Soil, Crop and Climate Sciences

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Open Access
Agroclimatological risk assessment of rainfed maize production for the Free State Province of South Africa
(University of the Free State, 2010-12) Moeletsi, Mokhele Edmond; Walker, Sue; Landman, Willem A.
English: The risks associated with climate and its variability over the Free State Province is the major determining factor for agricultural productivity, and has a major impact on food security across the province. To improve productivity of agricultural lands, producers and decisions makers have to be provided with relevant agrometeorological information that will enable them to make appropriate decisions. This has lead to the investigation of this agroclimatological risk assessment for maize production in the Free State. The ultimate goal was to characterize the agroclimatological risks impacting negatively on dryland maize production and develop a climate risk tool that will assist the stakeholders in their management of agricultural lands. First, meteorological data needed to perform this study was prepared by looking specifically at filling the missing data gaps and using alternative data in cases where measured data was not available to obtain good spatial distribution of weather stations. Frost was identified as one of the climate hazards affecting the maize plant in the Free State. Three frost severity categories were analysed, namely 2°C, 0°C and -2°C representing light, medium and heavy frost respectively. The onset of frost for all the thresholds was earlier over the northern, eastern and far southeastern parts of the Free State province while places over the western and southwestern parts of the province the first frost dates are later. The northern and eastern parts are also marked by late cessation of frost giving a shorter frost-free period (220-240 days at medium frost severity). The western and southwestern areas mostly have earlier cessation of frost resulting in relatively long frostfree period with ranges from 241 to 300 days at medium frost severity level. Cessation of frost occurring later than normal over the Free State can impact negatively on the maize crop if planted in October and early November, especially over the highlands. Productivity of the crops can also be hampered by earlier than normal onset of frost that affects maize at silking and grain-filling stages. The onsets and cessation of rains together with the duration of the rainy season also play an important role in agricultural planning. Over 300 stations across the Free State were analysed to characterize the rainy season. The onsets of rains were found to be early over the eastern parts of the province with median onsets on or earlier than 10 October. In most areas over the Fezile Dabi and Motheo districts, onsets are between 11 to 30 October while over the Lejweleputswa onsets are mostly between 21 October and 10 November. Most of the western parts of Xhariep experience later than 21 November at 50% risk level. The cessation of rains does not vary much over the Free State with most places having their median last rains between 21 April and 30 April. Rainy season lengths are longer over the Thabo Mofutsanyane district with over 200 days in some places. The ENSO episodes are related to Free State seasonal rainfall variability but only have slight effect on the cessation of rains while onsets of rains showed no differences between El Niño or La Niña phases as compared to all the years. In El Niño years the seasonal rainfall amount is lower than normal, being higher than normal in La Niña years which support findings from other studies. The cessation of rains occurs earlier in El Niño years and later than normal in La Niña years. Agricultural drought is one of the most devastating hazards affecting maize production in most growing periods depending on the location. It is important to plant during periods which minimise drought conditions. In this study a simple water balance model developed by FAO called WRSI was used to quantify drought risk. When using the 120-day maize cultivar as a reference, drought index over most parts of the Lejweleputswa, Xhariep and eastern parts of the Motheo district show high vulnerability (WRSI<40) for October planting dates while other areas have relatively low risk of drought. In December and January planting dates drought index over most parts of the province showed much improvement but places that showed low risk are over the Thabo Mofutsanyane, Fezile Dabi and pockets of northern Lejweleputswa district. Poone AgroClimatic Suitability Index (PACSI) was introduced to integrate all the climate hazards affecting maize production in the Free State. The index in made from the combination of frost probability over the growing period, non-exceedence probability of onset of rains and agricultural drought index. The index was further used to delineate the suitable areas across the Free State for planting maize variety requiring 1420 growing degree days (heat units) to maturity. The findings obtained from the resulting maps show areas of high maize production suitability over the Thabo Mofutsanyane district for mid-October to early November planting dates. Places over Fezile Dabi and northern parts of the Lejweleputswa district also showed high suitability of maize especially for planting from mid-November to end of December. The western and southern Xhariep district area is not suitable for planting maize while other marginal dryland maize production areas include western Motheo, southwestern Lejweleputswa and most parts of the central and eastern Xhariep. To conclude the study, the Free State Maize Agroclimatological Risk Tool (FS-MACRT) was developed to provide agroclimatological risk information important to the production of rainfed maize in the Free State Province. The tool is to be used by the farmers, extension officers, policy-makers and agricultural risk advisors. The tool has two main parts, 1) climatological risk and 2) forecasting. The climatological risk enables the user to obtain drought stress risk for the 100-day, 120-day and 140-day maize cultivars for planting window starting in October to January. The best planting dates based on the risk associated with the climatology onset and cessation of both rains and frost can be determined. Using climate forecasts obtained from the national forecasting centres, drought index can be predicted for different planting dates giving the farmer valuable information when planning for the coming season. The tool also has the functionality of predicting onsets of rains using weather and climate forecasts.
Open Access
Agroclymatic characterization of Lesotho for dryland maize production
(University of the Free State, 2004-11) Moeletsi, Mokhele Edmond; Walker, Sue; Barker, Charles H.
Agro-climatic characterization of Lesotho for dryland maize farming was performed using temperature and rainfall indices in a GIS environment. The temperature and rainfall meteorological parameters were patched for missing data using the UK method for the maximum and minimum temperatures. Missing daily rainfall data was patched using the inverse distance method. Statistical evaluation of the patching methods showed good performance. The spatial distributions of different temperature variables and indices were mapped. Important meteorological parameters were the frost occurrence (first day, last day and duration) and monthly and seasonal heat units. The onset of frost is early (March) over the highland areas while the low- lying areas onset can be as late as June. The last day of frost over the low- lying areas is mostly in August and on the other hand, the highlands last day of frost is in November/December at some places. Rainfall interpolation was done using the kriging method of the geostatistical analyst. Important aspects mapped include monthly averages, seasonal amounts, annual amounts and number of days of high daily rainfall. Wet season (October to April) rainfall was high (>800mm) over the north to northeastern parts of the country while some areas over the east and southern parts received less than 500mm of seasonal rainfall. Climatic potential of maize under dryland farming in Lesotho was investigated using five climatic suitability indices namely: probability of receiving heat units of greater than 1500GDD, probability of a frost-free growing season, probability of seasonal rainfall of more than 500mm, probability of 15-day dry spells during December to February and the slope of an area. For each of the above parameters a coverage layer was prepared in GIS environment and the layers were overlaid to obtain the agroclimatic suitability map of maize in Lesotho. The districts of Butha Buthe, Leribe and Berea are shown to have areas which are highly favorable for maize cultivation under dryland farming while the unsuitable areas are mostly over the high- lying areas (Mokhotlong, Thaba Tseka and Qacha’s Nek) together with other parts of the southern lowlands.
