visible, large inter-site variation and the shortage of data points above 20 years results in relatively low correlation coefficients and the curves being unreliable at their top end. Further research to obtain data from very old pastures is recommended, as well as ecotope specific research on benchmark ecotopes to define in a reliable way the shape of the organic matter restoration curve.