Open Access
Besproeibaarheid van gestruktureerde gronde
(University of the Free State, 1991-11) Nell, Johannes Petrus; Bennie, A. T. P.
Afrikaans: Data van 333 profiele se fisiese (breukmodulus, infiltrasie, hidrouliese geleivermoë, lugwaterdeurlatendheidverhouding, waterretensie en Atterberggrense), chemiese (KUK, UNP, NAV, pH, elektriese weerstand, elektriese geleivermoë, Fe-SBD, Al-SBD en organiese koolstof) , morfologiese (grondkleur, struktuur, kleiverhoudings en slik tot kleiverhoudings) en kleimineralogiese eienskappe was gebruik om neokutanies, pedokutanies, prismakutanies, rooi gestruktuurde en vertiese gronde te karakteriseer. Gronde van 15 dreineringstreke is in die ondersoek gebruik. Klem word egter op kalkryke gronde in die Sondagsrivier-, Klein-Visrivier- en Groot-Visrivierdreinerings- gebied geplaas, aangesien heelwat van die gronde deur verdere ontwikkeling van die Oranjerivierprojek geraak word. Daar bestaan regverdiging om verskille tussen diagnostiese horisonte as besproeiingsnorm te gebruik, aangesien duidelike fisiese en chemiese verskille tussen grondvorms, binne dreineringsgebiede voorkom. Daar is 'n afname in die geskiktheid van gronde vir besproeiing met 'n toename in graad van struktuurontwikkeling. Dit kan hoofsaaklik toegeskryf word aan 'n toename in opbou van skadelike soute en 'n verskuiwing van 'n oorwegend illietiese kleimineraalsamestelling na 'n smektietiese samestelling wat tot swelling en/of dispersie aanleiding gee. Laer infiltrasie en hidrouliese geleivermoë waardes, met 'n toename in struktuurontwikkeling, kan grootliks hieraan gewei word. Alhoewel die Na-inhoud ook 'n rol speel by die strukturele eienskappe van vertiese en rooi gestruktuurde gronde, het die tipe en hoeveelheid kleiminerale en seskwioksiedes 'n duideliker invloed by dié gronde se struktuurstabiliteit. Daar bestaan egter nie regverdiging om kleur volgens seriedifferensiasie as besproeiingsnorm by pedokutaniese gronde te gebruik nie. Die stabiliserende effek van die ysteroksiedes wat vir rooi grondkleur verantwoordelik is, word egter in sommige dreinerings- gebiede oorbeklemtoon. Die soort en hoeveelheid soute tesame met die tipe kleiminerale het 'n meer dominante invloed op hierdie gronde se struktuurstabiliteit. Die groot variasie in hidrouliese eienskappe van gestruktuurde gronde kan moontlik beter verstaan word, indien die beskrywing van die graad, tipe en grootte van die struktuureenhede met spesifieke parameters gekwantifiseer kan word. Die oorgang tussen horisonte is ook van belang. By dupleksgronde kom daar in die algemeen 'n drastiese afname in infiltrasie van die A- na die B-horisonte voor. Die lae infiltrasievermoë van die B- horisonte veroorsaak dat horisontale infiltrasie tot 'n groter mate as vertikale infiltrasie plaasvind. Aansienlike interkorrelasie tussen fisiese, chemiese en kleimineralogiese eienskappe kom voor wat tot lae verwantskappe aanleiding gee. Indien gronde egter in pedologiese eenhede gegroepeer word en die korrelasies vergelyk word, kan sommige daarvan verhoog of beter verstaan word.
Open Access
Bio-stimulatory properties of a Lupinus albus L. seed suspension
(University of the Free State, 2005-05) Van der Watt, Elmarie; Pretorius, J. C.; Van der Westhuizen, A. J.
English: The sustainable increase in crop yields on available land, in light of an ever growing world population, remains an issue of importance for agronomists and plant physiologists alike. With this in mind, the underlying study was aimed at investigating the bio-stimulatory properties of a Lupinus albus L. seed suspension (SS) as well as its potential to be applied as a natural plant growth regulator in the horticultural and agricultural industries. Preliminary bio-tests, including the respiration rate of monoculture yeast cells, seed germination and seedling growth, strongly indicated that the rather crude seed suspension possessed the ability to manipulate these aspects when applied exogenously. SS significantly increased the respiration rate of monoculture yeast cells and, although it did not have a significant effect on seed germination, stimulated seedling growth in terms of coleoptile and root length in a number of crops at an optimal concentration of 5 mg L-1. Subsequently, the effect of SS on the yields of a variety of vegetable crops, flowers and cash crops was tested under field conditions at a concentration of 5 mg L-1. This concentration proved to be optimal for most vegetables, flowers, maize and wheat as either flower formation or yield of different crops were significantly enhanced. The most significant yield increases, after a foliar spray treatment with SS at an early growth stage, was observed in beetroot, lettuce and carrots under drip irrigation by increasing the yield with 9.3, 20.0 and 24.3 ton ha-1 respectively. This supplied the rationale for activity directed isolation and purification of the active bio-stimulatory compounds involved using standard chromatography techniques. By means of NMR spectroscopy and mass spectrometry the active compound was identified as a triglyceride, glycerol trilinoleate, and its structural formula elucidated. It was concluded from this study that the development of a natural product with growth regulating and yield enhancing properties should be considered strongly from a commercial perspective.
Open Access
Biokatalitiese komponente in sade van enkele spesies uit die families Fabaceae en Caryophyllaceae
(University of the Free State, 2002-11) Du Plessis, Helena Aletta; Pretorius, J. C.; Potgieter, G. P.
English: In the south of Germany it was observed that species of the families Fabaceae and Caryophyllaceae survived an unknown environmental catastrophy while this was not the case with plant species from other families (Huster, Personal observation). Seed suspensions of some species belonging to these two families were applied to agricultural crops as foliar sprays, improving both the vegetative growth and yield of the crops. This supplied the rationale to test the seeds of South African species from these two families for its biocatalytic properties. Seeds of two species from each of the mentioned families namely Acacia karroo and Acacia erioloba from the family Fabaceae as well as Pollichia campestris and Dianthus basuticus from the family Caryophyllaceae, were subsequently screened for similar biocatalytic properties. Not much information on the chemical composition of these four plant species and nothing at all about their biocatalytic activity could be found in literature. In the study that followed biocatalytic activity was confirmed in a seed suspension of Acacia erioloba (Fabaceae) as it increased the respiration rate of a mono culture yeast cells as well as root growth of Cress-seedlings markedly. Activity directed liquid-liquid extraction of the A. erioloba seed suspension revealed that most of the biocatalytic activity was present in the ethyl acetate fraction. Further column chromatographic fractionation of the ethyl acetate fraction produced 13 combined column fractions of which two were active. Activity directed preparative thinlayer chromatographic purification of compounds in one of these fractions produced one active compound that was identified as 2-ethylhexylphtalate by means of nuclear magnetic resonans (NMR) spectroscopy. From the second column chromatography fraction six compounds were isolated but in such small amounts that NMR analysis was not possible. By means of colour reagents it was, however, established that three of the six compounds were alkaloids, two were terpenoides and one a coumarin. It is recommended that active components in this group should be identified in a follow-up study by initially extracting much more seed material.
Open Access
Causes and impact of desertification in the Butana Area of Sudan
(University of the Free State, 2006-11) Elhag, Muna Mohamed; Walker, S.
English: 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 vi 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 miimum 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 vii 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 3×3 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 ndicated 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 “Tashur”. 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.
Open Access
Changes in the phosphorus status of soils and the influence on maize yield
(University of the Free State, 2003-05) Schmidt, Christiaan Jan Jacob; Du Preez, C. C.; Adriaanse, F. G.
The principle objective of this study was to obtain P fertilizer guidelines for large maize producing areas west of the Drakensberg mountains according to a similar approach previously used in KwaZulu-Natal based on the sufficiency concept of soil extractable P. Data from 14 different P fertilizer trials at various localities in the Free State, Gauteng, Mpumalanga and North West provinces was used. Different phosphorus treatments were applied for all trials in order to establish differences in extractable soil P levels which were expected to have corresponding effects on maize yield. Long-term rainfall varied from 990 mm per annum for the Athole trial in the eastern maize producing region to 494 mm per annum for the Wolmaransstad trial in the western region. The duration of trials varied between one and nine seasons. Firstly, simple regression equations with high R²-values were obtained for relationships between Ambic 1 and Bray 1 extractions over soils, but since it was demonstrated that relationships for different soils differed significantly from each other the use of these equations may result in a very high degree of inaccuracy with respect to P fertilizer recommendations. Soil properties had a significant effect on the efficacy of the two extractants. Furthermore, slopes of relationships between Ambic 1 and Bray 1 could be predicted by using exchangeable Ca in simple regression relationships (R²-values of between 80 and 83%). Secondly, it was established that P requirement factors (PRF's) cannot be obtained over soils (R²-values varied between 10 and 54 %), but rather for different soils separately (R²-values varied between 75 and 99%). Differences between the PRF's in total soil volumes (1.7 to 63.2 for Ambic 1 concentrations and 0.8 to 27.3 for Bray 1 quantities) indicated that the soils used in this study differed in their behavior to applied P. Phosphorus requirement factors could be predicted by a simple regression equation using degree of leaching based on the clay content as input parameter (R²-values between 60 and 78 %) as well as six multiple regression equations using either one of exchangeable Ca, Mg, K, silt content or degree of leaching based on the clay content (R²-values between 52 and 99 %) as input parameters. The implementation of any of these regression equations should be practical since all the parameters are usually included in standard analysis. However, the simple regression with degree of leaching based on the clay content appears to be an obvious option above the multiple regression equations since it is based on five input variables, i.e. exchangeable Ca, K, Mg, Na and clay content. Lastly, threshold extractable P values were derived for 10 out of the 14 localities that have been included in this study with varying R²-values. These threshold extractable P values were related to soil properties and it was found that the degree of leaching and silt-plus-clay content were the parameters that explained most of the variation. However, it was decided to explore only the relationships between threshold extractable P values and silt-plus-clay contents in more detail. By excluding data from two localities of which the topsoil contained lime, the R²-values of the mentioned relationships improved substantially so that threshold extractable P values could be derived from the silt-plus-clay content range of the other eight localities. For example the threshold extractable soil P concentrations based on Bray I for the total soil volume to obtain 90 % relative yield varied from 33.5 mg kg-¹ at 13 % silt-plus-clay to 14.6 mg kg-¹ at 60 % silt-plus-clay. These P (Bray 1) thresholds are much higher on the sandy soils than the value of 19 mg P kg:' (Bray 1) for 95 % relative yield currently in use according to existing guidelines. This may not necessarily imply that more P fertilizers will be sold according to higher soil P thresholds obtained in this study, since the corresponding soil sampling procedure also measures more residual P from enriched zones over rows where P fertilizer was band placed. The soil sampling procedure according to existing guidelines excludes sampling from these zones.
Open Access
Characterisation and effects of different levels of water stress at different growth stages in malt barley under water-limited conditions
(MDPI, 2022) Bello, Zaid Adekunle; Van Rensburg, Leon D.; Dlamini, Phesheya; Tfwala, Cinisani M.; Tesfuhuney, Weldemichael
Malt barley is typically grown in dryland conditions in South Africa. It is an important grain after wheat, but little is known about its water requirements and, most importantly, how it responds to water stress. Determining when water stress sets in and how malt barley responds to water deficit during its growing season is crucial for improved management of crop water requirements. The objectives of this study were to evaluate the response of transpiration (T), stomatal conductance (SC), and leaf water potential (LWP) to water stress for different growth stages of malt barley and to characterise water stress to different levels (mild, moderate, and severe). This was achieved by monitoring the water stress indicators (soil- and plant based) under greenhouse conditions in well-watered and water-stressed lysimeters over two seasons. Water stress was characterised into different levels with the aid of soil water content ‘breaking points’ procedure. During the first season, at the end of tillering, flag leaf, and milk/dough growth stages, which represent severe water stress, plant available water (PAW) was below 35%, 56%, 14%, and 36%, respectively. LWP responded in accordance to depletion of soil water during the growing season, with the lowest recorded value to −5.5 MPa at the end of the milk/dough growth stage in the first season. Results also show that inducing water stress resulted in high variability of T and SC for both seasons. In the second season, plants severely stressed during the anthesis growth stage recorded the least total grains per pot (TGPP), with 29.86 g of grains. The study suggests that malt barley should be prevented from experiencing severe water stress during the anthesis and milk/dough stages for optimum malt barley production. Quantification of stress into different levels will enable the evaluation of the impact of different levels of stress on the development, growth, and yield of barley.
Open Access
Characterization and modelling of water use by amaranthus and pearl millet
(University of the Free State, 2013-01) Bello, Zaid Adekunle; Walker, Sue
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.
Open Access
Characterization of Yr15 resistance to stripe rust of wheat
(University of the Free State, 1999-12) Kotzé, Pieter Malan; Pretorius, Z. A.; Spies, J. J.; Viljoen, C. D.
English: Stripe (yellow) rust, caused by Puccinia striiformis Westend f. sp. tritici, is a major disease of bread wheat, Triticum aestivum L., in South Africa. The objective of this study was to evaluate the Yr15 gene as a potential resistance source for breeding purposes. Histological evidence was provided that Yr15 conditions a hypersensitive reaction typical of non-durable resistance. Fluorescence, phase contrast and scanning electron microscopy complemented each other and may be used to quantify various aspects of resistance expression in the wheat stripe rust pathosystem. The frequency of stomatal penetration was extremely low and no meaningful differences in early infection structures were observed between the resistant Yr15 and susceptible Avocet 'S' lines. Papillae were not involved in the resistance response to pathotype 6E16. In studies of defense-related proteins, an induction of β- 1,3-glucanase activity in both resistant and susceptible wheat genotypes was detected within 96 h post-inoculation (hpi). This induction, however, was constitutive and did not depend on the infection process. Likewise, no changes in chitinase activity were observed between resistant and susceptible plants. A pronounced increase of peroxidase activity between 0 and 168 hpi, in both infected and uninfected resistant and susceptible plants was observed, suggesting a constitutive expression of this enzyme. Using AFLP (Amplified Fragment Length Polymorphism) technology to detect possible molecular markers for Yr15, two primer combinations yielded polymorph isms between the bulked susceptible and resistant SST55 x Yr15/6*AvS F2 DNA, as well as between the susceptible and resistant parents. Fragment sizes ranged from 50 to 420 bp. Of these, one 95 bp fragment identified using primers MseCAA + Eco-ACA and one 205 bp fragment identified using primers MseCAA + Eco-AAC were detected in the resistant parent and the resistant F2 group. This indicates that these fragments are closely associated with Yr15 making them ideal candidates for further investigation.
Open Access
Chemical factors influencing dry bean yield
(University of the Free State, 2000) Molahlehi, Lebone; Pretorius, J. C.; Ceronio, G. M.
English: The abscission of the reproductive organs (flower buds, flowers and pods) of the common bean, Phaseolus vulgaris L., has proved to be a very serious problem affecting bean yields. The yield reductions are mainly significant under conditions of environmental stress, which aggravate production of the abscission causing hormones such as ethylene. The study therefore evaluated the effect of certain nutrients, other chemicals and ethylene (growth regulator) applied as foliar sprays to determine their effect in as far as abscission of the reproductive organs in beans is concerned. Glasshouse trials were carried out during the 1998/99 and 1999/00 seasons to investigate the possible preventative role of some micro and macro-nutrients, abscission inhibitors (e.g. Silver) and growth regulators (e.g. ethylene) all applied as foliar sprays. In another trial, three cultivars were compared where no treatments were applied. Data were collected and the following parameters were measured; number of flowers and pods that abscised, pods formed and abscised, pods that matured and could be harvested, dry pod mass at harvest, seed number per pod, dry seed mass per pod and total yield per hectare. Data were analyzed using a SAS software system and interpreted accordingly. The results of this study indicated that flower and pod formation might not be considered as the major factor affecting bean yields. The reason being that flowering percentage was not very different between the treated and the untreated plants. The abscission of flower buds, flowers and pods, however, was relatively high in general, indicating that this is possibly a primary factor affecting poor yields in beans. Ethrel, an ethylene releasing compound, applied as a foliar spray, aggravated the problem of flower and pod abscission. An ethylene antagonist (silver salt) had a slight alleviating effect on organ abscission but did not improve the final yield significantly. Molybdenum and potassium salts improved flower formation to a certain extent but had no real alleviating effect on organ abscission. This was also revealed in the lack of yield improvement. Foliar application of a copper salt as well as treatment with ComCat®, a natural product with biocatalytic properties, did not influence flower and pod abscission significantly, but increased the final yields to some extend. The latter indicates that other factors besides organ abscission could have had an effect on determining the final yield outcome. In the light of these findings, it is suggested that further studies be undertaken to test a wider range of chemicals for their potential to circumvent flower and pod abscission and improve bean yields. The latter could also include treatment at other growth stages, including post flowering. Other attempts could include an investigation into the effect of other known ethylene antagonists such as AVG (L-a.-(2-arninoethoxyvinyl)-glycinehydrochloride), an inhibitor of ethylene synthesis and NBD (2,S-norbornadiene), an inhibitor of ethylene action, on organ abscission and yield outcome in beans. As the results also showed that other factors besides organ abscission could have played a role in yield improvement, it might be worthwhile to investigate the effect of promising chemicals on physiological processes.
Open Access
Climate change impacts on temperature and chill unit trends for apple (malus domestic) production in Ceres, South Africa
(MDPI, 2021) Tharaga, Phumudzo; Steyn, Abraham Stephanus; Coetzer, Gesine Maria
Climate is an essential part of crop production, determining the suitability of a given region for deciduous fruit products such as apples (Malus domestica). It influences the yield and quality of fruits. There is strong evidence of global and regional-scale climate change since the advent of the industrial era. In South Africa, mean surface temperatures have revealed a warming trend over the last century. This study aimed to assess the impact of climate change on temperature and chill unit trends for apple production in Ceres, South Africa. The daily positive Utah chill units (DPCU) model was used as frequent high temperatures can lead to a high negation volume. Historically observed (1981–2010) and future projected (2011–2100) temperatures were obtained from the South African Weather Service (SAWS) and three ensemble members of the Cubic-Conformal Atmospheric Model (CCAM), respectively. The latter employed the RCP8.5 pathway. Linear trends were calculated for temperature and accumulated PCUs for the historical base period. The probability of accumulating specific threshold PCU values for both historical and future periods was assessed from cumulative distribution functions (CDFs). The historical change in minimum temperatures showed no significant trend. Ceres revealed a warming trend in maximum temperatures over the historical period. By the 2080s, the probability of not exceeding a threshold of 1600 PCUs was exceptionally high for all ensemble members. Future projections showed a decline in the accumulated PCUs of 2–5% by the 2020s, 7–17% by the 2050s, and 20–34% towards the end of the 20th century. Based on these results, it is clear that winter chill units are negatively influenced by climate change. The loss in yield and fruit quality of apples due to climate change can negatively impact the export market, leading to significant economic losses for apple production in the Ceres area.
Open Access
The compaction susceptibility of soils in the Free State
(University of the Free State, 1998-12) Nhantumbo, Alfredo Bernardino Julio Da Costa; Bennie, A. T. P.
Crop production in the Free State is mainly performed on sandy soils. The compaction of these apedal soils can present a management problem which affects crop yields and financial returns. In addition, environmental problems such as soil erosion and siltation of rivers may result. The main objective of the study was to see whether the easily measured soil properties, texture and organic matter, could be used to classify the compact ion degree of soils and to predict their compaction susceptibility. Additional objectives were to define critical densities beyond which crop growth would be impeded, to test the applicability of the relative bulk density concept proposed by Bennie & Van Antwerpen (1988) and to propose a procedure for the prediction and classification of the compactibility of the soils in the Free State. Finally, the results were compared to the data from forestry soils (Smith, 1995) for more humid conditions. Twenty two (22) samples of selected soils covering a large range of soil texture variation were submitted to a determination of the maximum bulk density (using the Proctor test), minimum bulk density, uniaxial compression test and penetrometer resistance measurements. A procedure for the determination of minimum bulk density was developed. Good quadratic relationships were obtained between the maximum bulk density and silt plus clay or organic matter content as loss on ignition. The minimum bulk density can be predicted from the silt plus clay percentage or loss on ignition. When the actual bulk density and the silt plus clay or loss on ignition percentages are available, the relationships allow the estimation of maximum and minimum bulk densities. The measured bulk density can then be used to calculate the relative bulk density of a soil. Threshold relative bulk density classes (5) were derived which can be used to evaluate the degree of denseness or looseness of the soil, and its suitability for different applications. It is proposed that compactibility be regarded as the difference between the maximum and minimum bulk densities, instead of simply the maximum bulk density. Different classes of compactibility were derived. The compression index of all the soils at the critical water content showed good relationships with the silt plus clay, clay or organic matter contents of the soils. Compressibility classes were also established. A combination of the compactibility and compressibility of soils was used to determine its susceptibility to compaction. Compaction susceptibility was classified into five groups varying from very low to very high degrees of susceptibility for apedal soils. The data set from this study when combined with the data reported from the study of forestry soils by Smith (1995), made it possible to derive equations with a more universal applicability for the prediction of the maximum and minimum bulk densities, critical water content as well as compression index. The results from this investigation, when properly transferred in popular terminology set guidelines for the evaluation of the degree of compactness and compaction susceptibility of soils. These guidelines will allow persons responsible for soil evaluation to draw more meaningful conclusions from bulk density measurements.
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.
Open Access
Conceptual hydrological response models of selected arid soilscapes in the Douglas area, South Africa
(University of the Free State, 2016-02) Tinnefeld, Martin; Le Roux, P. A. L.; Van Tol, J. J.
The conceptual hydrological response model (CHRM) is a powerful tool, able to transfer hydrological information of hillslopes. Soil, a first order control of partitioning of water flow, is often the only source of information of hillslope water flowpaths and storage mechanisms. Conceptual hydrological models applied at different scales, serve as the framework to understand and structure the hydrological response of sites, hillslopes and catchments. They complement decision making and planning of natural resource allocation and delineation for land-use change in ecology, agriculture, mining and urbanisation. Soil morphology, chemistry and hydrometrics are used singularly and complimented in combination as indicators and controllers of hydrology to construct CHRMs. The more accurate the input parameters of soil morphology, chemistry and hydrometrics, the more accurate the final CHRM. Arid soils of South Africa have been neglected to be investigated for their hydrological properties. This is due to the low rainfall, infrequent hydrological response and resulting low output of data. Where such hydrological studies have been performed under higher rainfall climatic zones, soil has been found to respond as a store and conduit of water. These are referred to as storage mechanisms and flowpaths within the soil and can be determined by studying the soils’ morphology, improved by soil chemistry and are verified by application of hydrometrics. Soil distribution patterns are not random and are influenced by hillslope hydrology. The vastness of the arid regions of South Africa, make it difficult to select singular representative hillslopes. However, by describing the soil distribution patterns of randomly distributed detailed soil maps within different land types, allowed for soil distribution trends to be identified in this study. These soil distribution trends were seen to coincide with terrain morphological units. Representative modal profiles were selected on dominant and representative terrain morphological units of 4 different land types on criteria that they are representative soils of the land type and sites. The soils have well developed horizons resulting in vertical pedological variation including different degrees of carbonate precipitation. Methodology of other research, to interpret morphology in higher rainfall regions of South Africa, was used to construct a conceptual hydrological response model for the arid hillslopes and the region in general. Hydrological properties of modal profiles, were used to confirm the concepts. A class 1 hillslope hydrological response: soil/bedrock interflow to wetland; dominates on all four sites. Individual sites and their respective soil distribution patterns showed fast pedon recharge to the soil/rock interface. Pedon interflow at higher lying topographical positions with associated steeper slope, contributed to carbonate as dominating morphological flowpath indicator lower lying in the landscape. This is primarily driven by a low rainfall/high evapotranspirative demand of arid climates. Topographically lower lying soils showed reduced infiltration due to high alluvial clay and silt deposits and/or soil matrix saturation with carbonate precipitation sufficient to reduce the permeability of the soil.
Open Access
Correlation between argronomic and environmental phoshorus analyses of selected soils
(University of the Free State, 2012-06) Nthejane, 'Mabatho Margaret; Du Preez, C. C.; Van Huyssteen, C. W.
English: In crop production phosphorus (P) is an essential nutrient for crop growth, and hence P fertilization is necessary to achieve optimum yields. However, this can induces in soil a P concentration which may contributes to eutrophication of fresh water bodies. Soil P tests are therefore considered very useful in setting threshold values important for both agronomic and environmental management purposes. Soil P tests developed from a water pollution protection point unlike agronomic P tests are not easily adapted for use on a routine basis because they are not considered, for this purpose, and this could make agronomic P tests more practical for routine environmental P assessment also. Determination of appropriate agronomic P tests for this purpose however, involves evaluating the potential use of the tests for environmental purposes. Hence, the objective of this study was to review the current methods used to determine the agronomic and environmental P status of soils, and to establish whether P extracted from a range of soils by various agronomic and/or environmental P determination methods are related or not. Soil samples from the orthic A horison were collected in three cropping areas in the Free State province, namely Jacobsdal, Bloemfontein, and Ficksburg. These samples were treated with K2HPO4 to induce different phosphorus concentration levels and then incubated at room temperature for three months. During incubation the samples were subjected to several wetting and drying cycles to ensure that the applied phosphorus equilibrated. The samples were then analysed for P using the extractants of Olsen, Bray 1, Truog, ISFEI and citric acid commonly employed for routine analysis to establish the agronomic P status of soils. In order to establish the environmental P status of the soils, the samples were analysed for using the extractants calcium chloride (CaCl2) and ammonium oxalate [(NH4)2C2O4.H2O]. The latter was used to calculate the degree of phosphorus saturation (DPSox). The results showed significant relationships among agronomic P tests when data of individual soils were analysed separately (r2=0.65-0.99) and, when data of all soils from a sampling area were pooled (r2=0.52-0.87). All the relationships were significant for the Ficksburg soils (r2≥0.55) and for the Bloemfontein soils (r2≥0.82) but not for the Jacobsdal soils. For the latter soils the Truog-P correlations with Olsen-P (r2=0.44), Bray 1-P (r2=0.42) and ISFEI-P (r2=0.35) were not significant, probably due to that they are calcareous. Significant relationships were also obtained for P extracted by the environmental P tests when regression analysis was done for each individual soil (r2≥0.80). However, when data of soils from a sampling area were pooled significant relationships were obtained for Bloemfontein soils (r2=0.92) and Ficksburg soils (r2=0.56) while Jacbosdal soils (r2=0.33) showed an insignificant relationship. Pooling data of all soils from the three sampling areas also resulted with a lower correlation coefficient (r2=0.40) implying a poor relationship between the environmental P tests. The correlation between P extracted by the agronomic tests and CaCl2-P showed positive relationships (r2 ≥0.57) except in a few instances. Truog-P and citric acid-P showed a poor correlation with CaCl2-P when the Jacobsdal soils’ data were pooled (r2=0.22 and 0.35 respectively). Pooling of all soils’ data resulted also in a poor correlation between CaCl2-P and Truog –P (r2= 0.28). The DPSox correlated significantly with the extractable P of all agronomic tests when the individual soil’s data were analysed separately (r2 ≥0.73). However, when data of all soils from a sampling areas were pooled for regression analysis, all relationships were significant for the Bloemfontein soils (r2 ≥0.70), but not for the Jacobsdal soils, and Ficksburg soils. Pooling data of all soils from the three sites resulted with a positive relationship between DPSox and the extractable P of all agronomic tests (r2 ≥0.50), except ISFEI (r2 ≥0.45). The threshold values estimated for agronomic tests with regression equations from CaCl2-P DPSox threshold values varied greatly between individual soils and even the soils groups of a sampling area. The threshold values for all soils when based on CaCl2 implied that if the extractable P status of cropped soils are maintained at optimum levels for Bray 1, Truog, ISFEI and citric acid the soils may be a threat to water pollution. The opposite is true with the estimated threshold values when based on DPSox. The results therefore showed that agronomic tests can be used also for environmental management of P although only the Olsen test showed the potential for developing a single threshold value for all soils.
Open Access
Developing a digital soil mapping protocol for southern Africa using case studies
(University of the Free State, 2013-06) Van Zijl, G. M.; Le Roux, P. A. L.
English: Although there is an increasing need for spatial soil information, traditional methods of soil survey are too cumbersome and expensive to supply in that need. Digital soil mapping (DSM) methods can fulfil that need. Internationally, DSM is moving from the research to the production phase. As soil-landscape interaction and availability of data varies between locations, local DSM research is needed to make its application practical. This research aims to produce a working DSM protocol which can be used for mapping large areas of land in southern Africa. The protocol must meet soil surveyors where they are at, being easy enough to follow, while also allowing for the creation of products needed by industry. To keep the link with industry’s needs, a case study approach was followed. Four case studies were done in succession, with the protocol being improved with every case study. The case studies cover an array of challenges faced by soil surveyors. In the first case study a baseline protocol was created when two land types near Madadeni were disaggregated in a series of soil maps. With each map, more information was incorporated when creating the map. For Map 1 only the land type inventory and terrain analysis were used. A reconnaissance field visit with the land type surveyor was added for the second map. Field work and a simplified soil association legend proved to improve the map accuracy for Maps 3 and 4, which were created using 30% and 60% of the observations points as training data respectively. The accuracy of the maps increased when more information was utilized. Map 1 reached an accuracy of 35%, while Map 4 achieved a commendable accuracy of 67%. Principles which emerged was that field work is critical to DSM, more data input improves the output and that simplifying the map legend improves the accuracy of the map. An unrealistic demand for a soil survey of 37 000 ha of land in the Tete Province, northern Mozambique, possibly infested with land mines, in 8 working days by two persons, created an opportunity to apply the soil-land inference model (SoLIM) as a digital soil mapping tool. Dividing the area into smaller areas where unique soil distribution rules would apply (homogeneous areas, HA’s) was introduced. A free survey was conducted along the available roads of the area. The final soil map for 15 000 ha had an accuracy of 69%. A principle which emerged was that inaccessible areas can be mapped, provided that they occur within surveyed HA’s. Near Namarroi, Mozambique, the potential of DSM soil survey methods to rapidly produce land suitability maps for a large area with acceptable accuracy was evaluated. Conditioned Latin hypercube sampling (cLHS) was introduced to determine field observation positions. SoLIM was used to run an inference with soil terrain rules derived from conceptual soil distribution patterns. A restriction of the expert knowledge based approach was found in that only six soil map units (SMU’s) could be determined per HA. The map achieved an overall accuracy of 80%. Land suitability maps were created based on the soil class map. In the Kruger National Park a soil map was used to create and extrapolate 2-dimensional conceptual hydrological response models (CHRM’s) to a 3-dimensional landscape. This is a very good example on how value could be added to a soil map. An error matrix convincingly identified problem areas in the map where future work could focus to improve the soil map. The current data indicates that at least 28 soil observations are necessary to create a soil map to an acceptable standard. When minimum observation criteria are met, observation density is irrelevant. The cLHS method to pre-determine observation positions improved the usability of observations. Although more research is needed to accurately determine the minimum observation criteria, an observation strategy is suggested. A 15 step protocol is produced with which it was shown that soil surveyors could produce a variety of maps in diverse situations. The protocol relies on the expert knowledge of the soil surveyor, combined with field observations. It has the advantages that fewer observations are necessary, map accuracy assessment is possible, problem areas are identified and under certain conditions unsurveyed areas can also be mapped. On the down side, there is a limitation of six SMU’s per HA. Further research needs to be done to determine the minimum criteria for soil observations, and soil distribution relationships between soil and remotely sensed covariates.
Open Access
Developing an index for phosphorus loss from sugarcane soils in Mauritius
(University of the Free State, 2015-01) Mardamootoo, Tesha; Du Preez, C. C.; Ng Kee Kwong, K. F.
Sugarcane is the major crop cultivated in Mauritius and currently occupies some 53,500 hectares of land with an average cane productivity of 74 t ha-1. Phosphorus (P) fertilization on deficient soils can increase annual cane production up to 24 t ha-1. Currently the sugarcane industry consumes some 3,350 tonnes of P2O5 in the form of soluble mineral fertilisers with an additional 1,110 tonnes P2O5 coming from filter muds. While the application of P to agricultural land is essential in maintaining crop productivity, non-point sources of P leaving the agricultural landscape can cause accelerated eutrophication of surface waters, thereby impairing water quality. Although these losses may not always be of economic importance to farmers, only small amounts of P can trigger eutrophic conditions in freshwaters. Past studies in Mauritius have shown that runoff P concentrations of limnological significance (> 0.1 mg P L-1) can occur, particularly during high flow events. Hence, this study was initiated to provide a better understanding of P mobilization from cane fields and to integrate factors influencing P movement into a P risk assessment tool. To achieve these objectives, rainfall surface runoff simulations were conducted on 20 sites representing the main soils under which sugarcane is cultivated. The tests were done on runoff plots (2.1m by 0.75m in duplicate) at three slope categories (0 to 8%, 8 to 13%, 13 to 20%) and under three different rainfall intensities (50, 100, 150 mm hr-1) for a duration of 30 minutes each. In an attempt to evaluate the impact of continuous fertilizer application on the environmental soil P status of sugarcane soils in Mauritius, it was found that 0.01M CaCl2-P of surface soils was linearly correlated to dissolved runoff P losses (r2= 0.92). Thus simple laboratory soil extractions with 0.01M CaCl2 is a suitable estimate for dissolved P losses when field experimentation is not possible. Since the use of the routine agronomic soil test (0.1M H2SO4 soil extraction) for sugarcane in Mauritius provided a more accessible analytical tool for P management, the relationship between the 0.01M CaCl2-P and 0.1M H2SO4-P was established and it was found that soils with 0.1M H2SO4-P above 160 mg kg-1 can potentially impair runoff water quality. While soil testing provides a reliable pointer of desorbable P in surface soils, it gives no indication of its potential for transport during runoff and erosion. Results from the simulation studies showed that runoff and erosion potential varied across the different soil types. It was also observed that with increasing rainfall intensities and field slopes, P mobilisation was enhanced due to increases in runoff and erosion rates. The results further showed that total runoff P was more strongly correlated with suspended sediments (r2=0.92) present in runoff waters than with runoff volume (r2=0.49) indicating that a greater proportion of the P transported in runoff occurred mostly as particulate P rather than dissolved P. Actually, about 90% of total P loss in runoff waters was mobilised in particulate forms regardless of field soil type, rainfall intensity and field slope. Using these research findings and historical data, the P index was developed to rank site vulnerability to P loss by accounting for source (dissolved P, particulate P, P application rate, method of application and application timing) and transport factors (soil erosion, surface runoff potential and precipitation) such that site-specific management practices can be implemented to critical source areas to minimize offsite P export. The proposed improved management practices to reduce P loss from fields in the P index include terracing, construction of diversions, field borders, field strips, grassed waterways, forest buffers and herbaceous cover. Sensitivity analysis and edge-of-plot field testing were used to assess the behavior and performance of the P index. The results indicated that further evaluations at a watershed scale would be more insightful about the strengths and weaknesses of the P index as a risk assessment tool. Besides, further evaluations of this tool will eventually lead to improvements in estimating the impacts of agricultural P management on downstream water quality.
Open Access
Development of an index for wheat stripe rust infection
(University of the Free State, 2001-11) De Wet, Linda; Walker, S.; Pretorius, Z. A.
English: The main objective of this project was to develop an early warning index for infection of stripe rust (Puccinia striiformis f. sp. tritici) in susceptible cultivars of wheat (Triticum aestivum) for the main wheat growing areas of the R.S.A. Various authors are in agreement that temperature and moisture are the major climatic factors necessary for infection by stripe rust. Temperature limits from the literature vary between -4 and 30°C and no cut-off values were available. It was therefore decided to run an experiment to observe infection of stripe rust under conditions of high relative humidity and a range of temperatures, namely 5°C, 10°C, 15°C and 20°C. Results from the experiment indicated that little or no infection occurred for the 20°C temperature level and for exposure time periods of one hour or less. Exceptions occurred when sudden drops in temperature with removal of plants from growth chambers to the glasshouse took place. For both incidence and severity, it was found that temperatures greater than 22°C inhibited infection. The Pan 3349 cultivar was found to exhibit higher incidence than Karee. The statistical analysis shows significance for severity for Experiment 1 and 2, but not for incidence. Temperatures for Experiment 2 were slightly lower than those in Experiment 1 and could be explained by the presence of the fans in Experiment 2, to allow evaporation of dew from the sensors, allowing the sensors to register correct readings. The resulting higher moisture conditions in the still air In Experiment 1 could have contributed to the higher severity observed in Experiment 1. Conditions in the laboratory are not the same as in the field and possibilities for infections at higher temperatures in the field have been reported by Park (1990). He warned against extrapolation of results from the laboratory to the field, but it was nevertheless decided to use the results from the experiment as a basis for the development of an index for stripe rust infection of susceptible wheat cultivars in South Africa. Values of 14 day total degree days (TDDI4) were calculated from total degree hours (TDH) acquired from the experiment. TDDl4 for the experiment was calculated by summing degree days (DD) from inoculation until 14 days and relating them to average incidence observed on day 14. Two linear regression lines were obtained, one for 5°C 15°C temperature level and the other for the 15 °c - 20°C temperature levels. A cut-off TDDl4 value of 227 where the two lines crossed indicated 67 % cut-off value for incidence. TDDl4 was developed from this information and reads as follows: IfTDDI4> 227, then risk of incidence is low at < 67 %. IfTDDl4 < 227, then risk of incidence is high at > 67 %. TDDl4 was validated by testing on 1996 - 1998 and 1999 - 2000 data. Correct prediction values for highest and average incidence observation were 50 % and 29 % for 1996 - 1998 data (Table 4.7) and 40 % and 33 % for 1999 - 2000 data (Table 4.8). TDDl4 however, was thought to be impractical, so TDD7 was developed by using TDD7 for 7 days from a wet period. TDD7 = 128 was found to be 1.77 times less than the value for TDDl4 = 227 and so this value of 128 was used as the cut-off value. TDD7 thus reads as follows: If TDD7 > 128, then risk of incidence is low at < 67 %. If TDD7 < 128, then risk of incidence is high at > 67 %. TDD7 was validated using the 1999 - 2000 data, with total correct predictions of 53 % and 40 % (Table 4.10). It was therefore decided that TDD7 could be used by the producer as an early warning index, although the index would have to be tested in the field so that necessary improvements could be made. It is recommended that research on stripe rust-environment interaction be continued, also to include other diseases and the recommendations made in Chapter 2. Another possible recommendation would be to plan a refined laboratory experiment with a mechanistic approach to use a constant temperature data determined model in real life situations. Temperature and RH would vary and the results should be useful to producer, as well as the researcher. A model suitable for various other diseases as well, could be of great benefit when the cost of pesticides and fungicides are taken into account. The successful application of such a model would be of great benefit to all. After all, food is our fuel and sustainable production of high quality foodstuffs is essential to our survival.
Open Access
Die geohidrologie in die opvanggebied van die Moloporivier in die Noordelike Kalahari
(University of the Free State, 1977) Smit, Petrus Jurgens; Grobler, N. J.
Afrikaans: Die gebied wat 58 153 km2 beslaan en waarvan ongeveer 75 persent met sand bedek is, is uitsluitlik van ondergrondse water afhankliko Die gesteentes le hoofsaklik in die westelike en suidelike dele blootgele Die Kalaharilae is tot 180 m dik en die geologiese formasies onder die Kalaharilae is volgens boorgatgegewena vasgestel Die diepte van die grondwatervlak varieer tussen 15 m in dagsoomgebiede en 150 m in gebiede waar die Kalaharilae die dikste is. In die westelike dele Is die Kalaharilae waterdraend. In stollingsgesteentes lewer 34 tot 50 persent van die boorgate meer as 450 l/h en in sedimentire geateenteep behalwe die Formasie Dwyka, 50 tot 80 perssnte Boorgate wat wetenskaplik op Voor-Kalaharigeeteentes gekies is lewer 10 persent hoer suksease (lewering meer Ba 450 1/h) 88 boorgate wat op ander metodes gekies is. In 38 boorgate is watervlakakommelinge sedert 1967 gereeld gemeeto Seisoensveranderinge is ongeveer 3 keer groter in boorgate in dagsoomgebiede aa in boorgate waar'die Kalaharilae tussen 0 en 15 m dik is. Waar die Kalaharilae dikker as 15 m is, is geen seisoensveranderinge waarneembaar nie. Die gebied kan volgens die hoogtekontoere van die grondwatervlak in 8 grondwatereenhede r ingedeel word wat elk 'n afsonderlike grondwaterkompartement vorm. Die temperatuur van die grondwater varieer volgens die diepte van die grondwatervlak. Die geotermiese gradient is hiervolgens bereken as 54 m/°C in graniet, 50 m/°C in dolomiet, 35 m/°C in jaspiliet, 30 m/°C in lawa en 45 rn/°C 1n Kalaharilae. In totaal ia 204 grondwatermonsters chemies ontleed en totale opgeloste stowwe ia In 655 monsters bepaelo Die konsentrasie van T.O.S. in die grondwater in Voor-Kalaharigesteentea word progressief hoer met 'n toename in die dikte van die Kalaharilae. Die Kalaharilae veroorsaak nie 'n verhoging in die konsentrasie van totale opgeloste stowwe in die grondwater nie maer ioonuitruiling vind wel plaas waar grondwater deur die kleilae beweeg. Die grondwater in die Voor-Kalaharigesteentes en in die Kaleharilae kan elk in 4 tipes ingedeel word volgens die chemiese samestelling en oorsprong. Die grondwateraanvulling in dagsoomgebiede varieer tussen 2,2 persent in die weste by ‘n reenval van 220 mm per jaar, tot 3,8 persent in die ooste, by ‘n reenval van 550 mm per jaar. In gebiede waar die bedekking tussen 0 en 15 m is, is aanvulling moontlik 0,5 persent van die jaarlikse reenval en waar die Kalaharilae dikker as 15 m is, vind geen direkte aanvulling plaas nie. Die totale jaarlikse aanvulling tot die grondwater in die hele gebied is 227,2 x 106m3• Die totale grondwaterverliese in die gebied is 52,498 x 106m3 per jaar, waarvan 70 persent uit boorgate en 30 persent uit fonteine afkomstig is. Die jaarlikse toename in die waterverbruik word beraam op 1,5 persent in Blanke gebiede en 8 persent in Tuialandgebiede. Die waterverbruik deur mense en diere verteenwobrdig 39 persent van die totale gl'ondwaterverliese, verbruik deur besproeiing 36 persent, deur dorpe en myne 20 persent en deur verdamping en transpirasie 5 persent. Net 62 persent van die totale fonteinvloei word direk benut. Die totale grondwaterverliese in, die gebied verteenwoordig 23 persent van die jaarlikse bruto aanvulling, maar varieer tussen 91 en 17 persent in die verskillende grondwatereenhede. Die totale hoeveelheid water wat uit boorgate onttrek word is 36,856 x 106m3/j of 16 persent van die totale jaarlikse aanvulling. Die toekomstige waterverbruik in die gebied word beraam op 114,l4 x l06m3/j vir die jaar 2000, wat ongeveer 50 persent van die jaarlikse potensiele aanvulling verteenwoordige Die benutting varieer egter tussen 31 en 132 persent in die' verskillende grondwatereenhede. Ten einde die grondwaterpotensiaal beter te bepaal is verdere navorsing nodig op die natuurlike aanvulling tot die grondwater in dagsoomgebiede, die optimale dikte van die bedekking in verhouding tot aanvulling, die ondergrondse vloei deur die dolerietgange in dolomiet en op die ondergrondse vloei uit sommige van die grondwatereenhede. Ondersoek is ook nodig op die ontginningspotensiaal en stoorkapaaiteit van die verskillende formasies insluitende die Kalaharilae wat oor groot gebiede ‘n belangrike akwifeer vorm. Die ontginning van grondwater vernaamlik in granietgelJiede is 'n wesenlike probleem en navorsing op metodes en tegnieke van boorplek aanwysing sowel as boorgatontwikkellng is nodig. In groot gebiede in die noordweste is die grondwater onbruikbaar sout en ontsouting van grondwater is ‘n baie belangrike aspek vir toekomstige benutting. Navorsing behoort ook gedoen te word op die ontwerp en beplanning van kunsmatige opvanggebiede ten einde reenwater in sekere gebiede direk te benut.